Lincoln University Digital Thesis Copyright Statement The digital copy of this thesis is protected by the Copyright Act 1994 (New Zealand). This thesis may be consulted by you, provided you comply with the provisions of the Act and the following conditions of use: 
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you will use the copy only for the purposes of research or private study you will recognise the author's right to be identified as the author of the thesis and due acknowledgement will be made to the author where appropriate you will obtain the author's permission before publishing any material from the thesis. Lincoln University Digital Thesis Copyright Statement The digital copy of this thesis is protected by the Copyright Act 1994 (New Zealand). This thesis may be consulted by you, provided you comply with the provisions of the Act and the following conditions of use: 
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you will use the copy only for the purposes of research or private study you will recognise the author's right to be identified as the author of the thesis and due acknowledgement will be made to the author where appropriate you will obtain the author's permission before publishing any material from the thesis. THE INFLUENCE OF THE DIET OF THE ARGENTINE STEM WEEVIL, Listronotus
bonariensis (KUSCHEL) (COLEOPTERA: CURCULIONIDAE) ON THE FITNESS OF ITS
P ARASITOID, Microctonus hyperodae LOAN (HYMENOPTERA: BRACONIDAE):
PROSPECTS FOR 'INDIRECT' CONSERVATION BIOCONTROL
I
._:-l_.;r__ ~'_.:...-:~
A thesis submitted in partial fulfilment
.,-,---_ ..
of the requirements for the Degree of
Master of Applied Science
at
Lincoln University
by
Mauricio Alvaro Urrutia Correa
Lincoln University
2005
Abstract of a thesis submitted in partial fulfilment of the
requirements for the Degree of M. Appl. Sc.
THE INFLUENCE OF THE DIET OF THE ARGENTINE STEM WEEVIL, Listronotus
bonariensis (KUSCHEL) (COLEOPTERA: CURCULIONIDAE) ON THE FITNESS OF ITS
PARASITOID, Microctonus hyperodae LOAN (HYMENOPTERA: BRACONIDAE):
PROSPECTS FOR 'INDIRECT' CONSERVATION BIOCONTROL
By M. A. Urrutia Correa
The Argentine stem weevil, Listronotus bonariensis (Kuschel) (Coleoptera: Curculionidae), is
the most important pest species in New Zealand, costing the pastoral industry an estimated
NZ$78-251 m per year (Prestidge et al., 1991). The parasitoid wasp Microctonus hyperodae
Loan (Hymenoptera: Braconidae) was released in New Zealand in 1991 as a potential
biological control agent of the weevil (Goldson et al., 1994). As M hyperodae is a proovigenic species, its lifetime complement of eggs is set at adult emergence (Phillips, 1998).
Adult wasp fitness is likely to be affected by host nutrition, as resources carried over from
immature stages to the adult stage of parasitoids can vary with the size and nutritional quality
of the host (Jervis et al., 2001). The fecundity of Argentine stem weevil can increase after the
addition of pollen to its diet (Evans. and Barratt, 1995). Conversely, a decrease in gonad
development (i.e., vitellarium development and percentage of females with oocyte resorption)
can occur after consumption of ryegrass containing endophyte (Barker and Addison, 1996).
This research investigates tri-trophic-Ievel interactions on parasitoid fitness (i.e., egg load and
body size) by assessing field-collected adult weevils, and by providing bee-collected pollen,
buckwheat (Fagopyrum esculentum Moench cv. Katowase) pollen, ryegrass-only (Lolium
multiflorum L. cv. Tama) , or ryegrass (Lolium multiflorum Lambrechtsen cv. Aries)
containing endophyte (Neotyphodium lolii (Latch, Christensen and Samuels) Glenn, Bacon
and Hanlin in the host diet in the laboratory.
A survey was conducted in the Lincoln area to what extent pollen was part of the diet of fieldcollected Argentine stem weevils. The frequency and types of pollen grains consumed were
ii
analysed. Also, gut fullness (in terms of ryegrass particle content), parasitoid larval presence,
weevil gender and weevil gonad development were also assessed. Argentine stem weevil
adults fed on pollen in the field, although a low proportion of them contained pollen in their
gut. Gonad development was positively correlated with the extent of pollen consumption (i.e.,
number of individuals with sexually mature gonads).
A cage experiment was conducted in the laboratory. Potentially parasitised adult Argentine
stem weevils were collected from ryegrass on roadsides in the Lincoln area during 2004. They
were randomly allocated to four treatments: ryegrass-only (control), ryegrass plus beecollected pollen, ryegrass plus buckwheat pollen, and ryegrass with endophyte, each
replicated four times. Water was provided in every treatment. The parasitoids that emerged
were assessed for size (i.e., hind leg tibial length) and potential fecundity (i.e., egg load) (see
Phillips and Baird, 2001). Despite the fact that the addition of bee-collected or buckwheat
pollen to the host diet had no significant effect on the number of adult parasitoids emerged,
their body size or egg load (P > 0.05), some factors that could have affected a potentially
positive correlation between parasitoid fitness and pollen consumption are discussed.
Similarly, the addition of ryegrass with endophyte had no significant effect on the number of
adult parasitoids emerged, their body size or egg load (P > 0.05). This leads to the suggestion
that biological control (i.e., M hyperodae) and plant resistance (i.e., endophyte) strategies can
work in a complementary way in integrated pest management programmes.
The concept of benefiting the third trophic level with 'resource subsidies' made available
selectively to the second level has the potential to lead to the suggestion of a new mechanism
for enhancing parasitoid fitness, that is 'indirect' conservation biocontrol.
Keywords: Argentine stem weevil, Listronotus bonariensis, Microctonus hyperodae, ryegrass,
buckwheat, pollen, endophyte, conservation biological control, tri-trophic interactions.
iii
To my wife and daughters .. . ........ ,
" ......... far from being a purely passive victim , obliterated without a trace, the host is o ften able
to impress its mark ...... upon the insect parasito id that destroys it" (Sa lt, 1941 ).
iv
Acknowledgements
I would like to thank my wife Denise for her constant support and advice. Thanks as well to
my daughters Valentina and Magdalena, who despite their young age allowed Daddy to read,
write and think during those peak times. Thanks as well to the NZAID scholarship granted by
the New Zealand Government for the opportunity to study this Master's Degree.
Also I would like to thank my supervisor, Professor Steve Wratten, from Lincoln University
for his unconditional professional and personal support. Thanks also to my co-supervisor,
Craig Phillips, from AgResearch for sharing his expertise. Thanks as well to Bruce Chapman
from Lincoln University, for his generous knowledge and important contribution in this
Masters Degree. Many thanks also' to Mark Wade for his technical and intellectual
contribution to this thesis. Also, thanks to David Baird, from AgResearch for his help in the
statistical analysis of the data. Also thanks to Neville Moar from Manaaki Whenua Landcare
Research for his help in the pollen identification. Finally, thanks to all the staff and students
aligned to the National Centre for Advanced Bio-Protection Technologies who helped me in
several aspects of the present work.
v
Contents
Page
Abstract / keywords
11
Acknowledgements
v
Contents
VI
Tables
x
Figures
x
Plates
X111
Chapter 1: General Introduction
1
Biological control background
1
The Argentine stem weevil and current control practices background
3
Argentine stem weevil
3
Graminaceous crops and Argentine stem weevil in New Zealand
3
Management tools for Argentine stem weevil
6
Chemical control
6
Cultural control
6
Plant resistance
7
Biological control
8
Goal and Objectives
12
Hypotheses
12
Chapter 2: Gut contents of field-collected Argentine stem weevils
13
Introduction
13
Materials and Methods
14
1. Pollen content
14
2. Ryegrass particle content
15
vi
3. Gonad development in Argentine stem weevil
15
4. Parasitism
17
5. Statistical analyses
17
17
Results
17
Pollen content
Amount of pollen in the weevils' gut
17
Effects of pollen consumption on gonad development
19
Types of pollen
22
Ryegrass particle content
24
Parasitism
25
Effects of parasitism on weevils' gonad development and rye grass intake 25
28
Discussion
28
Pollen content
Argentine stem weevil diet breadth
28
Quantity of pollen in the weevils' gut
28
Effects of pollen consumption on gonad development
29
Types of pollen
32
Ryegrass particle content
34
Parasitism
35
Effects of parasitism on weevils' gonad development and ryegrass intake 36
Chapter 3: Effects of host diet on components of fitness of Microctonus hyperodae
Loan
38
Introduction
38
Materials and Methods
40
Experiment 1
40
1. Experiment set up
40
vii
2. Rearing plants for Argentine stem weevil in experiment 1
40
3. Argentine stem weevil cages
40
4. Parasitoid morphological measurement
41
5. Quantifying egg load of the parasitoids
41
Experiment 2
43
6. Experiment set up
43
7. Methods during and after exposure to parasitoids
44
8. Rearing plants for Argentine stem weevil in experiment 2
45
9. Gonad development in the Argentine stem weevil
45
10. Gut fullness of the Argentine stem weevil
46
11. Exposure of weevils to adult parasitoids
47
12. Statistical analyses
47
47
Results
47
Experiment 1
Parasitoid tibia length and egg load
Experiment 2
47
50
Diet consumption by the weevil
50
Survival and fitness-related data of Argentine stem weevil
52
Components of fitness of the parasitoid
56
Number of adult parasitoids emerged
60
60
Discussion
Experiment 1
61
Parasitoid body size and egg load
Experiment 2
61
63
Diet consumption by the weevil
63
Survival and fitness of Argentine stem weevil
65
viii
Fitness of the parasitoid
68
Number of adult parasitoids emerged
72
Chapter 4: General discussion
75
The host's mark
75
Implications on biological control
77
Selectivity issues
78
Is there room for improvement?
78
References
80
ix
Tables
Page
Table 1. Combination of treatments of experiment 2
43
Table 2. The relationship between tibia length and egg load for parasitoids emerged
from weevils fed with four treatments
58
Table 3. Combination of possible effects on the survival and/or fitness of host
and parasitoid produced by changes in the host's diet
78
Figures
Page
Fig. 1. (A) Map of New Zealand showing sites where M hyperodae was released in 1991
(adapted from Phillips et ai., 1997). (B) Map of South America showing the
approximate positions of the eight Microctonus hyperodae collection sites
(adapted from Phillips et ai., 1997)
9
Fig. 2. Reproductive morphology and anatomy of male and female Listronotus bonariensis
(from: Barker, 1989)
16
Fig. 3. Percentage of weevils from a range of collection dates that had consumed pollen
18
Fig. 4. Percentage of weevils that had consumed pollen in different abundance categories
18
Fig. 5. Percentage of weevils that had consumed pollen in different abundance categories in
both genders
19
Fig. 6. Percentage of sexually mature and immature weevils that had consumed pollen
over a range of abundance categories
20
Fig. 7. Percentage of sexually mature weevils over a range of collection dates
20
Fig. 8. Percentage of weevils of both genders with mature/immature gonads
21
x
Fig. 9. Sex ratio of weevils over collection dates
22
Fig 10. Pollen types and their percentages within Argentine stem weevils' gut
23
Fig. 11. Pollen types and their percentages on Argentine stem weevils' exoskeletons
23
Fig. 12. Percentage of weevils from both genders with three states of gut fullness
24
Fig. 13. Percentage of weevils with sexually mature and immature gonads with three
states of gut fullness
25
Fig. 14. Percentage of weevils that were or were not parasitised that had consumed
pollen in several abundance categories
Fig. 15. Percentage of parasitised weevils over a range of collection dates
26
26
Fig. 16. Percentage of weevils with sexually mature and immature gonads that were or
were not parasitised
27
Fig. 17. Percentage of weevils with three states of gut fullness that were or were not
parasitised
27
Fig. 18. Cumulative number of eggs per female Argentine stem weevil with a diet of
ryegrass-only or ryegrass plus bee-collected pollen (From Evans and Barratt,
1995, used with permission)
39
Fig. 19. Mean ± SE hind tibia length (mm) of M hyperodae from hosts fed
either ryegrass-only or ryegrass plus bee-collected pollen
48
Fig. 20. Mean ± SE egg load of M hyperodae from hosts fed either ryegrass-only
or ryegrass plus bee-collected pollen
49
Fig. 21. Correlation between egg load and tibial length (mm) of M hyperodae
parasitoids emerged from hosts fed ryegrass-only or ryegrass plus bee-collected
pollen
49
Fig. 22. Percentage of weevils with pollen in their gut at the beginning of the
experiment, and at the end of experiment which comprises non-exposed
and exposed weevils to adult parasitoids
xi
50
Fig. 23. Percentage of weevils with different gut fullness from four treatments
51
Fig. 24. Percentage of weevils with different ryegrass and pollen gut fullness
52
Fig. 25. Mean ± SE number of weevils exposed to adult parasitoids after the termination
of parasitoid emergence, from different treatments
53
Fig. 26. Percentage of weevils with sexually mature gonads at the beginning of the
experiment, and from four different treatments at the end of the experiment
54
Fig. 27. Percentage of weevils with sexually mature and immature gonads in respect to
different pollen abundance categories
54
Fig. 28. Percentage of weevils with sexually mature and immature gonads from different
55
genders
Fig. 29. Sex ratio of weevils from different treatments over time
56
Fig. 30. Mean ± SE hind tibia length (mm) of M hyperodae parasitoids emerged from
different treatments
56
Fig. 31. Mean ± SE egg load of M hyperodae parasitoids emerged from different
treatments
57
Fig. 32. Correlation between egg load and tibia length of M hyperodae parasitoids
58
emerged from different treatments
Fig. 33. Correlation between egg load and days after M hyperodae emergence
59
Fig. 34. Mean ± SE days to 50% of parasitoid emergence from different treatments
59
Fig. 35. Mean ± SE number of adult parasitoids emerged from different treatments
60
Fig. 36. Hypotheses about how the diet manipulation of the host could affect its
parasitoid. Possible effects on number of individuals and/or their fitness are
represented by:
i
=
positive effect,
!=
xii
negative effect
61
.-_'_~-_-_-..i
Plates
Page
Plate 1. Listronotus bonariensis larva. Ryegrass tiller was cut longitudinally
5
Plate 2. Microctonus hyperodae ovipositing on Listronotus bonariensis
5
Plate 3. Microctonus hyperodae larva inside female Listronotus bonariensis
5
Plate 4. Microctonus hyperodae larva emerging from Listronotus bonariensis
5
Plate 5. Listronotus bonariensis feeding on bee-collected pollen
42
Plate 6. Tibia from the hind leg of Microctonus hyperodae ready for measurement
42
Plate 7. Dissected abdomen of Microctonus hyperodae ready for egg counting
42
Plate 8. Pollen in Listronotus bonariensis gut
42
xiii
CHAPTER 1
GENERAL INTRODUCTION
BIOLOGICAL CONTROL BACKGROUND
Early agricultural production systems were probably designed partly to maximise the impacts
of the natural enemies of the herbivores that were attacking crops. For example, in ancient
China, colonies of ants were introduced into citrus groves to control pests (Gurr et al., 2000a).
However, it was around 1900 when the first major episode of biological control of an
arthropod by an arthropod occurred; it was the control of cottony-cushion scale (lcerya
purchasi Maskell; Homoptera: Margarodidae) on citrus in California following the
importation of a predator, the vedalia beetle (Rodolia cardinalis (Muls)), from Australia
(Doutt, 1964).
However, biological control of arthropods by arthropods has its problems and critics (Gurr et
al., 2000b). A success rate of only 10% has been recorded in classical biological control, with
no major improvement occurring with time (Greathead and Greathead, 1992). This is mainly
due to a "shot-gun" approach of releasing as many species of agents as were available in the
hope that control would be achieved. However, the intentional introduction of alien species
into complex biological communities is a threat to their structure and dynamics (Louda et al.,
2003). Indeed, any method that reduces the population of an abundant pest organism to below
an economic threshold will have inherent environmental risks (Simberloff and Alexander,
1998; Howarth, 2000). Because the release of non-indigenous agents to control pests is
usually irreversible, and because the introduced agents can attack non-target organisms,
reproduce, evolve, and spread away from the point of release, their potential for causing
damage is high (Simberloff and Alexander, 1998; Howarth 2000; Louda et al., 2003).
Subsequently, there has been increasing emphasis on pre-release experimentation to select the
best agents and increase the subsequent likelihood of success (Gurr et al., 2000b). Also,
environmental and economic costs of inaction and of alternative actions must be weighed
before the release ofa biocontrol agent (Frank, 1988).
The first attempt at biological control of Argentine stem weevil, Listronotus bonariensis
(Kuschel) (Coleoptera: Curculionidae), in New Zealand was made in the 1960s. The
Department of Scientific and Industrial Research introduced an egg parasitoid, Patasson
(Anaphes) atomarius (Brethes) (Hymenoptera: Mymaridae) as a potential biological control
agent, but it failed to establish (Dymock, 1989). In 1991, the South American parasitoid wasp
Microctonus hyperodae Loan (Hymenoptera: Braconidae) was released in New Zealand as a
potential biological control agent of the weevil and, within three years, maximum parasitism
rates of >90% were being recorded (Goldson et at., 1994a). M hyperodae oviposits in adult
Argentine stem weevils, and a solitary parasitoid larva develops within the living, active host.
The mature larva then emerges to pupate, while the host dies as a result of the parasitism
(Phillips and Baird, 2001).
THE ARGENTINE STEM WEEVIL AND CURRENT CONTROL PRACTICES
BACKGROUND
Argentine stem weevil
Argentine stem weevil was first recorded in New Zealand in 1927 (Marshall, 1937), and was
first recognised as a pest of wheat in 1933 (Pottinger, 1961a). It was not until the late 1950s
that this weevil's potential for damaging pasture was recognised (Pottinger, 1961 b). Since
then, it has become regarded as New Zealand's most damaging pest of ryegrass (Lotium spp.),
and before the introduction of a biological control agent, was estimated to cost the pastoral
industry between NZ$78-251 m per year (Prestidge et at., 1991). Pasture covers over 10
million hectares (38%) of New Zealand and is the largest single land use in the country (Land
Cover Data Base, 1997). Perennial ryegrass (Lotium perenne L.) and Italian ryegrass (Lotium
muttiflorum Lambrechtsen) are the most widely used pasture species (Charlton and Stewart,
2000).
Argentine stem weevil is a South American species and is widespread in Argentina, Uruguay,
Chile, Bolivia and Patagonia (Barker and Pottinger, 1982). It has been recognised as a minor
pest in Tasmania and New South Wales (Australia) (Barker and Pottinger, 1982). Its status as
a key or primary pest in New Zealand (Barker and Pottinger, 1982), but not being a notable
problem elsewhere, is similar to several other species (Goldson et at., 1998b). Goldson et at.
2
(1997) attributed such eruptions of relatively minor pests to a lack of natural enemies in New
Zealand's highly modified pastoral ecosystem.
In New Zealand, the weevil is summer-active (Goldson et al., 1998a) and has two complete
generations each summer in the South Island (Barker and Pottinger, 1982), though three may
occur in the warmer parts of the North Island (Pottinger, 1961a). In 1982, Barker and
Pottinger (1982) observed that eggs and larvae of the first summer generation were
considerably more abundant than those in the second. However, by 1995-96, this trend had
disappeared and this coincided with a growing impact of M hyperodae on the overwintering
survival and. reproductive capability of the weevil, from the 1993-94 season onward (Goldson
et al., 1998b).
The weevil overwinters in a state of photoperiodically-induced diapause (Goldson and
Emberson, 1980), with spring egg-laying commencing in late September (Goldson et al.,
1998b). By the end of December, the remaining now senescent overwintered population has
died and is replaced by the first summer generation (Goldson et al., 1998b). Oviposition by
the first summer generation peaks in early February, with second-generation adult emergence
starting in early March and persisting until mid June (Goldson et al., 1998b).
Graminaceous crops and Argentine stem weevil in New Zealand
Argentine stem weevil is a major pest of graminaceous crops in New Zealand (Barker et al.,
1984), and attacks maize, sweetcorn (Barker et al., 1988b), wheat, barley and oats, as well as
most pasture grasses (Timlin, 1964). However, the weevil shows marked preferences for some
species such as short-rotation ryegrass (L. multiflorum hybrid) and Italian ryegrass (L.
multiflorum) (Timlin, 1964).
In ryegrass, the female weevil deposits 1 - 8 eggs in the leaf cuticle (pottinger, 1961a;
Goldson et al., 1998b); then the newly hatched larvae typically tunnel downward within the
tiller (Plate 1) (Barker et al., 1984). More than one larva may feed on a tiller, particularly
during the early instars (Barker et al., 1984). Later, the resulting stem-mining larvae feed on
3-8 tillers of rye grass before pupating in the soil (Goldson et al., 1998b).
There are three types of damage caused by the weevil on ryegrass: a) feeding by adults gives a
silvery appearance to leaves, but recovery from this type of damage is usually good (Kelsey,
3
1958). However, the survival of summer-sown rye grass seedlings may be reduced by adult
feeding (Chapman, 1984), b) the second and most severe damage is caused by the larvae
tunnelling in tillers (Kelsey, 1958; Pottinger, 1961a), c) the third type of damage is to seed
heads. When these are being produced, larvae nearly always move upwards in stems and the
result is a "white head" that does not set seed (Kelsey, 1958).
Generally, newly-established pastures «5 years-old) are most favourable for the weevil and
are most prone to damage. There have been a number of estimates of damage levels, either by
direct examination of tillers or by measuring. pasture production in insecticide-treated and
untreated areas (Barker et ai., 1984). Based on tiller dissections, damage to annual ryegrass
has been as high as 98%, whereas perennial ryegrass suffered 30-40% damage (Barker et ai.,
1984).
The weevil is also a major problem in seedling maize sown soon after the cultivation of
pasture, or in previously-cropped ground where grass weeds, including Poa annua L. and
couch, Eiytrigia repens (L.) Beauv., are present. The weevil is the greatest cause of seedling
losses in maize in the Waikato region of the North Island of New Zealand (Watson and Hill,
1985).
4
Plate l. Listronotus bonariensis larva.
Ryegrass ti ll er was cut longitudinally
(Photo from AgResearch, used wi th
permission).
Plate 2. Microctonus hyperodae
ovipositing on Listronotus bonariensis
(Photo from AgResearch, used wi th
permission).
Plate 3. Microc/onus hyperodae larva
inside female Listrono/us bonariensis.
Plate 4. Microctonus hyperodae larva
emerging from Listrono/us bonariensis
(Photo from AgResearch, used with
permission).
5
Management tools for Argentine stem weevil
Chemical control
Over the past 40 years there has been continual investigation into the use of insecticides to
control Argentine stem weevil. However, the mining habit of the larvae coupled with
considerable flight activity of the adults (Goldson, 1981a), especially in dry east coast areas of
the South Island of New Zealand, make cost-effective chemical control unlikely (Goldson et
al., 1998a). When establishing pastures, drilling with a systemic granular insecticide
(Chapman, 1984) or using seeds treated with chemicals (Goldson et al., 1994b) can offer
protection to the seedling. However, in established pasture, control by insecticides is highly
variable (Chapman, 1984), with increases in pasture yields due to Argentine stem weevil
control from 0% to 91 % in the first spring-autumn period of the pasture (Barker et al., 1984).
Cultural control
Good pasture management may also assist in alleviating the effects of weevil damage. Some
aspects of pasture management that may be manipulated for pest control include the timing
and method of establishment, the timing and intensity of grazing or cutting, and the use of
fertiliser and irrigation (Gregg, 1997).
Time of sowing of crops may be important in limiting damage. Spring-sown grasses and
cereals are more severely affected than autumn-sown crops. In some areas of New Zealand,
December-January-sown ryegrass crops have suffered heavy second-generation attack due to
both adult and larval feeding (Pottinger, 1961a; Matthews et al., 1999). Sowing of grain
maize and sweet corn in South Auckland extends from early October to mid December. Crops
sown at this time are particularly at risk unless sowing is preceded by adequate cultivation to
eliminate previous pasture residual populations (Barker et al., 1988a). Independent of plant
species, young plants are more susceptible and do not have the same ability to recover as
mature plants (Pottinger, 1961a; Matthews et al., 1999). In addition, ploughing in spring can
reduce weevil larvae populations up to 70%, whereas autumn ploughing is less effective
because larvae are less susceptible to injury than pupae (Matthews et al., 1999).
Grazing livestock, for instance, affects pests by trampling or ingesting them, or by changing
the microclimate or food supply (Gregg, 1997; Matthews et al., 1999). Cutting also removes
pests and changes the microclimate, often to the detriment of the pest, though damage to
natural enemies may lead to a reduction of biological control (Gregg, 1997). Fertiliser
6
application and irrigation generally has little direct effect on pests, but can allow pastures to
recover more rapidly from insect attack (Gregg, 1997; Matthews et al., 1999). However, they
also can make plants more attractive to pests (Gregg, 1997).
On the other hand, crop rotation is another cultural control practice that involves measures
such as growing potato crops after cereals and crucifers, before sowing pasture again, the idea
being to eliminate the existing population of weevils before sowing the next susceptible crop
(Pottinger, 1961a; Watson and Wrenn, 1978; Barker et al., 1988a). If maize is to be sown in
land that was previously in permanent pasture or short rotation ryegrass winter green feed, a
fallow of 4 - 6 weeks between cultivation and drilling the maize should be allowed (Chapman,
1984).
Plant resistance
Ryegrass may contain an endophytic fungus, Neotyphodium lolii (Latch, Christensen and
Samuels) Glenn, Bacon and Hanlin (Keogh, 1973), which produces three major toxins within
the plant, especially near the shoot base and seed-heads: a) peramine, which deters feeding
and oviposition by the weevil and some other pests, b) lolitrem B, that causes "ryegrass
staggers" in grazing animals and c) ergovaline that causes heat stress and blood circulation
problems in grazing animals (Charlton and Stewart, 2000). Harm to animals can be
diminished by using mixtures of ryegrass with endophyte-free cultivars or other pasture
species, or by using low-endophyte cultivars, which are used especially for deer and horses,
because these animals are more sensitive to ryegrass staggers (Charlton and Stewart, 2000).
A high-endophyte cultivar has an endophyte percentage over 70% and a low endophyte
cultivar contains about 5% of the fungus (Charlton and Stewart, 2000). The endophyte ARI
produces peramine that provides resistance to the weevil but lacks the mammalian toxins
ergovaline and lolitrem B (Popay and Baltus , 2001). However, factors such as host plant
genotype and variations in hyphal content in individual plants, as well as infection by other
foliar endophytes and preferences amongst individual weevils may influence the level of
resistance. Moreover, the level of resistance is significantly reduced in the presence of
infection by the mycorrhizal fungus Gliocladiumfasciculatum (Barker, 1987).
7
Biological control
Microctonus hyperodae as a bioiogicai controi agent
.-·-.... ··:-..-·-·1
Microctonus hyperodae was imported from eight South American locations in Argentina
(Ascasubi, Mendoza, General Roca and S.C. de Bariloche), Brazil (Porto Alegre), Chile
(Concepcion and La Serena) and Uruguay (Colonia) (Goldson et ai., 1990; Fig 1).
Quarantine-based host specificity testing suggested M hyperodae was oligophagous, and the
results indicated it was unlikely to threaten New Zealand species of Curculionidae (Goldson
et ai., 1992).
Permission to release the parasitoid was granted by the Ministry of Agriculture and Fisheries
(Goldson et ai., 1992) and the parasitoid was released in the form of parasitised weevils
(Barlow et ai., 1994) at nine sites throughout New Zealand during 1991 (McNeill et ai., 1993;
',~.-." • ~ *-",
Fig. 1), in approximately equal numbers of each South American geographic population
(Phillips et ai., 1996). After release, the parasitoid species successfully established in
Canterbury and Northland (McNeill et ai., 1993).
Based on models, high parasitism rates (i.e., > 60%), are a significant estimate of the
probability that a parasitoid introduction will reduce host densities (Hawkins et al., 1993;
Hawkins and Cornell, 1994; Williams et ai., 1994). Hawkins and Cornell (1994) noted that
there is a threshold for success in the introduction of a parasitoid; when the parasitism rates
are below 32% in the native range, no control of the pest is achieved with the parasitoid.
Moreover, Myers et ai. (1994) stated that host parasitism rates below 30% in the native range
~
"-
~.
..
,:.-
~
are significant estimates that the host will not be amenable for biological control using
parasitoids.
Once introduced to New Zealand, M hyperodae achieved parasitism rates that exceeded 80%
in some cases. This parasitoid was liberated between 1991 and 1993, at 12 New Zealand
.
'~--,.; --'.-:.~
locations and established itself at 10 of the sites (Goldson et al., 1994a). Today it is
established at over 80 locations in the North and South Island (Phillips et ai., 1998), being
widely distributed throughout the North Island and in more limited coverage across the South
Island (mainly Canterbury Plains) (McNeill et ai., 2002a).
8
~.:
B
Brazil
....
)."-,,,~.,-.
..1.-*---Uruguay
Mendoza - - + +
"':k':":"-~_'~
•
South Chlle•
~
(?L'
~
Buenos Aires
~
Bahia Blanca
Rio Negro
O--SOOkm
Fig. 1. (A) Map of New Zealand showing sites where M hyperodae was released in 1991
(adapted from Phillips et al., 1997). (B) Map of South America showing the approximate
positions of the eight Microctonus hyperodae collection sites. The Andes form the border
between Chile and Argentina (adapted from Phillips et ai., 1997)
Biology
Microctonus hyperodae was first found in 1961 as a parasitoid of adult weevils at Colonia,
Uruguay, where four specimens emerged from 750 Hyperodes (=Listronotus) bonariensis,
which is a native species of southern South America (Loan and Lloyd, 1974). Loan and Lloyd
(1974) were the first to describe the taxonomy and biology of this parasitoid, and its
development within its host. They found that only females emerged from parasitised weevils,
indicating that the species is thelytokous (Loan and Lloyd, 1974).
Microctonus hyperodae females oviposit between 30 and 60 eggs (McNeill et ai., 1993)
..
" ~- '
laying one egg per weevil in several adult weevils (Barlow et ai., 1994; Plate 2) and reach the
maximum egg laying potential at 24 hours old (Phillips et ai., 1996). The larva ofthis solitary
endoparasitoid develops within the living host until maturity (McNeill et ai., 1993; Phillips et
ai., 1996; Plate 3). The mature larva then emerges to pupate, while the host dies as a result of
the parasitism (Phillips et al., 1996; Plate 4).
9
During winter, the parasitoid enters photoperiodically-induced diapause independent of the
host condition (McNeill et al., 1993; Barlow et al., 1994) as an arrested egg or larva (Barlow
!
i
'__'~J~·I
.
i
et al., 1994). Development resumes in the spring with increase in temperature. It probably
completes three generations per year, with adults found in the field in late December through
to May (McNeill et al., 1993; Barlow et al., 1994). It seems likely that each adult generation
lives less than two weeks in the field, since their adult longevity under ideal conditions in the
laboratory was 17 days (Phillips, 1998).
__-.-.-f.' -.-:
.r_· ___ _I
~.
Egg load
In contrast to syn-ovigenic parasitoid species, where the parasitoid is able to mature eggs
throughout its reproductive life (Jervis et al., 2001), M hyperodae is described as a proovigenic species (Goldson et al., 1995; Phillips and Baird, 2001), so its lifetime complement
of eggs is set at emergence from the host. This can limit parasitoid fecundity if hosts are
abundant (Heimpel and Rosenheim, 1997), because the total set of eggs can be laid before
death. M hyperodae adults in the field were estimated to have a mean pre-oviposition egg
load of 67 eggs (Phillips et al., 1998). However, parasitoids reared in the laboratory had a
mean egg load of 47 (Phillips and Baird, 2001), which corresponded well to the fecundity of
48 eggs estimated by Goldson et al. (1995). Furthermore, when parasitised weevils are
collected in the field and maintained in the laboratory, the emerging parasitoid adults also had
a mean egg load of 47 eggs (C.B. Phillips, personal communication, February 2003). This
variation between field-collected and laboratory-reared parasitoids suggests unidentified
environmental factors have a major influence on parasitoid egg load. Identifying such factors
is the main goal of the research of this thesis. Indeed, models have predicted that the fecundity
of a parasitoid may have less influence on prey density than its searching ability, although egg
load may be far more amenable to enhancement than searching efficiency. Subsequently,
parasitoids with higher egg loads would be potentially good candidates for reducing prey
populations (Kean et al., 2003).
Host nutrition is likely to have an effect on the ability of a parasitoid to develop normally
(Beckage and Riddiford, 1983). The weevils used in the laboratory experiments of Phillips
and Baird (2001) were fed only ryegrass (C.B. Phillips, personal communication, February
2003), but weevils in the field may have access to additional food sources. For example,
pollen is a rich source of nutrients and enhances, in the laboratory, the survival, body fat,
gonad development, and reproduction of boll weevil (Anthonomus grandis Boheman;
10
Coleoptera: Curculionidae) in Mexico (Jones et ai., 1993), and eggs per female of Argentine
stem weevil in New Zealand (Evans and Barratt, 1995).
In parasitoid wasps, the resources carried over from the larval to the pupal and adult stages
vary with the size and nutritional quality of the host and the time available for larval
development (Beckage and Riddiford, 1983; Harvey et ai., 1995; Jervis et ai., 2001). This
carry-over of resources from the host is positively correlated with wasp body size within
parasitoid wasp species (Harvey et ai., 1995; Jervis et ai., 2001), and adult body size is
generally correlated with egg load at emergence (Jervis et ai., 2001). In this way, the type of
food consumed by hosts could influence the reproductive success of their parasitoids (Harvey
et ai., 1995). However, M hyperodae is an unusual case where some geographic populations
(e.g., those from Chile and from s.C. de Bariloche in the Argentinean Andes) exhibit
....-.:;; ..:...:-.
relatively strong correlations between body size and egg load, yet other geographic
populations from east of the Andes do not (Phillips and Baird, 2001).
Following these ideas, egg load variation between M hyperodae adults collected in the field
and those reared in the laboratory could be due to differences between the diets of host
weevils in the field and laboratory. Improving the understanding of the environmental factors
which influence egg load could assist biological control in several ways. For example, the rate
of establishment of newly-introduced biological control agents might be improved by
providing conditions that optimise parasitoid egg load both during laboratory mass rearing
and in the field during and after parasitoid release. Knowledge of environmental effects on
parasitoid egg load might also support the development of new habitat manipulation strategies
to enhance the suppressive effects of established parasitoids on pest populations. For example,
provision of plants that produce pollen available to weevils at times of the year when weevil
adults contain parasitoid larvae could result in adult parasitoids with increased egg loads, thus
potentially enhancing parasitism rates. Another possible approach involves augmentation
biological control. For instance, parasitised Argentine stem weevils, which had been fed
pollen, could be released in organic sweet corn fields. The adult M hyperodae, which
emerged from the pollen-fed weevils, might achieve higher parasitism rates than those
parasitising weevils that had fed on only ryegrass.
To achieve successful biological control there is a need for selective floral resources (Wratten
et ai., 2004). Wratten et ai. (2004) stated that selective floral resources have a hierarchical
11
component. This is: 1). parasitoid benefits but pest does not. 2); parasitoid and pest benefit,
but the pest has higher longevity only; 3). parasitoid and pest benefit but parasitoid benefits
relatively more; and 4). parasitoid and pest benefit but the improved fitness of the pest leads
to a further improvement in the parasitoid's fitness because the developing parasitoid larva
derives a higher level of nutrients from a host that has fed on, e.g., pollen (Wratten et
at.,
2004). The last of these possibilities represents the novel concept of "indirect resource
subsidies", which was exemplified previously.
GOAL AND OBJECTIVES
The goal of this study is to elucidate effects of host diet on parasitoid fitness to increase
knowledge of plantlparasitoidihost interactions.
Objective 1. To determine if the addition of bee pollen to the weevil's diet influences
parasitoid egg load and parasitoid body size, and to measure the relationship between
parasitoid body size and parasitoid egg load.
Objective 2. If the addition of pollen to the weevil's diet influences parasitoid egg load, then
Objective 2a will further examine in the laboratory the effects of different components
of the weevil diet (i.e., buckwheat pollen) on parasitoid egg load, survival, and
reproduction (i.e., oviposition rate). In addition, weevils captured from the field will
be assessed for the amount and types of pollen within their guts.
If pollen does not influence parasitoid egg load, then Objective 2b will further
examine in the laboratory the effects of other components of the weevil diet (i.e.,
endophyte ryegrass and buckwheat pollen) on parasitoid egg load. In addition, weevils
captured from the field will be assessed for the amount and types of pollen within their
gut.
HYPOTHESES
1. Parasitoids reared from weevils fed with pollen and rye grass will have higher egg loads
than those reared from weevils fed only with ryegrass.
2. Parasitoids reared from weevils fed with no-endophyte ryegrass (without pollen) will have
higher egg loads than those reared from weevils fed endophyte ryegrass (without pollen).
3. Weevils collected from the field will have pollen in their gut.
12
CHAPTER 2
GUT CONTENTS OF FIELD-COLLECTED ARGENTINE STEM
WEEVILS (Listronotus bonariensis (Kuschel)) (COLEOPTERA:
CURCULIONIDAE)
INTRODUCTION
Argentine stem weevil feeds on the leaves of plants of the family Poaceae (Morrison, 1938;
. Kelsey, 1958; Barker et ai., 1989). However, pollen is also a food source for this weevil
(Evans and Barratt, 1995) and for at least four other species from the same family (i.e.,
Curculionidae) (Haegermark, 1980; Nanda and Pajni, 1991; Jones et ai., 1993; Gultekin et ai.,
2003).
After weevil feeding activity, pollen can be found externally on an insect's cuticle and/or
internally in the digestive system (i.e., gut). Several authors have examined the pollen found
on the exoskeleton of insects to assess their cuticle pollination capacity (de los Mozos and
Martin, 1988), or migration patterns (Gregg, 1993; Silberbauer et ai., 2004). Others have
examined the pollen content of the gut of insects to assess diet breadth (Rummel et ai., 1978;
Jones et ai., 1993; Hickman et ai., 1995; Irvin et ai., 1999), migration patterns (Del Socorro
and Gregg, 2001), or habitat manipulation management strategies (Lovei et ai., 1993; Wratten
et ai., 1995; Hickman and Wratten, 1996; Silberbauer et ai., 2004).
Pollen analysis of field-collected insects has become an important tool in the study of insect
biology and ecology because pollen grains are distinctive and can often be identified to genus
(Wratten et ai., 1995; Del Socorro and Gregg, 2001), even after being ingested by the insect
(Gregg, 1993; Wratten et ai., 1995; Del Socorro and Gregg, 2001; Hickman et ai., 2001;
Silberbauer et ai., 2004). Also, assessing pollen as a self-marking material is inexpensive
(Silberbauer et ai., 2004).
13
Pollen is also a rich source of nutrients and its consumption by insects, either in the laboratory
or in the field, can increase their fecundity (Pesho and van Houten, 1982; Annis and
Q'Keeffe, 1984; Jones et al., 1993; Evans and Barratt, 1995; Hickman and Wratten, 1995;
Irvin et al., 1999; Landis et al., 2000). However, pollen consumption can vary between
genders (Irvin et al., 1999; Takakura, 2004), and can influence the consumption of other
components of the insect diet (Evans and Barratt, 1995; Barratt et al., 1996). Furthermore, in
theory, pollen consumption could influence the nutritional quality of the host and hence make
it more suitable for parasitism, compared with a host which has not fed on pollen.
It was hypothesised that pollen is part of the Argentine stem weevil diet and that its
consumption would vary depending on seasonal availability. It was also hypothesised that
pollen consumption would increase the proportion of sexually mature weevils, and decrease
ryegrass intake in both genders. Finally, it was hypothesised that weevils that fed on pollen
would constitute better quality hosts and consequently generate parasitoids with greater
fitness.
In order to assess these hypotheses, an experiment was designed with the objective of
analysing the frequency and types of pollen grains consumed by field-collected weevils. Gut
fullness, parasitoid presence, weevil gender, weevil gonad development and the interactions
of all these variables with pollen consumption were also assessed.
MATERIALS AND METHODS
Argentine stem weevil adults were collected from ryegrass pastures in the Lincoln area on
five dates (December
It\ 2003 and February 11 th, March 26th , June 15th, and September 30th
of 2004), using a sweep net. As soon as possible after capture (generally within 4 hours), the
weevils were separated from the debris and frozen at -80 D C.
1. Pollen content
To assess the pollen content of weevils' gut, the weevils were washed for at least one minute,
by shaking them with water and detergent, to eliminate pollen grains present on the
exoskeleton of the insects (J. Martin, personal communication, November 2003). Then the
weevils were dissected on dark purple paraffin wax plaques moulded in 9 cm diameter plastic
Petri dishes. Each weevil was secured for dissection by pressing its ventral surface into a
14
small area that had been melted usmg a hot needle. All work was performed under a
microscope at 20X, 40X or 63X magnification. Using a pair of fine forceps, the elytra were
lifted off, the folded wings were removed, and the dorsal surface of the abdomen was opened
and pulled back. This revealed the gut, overlying the gonads. The gut was removed after
severing its anterior and posterior ends and placed on a slide. Then, the method of Wratten et
al. (1995) was used, which is described below.
The gut was spread over part of the slide, then a drop of saffranin (i.e., 0.1 % w/v), and a drop
of heated jelly were added, followed by a coverslip. The jelly comprised 7 g gelatine, distilled
water to 19 ml, 33 g 82% glycerine, and 1 g phenol crystals. The slides were subsequently
scanned under a microscope at 63X magnification. Pollen numbers were estimated on a semiquantitative scale, without detailed counting at the higher pollen categories, into one of seven
frequency classes (1 = no pollen grains, 2 = 1-10 grains, 3 = 11-100 grains, 4 = 10 1-1 000
grains, 5
=
1001-3000 grains, 6
=
3001-5000 grains, 7 > 5000 grains). To prevent pollen
contamination between samples, the forceps and needles were sterilised using alcohol and a
Bunsen flame after each dissection. All pollen was identified at least to family level. The
percentage of each pollen type per gut was also estimated from the average of counts from
nine graticule quadrants (Wratten et al., 1995).
2. Ryegrass particle content
The presence or absence of ryegrass in the gut was also recorded. At the same time as gut
pollen content was assessed, gut fullness was recorded as 'full gut', when all ofthe alimentary
canal was full of ryegrass particles, 'little grass' when ryegrass particles were present only
either in the foregut or midgut, and 'no grass' when no ryegrass particles were observed.
3. Gonad development in Argentine stem weevil
In females, there are two ovaries located dorsolaterally in the abdominal cavity (Fig. 3). Each
consists of a pair of ovarioles. In reproductively mature females, there are as many as 12
developing oocytes in each ovariole and up to 4 eggs in each calyx, which is enlarged. In
males, there is one pair of testes, which are located dorsolaterally in the abdominal cavity,
they are divided by sutures into 8-10 wedge-shaped testicular follicles (Fig. 2). Seminal
vesicles and prostate glands are contiguous with the testes and vary in size according to the
level of reproductive activity. Sexually mature males have larger testes, vesicles and glands
than do sexually immature males. Weevils with enlarged calyxes (and eggs present) or with
15
fully-grown testes. seminal ves icles and prostate glands were considered as sexuall y ' mature'
indiv iduals. The oppos ite condition was considered as sex ua ll y ' immature' indi viduals.
B
A
.... \ J
I
AC . GL .
c
D
L
.......-
Fig. 2. Reproducti ve morpho logy and anatomy of male and female Lis/rono/us bonariensis
(From : Barker, 1989). (A) Gross morpho logy o f organs from immature male adult, dorsa l
view. (B) Gross morpho logy of organs of mature, sexually mature male adult, dorsal view.
(C) Stages in the development of the ovari oles o f sexually immature female Lis/rono/us
bonariensis. dorsal view. (D) Organs of gravid female. Legend : AC. G L. = accessory gland ;
Bur. Cop. = Bursa copulatri x; E. = egg; EJ . D. = ejacul atory duct; GER. = germinarium ; L.O.
= lateral oviduct; M.O. = medi an oviduct: PST. = prostate gland; R.C. = resorption crystal;
SEM. VES . = seminal vesicl e; SP. = spennatheca; SP. GL. = spennatheca l gland ; TES. =
testi s; V.D. = vas de fe rens; VIT. vite llarium .
16
,_
4. Parasitism
-_~_-~._"W _"~
At the same time that gonad development was assessed, the presence or absence of eggs and
larvae of M hyperodae was recorded.
5. Statistical analyses
A log-linear model was used to analyse the statistical significance of the interactions between
~---.~~.'j
.~:~:'~-J
the categorical variables measured in the present experiment (P < 0.05). The variables were
analysed for second-order interactions between each other, and then for third-order
interactions where significant second-order interactions were observed. Also, a Chi-squared
(X2) analysis was used to look for further statistical differences between pollen categories.
RESULTS
Pollen content
Amount o/pollen in the weevils' gut
The proportion of field-collected weevils that had consumed pollen varied significantly over
the collection dates (F 16 = 0.043, P < 0.05; Fig. 3). In June (i.e., winter), the percentage of
weevils that consumed pollen was 1.96%, whereas it was 5.12% in December (i.e., early
•• - r ,..--,-•• ~- "
summer), 6.25% in February (i.e., late summer), 10.71% in March (i.e., early autumn), and
5.92% in September (i.e., spring). Furthermore, in June no weevils contained more than 100
grains of pollen in their gut, though, in December and September, some weevils had up to
3000 pollen grains in their gut. However, there were no significant differences between the
amounts of pollen consumed (F3 = 0.62, P > 0.05,X
2
=
7.33; Fig. 4). In addition, there was no
significant second-order interaction between pollen content and gender (F4 = 0.23, P > 0.05;
Fig. 5).
17
c:
Q)
12
"0 11
c.
10
.3: 9
!!!. 8
-=
.>
Q)
Q)
:;:
0
,
'::;-<;":~--:"'';''''I
!
c
~i
"-' __·'-1
-~-'''.-'
Q)
Cl
j9
c:
Q)
...0
Q)
0..
7
6
5
4
3
2
1
0
N
D
E
F
A
M
M
J
J
A
S
0
Month
Fig. 3. Percentage of Argentine stem weevils from a range of collection dates that had pollen
in their gut. There were significant differences between two or more bars at P < 0.05.
Analysis was a log-linear model of categorical data. Pair-wise comparisons of
dates/categories, etc., were not possible.
,
. .
,; -_ .... ':'1
~
-;!
1 to 10 pollen
grains
11 to 100
pollen grains
101 to 1000
pollen grains
1001 to 3000
pollen grains
Fig. 4. Percentage of Argentine stem weevils that had consumed pollen in different abundance
categories. There were no significant differences between bars atP = 0.05.
18
"
~
• 100 1 to 3000 pollen grains
8
. 101 to 1000 pollen grains
<5 7
c.
• 11 to 100 pollen grains
,s 6
o 1 to 10 pollen grains
.~
!!!. 5
"4
OJ
OJ
~
'0 3
OJ
'"
g 2
"~
OJ
"- 0
Females
Males
Fig. 5. Percentage of Argentine stem weevil s that had consumed pollen in different abundance
categories [or both genders. There were no signifi cant differences between genders at P ~
0.05.
Effects of pol/el/ COl/sumptioll 011 gOliad developmellt
There was a signi fi cant second-order interaction between pollen content and gonad
development (F4
~
<0.00 I, P < 0.05 ; Fig. 6) . The proportion of weevil s with sexuall y mature
gonads increased with the number of pollen grains consumed. With I to 10 pollen grains, the
proportion of sex ually mature indi viduals was 78%, while 100% of weevi ls w ith more than 10
pollen grains in their gut were sex ually mature. A lso, there was a signifi cant second-order
interaction between collection date and gonad development (F3 ~ 0.06, P < 0.05; Fig. 7).
About 44% of weevil s were sex ually mature except in winter (i.e., June) when on ly 20% of
individual s were sex ually mature. Finally, there was a non-significant third-order interaction
between co llection date, pollen content, and gonad development (F '2 ~ I, P > 0.05).
19
• Sexually mature 0 Sexually irrmature
c:
~
(5
100
a.
£
o~
80
I/)
0>
60
Q)
Q)
~
40
o
Q)
20
0)
ctI
1:
Q)
o
e
o pollen
Q)
a.
1 to 10
pollen
grains
grains
11 to 100
pollen
grains
101 to
1000
pollen
grains
1001 to
3000
pollen
grains
Fig. 6. Percentage of sexually mature and immature Argentine stem weevils that had
consumed pollen over a range of pollen abundance categories. There were significant
differences between two or more bars at P < 0.05. Analysis was a log-linear model of
categorical data. Pair-wise comparisons of dates/categories, etc., were not possible.
III Males
• Females
45
!Eo 40
o~ 35
~
'0
30
25
Q)
~ 20
1:
15
Q)
e
Q)
a.
10
5
o +----r--"
J
F
M
A
M
J
J
A
s
Month
Fig. 7. Percentage of sexually mature Argentine stem weevils over a range of collection dates.
There were significant differences between two or more bars at P < 0.05. Analysis was a loglinear model of categorical data. Pair-wise comparisons of dates/categories, etc., were not
possible.
20
Gender and gonad development
There was a significant second-order interaction between gender and gonad development (Fl
=
<0.001, P < 0.05; Fig. 8); the proportion of individuals with sexually mature gonads varied
significantly between genders. The proportions of sexually mature and immature males were
similar (i.e., 18.64% and 18.99% respectively), but the proportion of sexually mature and
immature females differed (i.e., 21.86% and 40.5% respectively; F4 = 0.291, P > 0.05; Fig. 9).
The sex ratio did not significantly vary between collection dates .
• Sexually mature 0 Sexually irrmature
70
!!!.
60
Q)
Q)
50
.s;
.....0~
Q)
40
Cl
30
Q)
20
j!!
c:
u
....
Q)
c.
10
0
Male
Female
Fig. 8. Percentage of Argentine stem weevils of both genders with mature/immature gonads.
There were significant differences between bars at P < 0.05. Analysis was a log-linear model
of categorical data. Pair-wise comparisons of dates/categories, etc., were not possible.
21
0.9
o 0.8
~ 0.7
~ 0.6
m
E 0.5
~
0.4
Q)
m 0.3
~ 0.2
0.1
o
o
J
F
M
A
M
J
J
A
S
Month
Fig. 9. Sex ratio of Argentine stem weevils over collection dates. There were no significant
differences at P = 0.05.
Types ofpollen
The types (i.e., pollen identified to family level) and frequencies of pollen found in the gut of
field-collected Argentine stem weevils are shown in Fig. 10. Pollen from a wide range of
plant families had been ingested by the weevils, the most important were Plantaginaceae,
29.8%, and Poaceae, 8.7%, of the pollen found in the gut. Also, 58.3% of the pollen grains
were unidentifiable (Fig 10).
~_-:J_'_'_-'-'_
The types and frequencies of pollen found on the exoskeleton of the same weevils are shown
in Fig. 11. There was a wide range of families present on the exoskeleton of the weevils, the
.:- :-
most important were Betulaceae 32.1 %, Fagaceae 8.6%, Pinaceae 11.1 % and Plantaginaceae
8.6%. Also, 12.3% of the pollen grains were unidentifiable (Fig 11).
22
Betulaceae
Agavaceae
"
(0.2%)
Fabaceae
(0.1%)
Pinaceae
(0.1%)
(0.1%)
;.'~-.--'''=---.-1
Cupressaceae
(0.7%)
Plantaginaceae
(29.8%)
Poaceae
(8.7%)
Unidentified ~
(58.3%)
Ranunculaceae
(0.1%)
Rosaceae
(1.5%)
Salicaceae
(0.5%)
Fig 10. Pollen types and their percentages within Argentine stem weevils' gut.
Unidentified
(12.3%)
Acerarceae
(1.2%)
Rosaceae
(2.5%)\
Polygonaceae
(2.5%)
Asteraceae
(3.7%)
Betulaceae
~
(32.1%)
Plantaginaceae
----/
(8.6%)
Brassicaceae
(2.5%)
Pinaceae
(11.1%)
JUglandaCeae_~
(1.2%)
Hippocastanac
Fagaceae Fabaceae
(1.2%)
eae (2.5%)
(8.6%)
Chenopodacea
e (3.7%)
Cupressaceae
(6.2%)
~-
Fig. 11. Pollen types and their percentages on Argentine stem weevils' exoskeletons.
23
Ryegrass particle content
There was a non-significant interaction between pollen content and ryegrass content in the gut
(F2 = 0.07, P > 0.05). However, there was a significant second-order interaction between
gender and ryegrass content in the gut (F2 = 0.02, P < 0.05; Fig. 12). Many males (i.e., 47%)
had ryegrass either in the foregut or midgut ('little grass' Fig. 13), and only 9.2% of males
had a full gut (,full grass' Fig. 13). However, many females had ryegrass either in one ('little
grass') or both (,full grass') foregut and midgut (i.e., 48% and 45% respectively).
Furthermore, there was a significant second-order interaction between gonad development
and ryegrass content (F2 = 0.01, P < 0.05; Fig. 13), where weevils with sexually mature
gonads generally had their gut full of ryegrass (i.e., 58% of weevils), but only 30% of
sexually immature weevils had a full gut.
o No grass. Little grass • Full grass
!!l
'S;
Q)
Q)
3:
0
Q)
C)
j9
100
90
80
70
60
50
u
40
30
c..
20
c
Q)
W
10
0
Female
Male
Fig. 12. Percentage of Argentine stem weevils from both genders with three states of gut
fullness. There were significant differences between bars at P < 0.05. Analysis was a loglinear model of categorical data. Pair-wise comparisons of dates/categories, etc., were not
possible.
24
o No grass • Little grass • Full grass
100
90
80
70
60
50
40
30
20
10
o -f---Sexually mature
Sexually im mature
Fig. 13. Percentage of Argentine stem weevils with sexually mature and immature gonads
with three states of gut fullness. There were significant differences between bars at P < 0.05.
Analysis was a log-linear model of categorical data. Pair-wise comparisons of
dates/categories, etc., were not possible.
Parasitism
There was a non-significant second-order interaction between pollen content and parasitism
(F4 = 0.74, P> 0.05; Fig. 14); both parasitised and non-parasitised weevils contained the same
amounts of pollen. However, there was a significant second-order interaction between
collection date and parasitism (F3 = <0.001; P < 0.05; Fig. 15). The parasitism rate varied
widely between collection dates, ranging from zero to 50% (Fig. 15).
Effects ofparasitism on weevil's gonad development and ryegrass intake
There was a significant second-order interaction between parasitism and gonad development
(FI = <0.001, P < 0.05; Fig. 16), where the presence of a parasitoid larva was associated with
sexually immature gonads. The percentages of sexually mature and sexually immature nonparasitised weevils was 54% and 46% respectively, while parasitised weevils had percentages
of 11 % and 89% respectively. Also, there was a significant second-order interaction between
parasitism and ryegrass content (F2 = < 0.001, P < 0.05; Fig. 17), where the presence of a
parasitoid larva was associated with lower ryegrass consumption. With the presence of a
parasitoid larva, 54% of weevils had ryegrass either in the foregut or midgut ('little grass',
Fig. 17), whereas only 28.8% of parasitised weevils had ryegrass in both foregut and midgut
(,full grass'). In the absence of parasitism, 55% of weevils had ryegrass in both foregut and
midgut (,full grass', Fig. 17).
25
o 1 to 10 pollen grains
• 11 to 100 pollen grains
• 101 to 1000 pollen grains
• 1001 to 3000 pollen grains
25
~
'S;
Q)
Q)
3:
0
20
15
Q)
C>
.mc
10
Q)
u
Q)
0..
5
0
Non-parasitised
Parasitised
Fig. 14. Percentage of Argentine stem weevils that were or were not parasitised that had
consumed pollen in several abundance categories. There were no significant differences
between bars (P> 0.05).
~
'S;
Q)
Q)
3:
0
Q)
C>
en
'E
Q)
~
Q)
0..
50
45
40
35
30
25
20
15
10
5
0
0
J
F
M
A
M
J
J
A
S
0
Month
Fig. 15. Percentage of parasitised Argentine stem weevils over a range of collection dates.
There were significant differences between two or more bars at P < 0.05. Analysis was a loglinear model of categorical data. Pair-wise comparisons of dates/categories, etc., were not
possible.
26
• Sexually mature 0 Sexually immature
o
Q)
C)
.19
c
~
Q)
a.
100
90
80
70
60
50
40
30
20
10
o -+-----
--
Non-parasitised
Parasitised
Fig. 16. Percentage of Argentine stem weevils with sexually mature and immature gonads that
were or were not parasitised. There were significant differences between bars at P < 0.05.
Analysis was a log-linear model of categorical data. Pair-wise comparisons of
dates/categories, etc., were not possible.
o None. Little grass • Full grass
!!!.
"S
Q)
Q)
~
0
Q)
C)
.19
c
Q)
~
Q)
a.
100
90
80
70
60
50
40
30
20
10
0
Non-parasitised
Parasitised
Fig. 17. Percentage of Argentine stem weevils with three states of gut fullness that were or
were not parasitised. There were significant differences between bars at P < 0.05. Analysis
was a log-linear model of categorical data. Pair-wise comparisons of dates/categories, etc.,
were not possible.
27
DISCUSSION
Pollen content
Argentine stem weevil diet breadth
These results show evidence that pollen is part of the natural diet of the Argentine stem
weevil. Weevils fed on pollen over all collection dates, however, the proportion of weevils
that had consumed pollen was only about 6%. Evans and Barratt (1995) reported that
Argentine stem weevil fed on bee-collected pollen in the laboratory, but they measured
neither the percentage of weevils that consumed pollen, nor the amount of pollen taken.
However, up to 34.7% offield-collected boll weevils had pollen in their gut when captured in
pheromone traps (Jones et al., 1992), and this figure increased to 80% when the weevils
where captured feeding on flowers (Rummel et al., 1978).
Further research is needed to explain why such a low proportion of Argentine stem weevils
contained pollen. It is possible pollen may not have been available in sufficient quantities in
the pasture, especially during winter. In theory, pollen could have been consumed by a large
proportion of individuals, but only sporadically, and the low proportion of weevils that
contained pollen in their gut was of those weevils that had consumed pollen recently. It is also
possible that pollen grains could have been digested within a few hours, making it impossible
or difficult to detect. Although pollen's rigid exterior is composed of one of the most enduring
protein materials, pollenium (Hagler and Jackson, 2001), Jones et al. (1993) had problems
identifying pollen as a result of the boll weevils' digestive processes, which were apparently
capable of breaking down the exine wall of some pollen grains. Indeed, pollen often may only
remain detectable in the gut for short periods, less than eight hours in some cases (Wratten et
al., 2003). Further research should involve laboratory experiments to ascertain how long
pollen remains detectable after it has been ingested.
Quantity 0/pollen in the weevils' gut
The proportion of field-collected Argentine stem weevils that consumed pollen varied over
the collection dates. In winter, pollen consumption was much lower compared with spring,
summer and autumn (Fig. 3). These results agree with those of Cuadradro and Garralla
(2000), who observed that field-collected boll weevils contained fewer pollen grains in their
gut during winter compared with other seasons. The same trend was found in field-collected
28
hover flies, Melanostoma Jasciatum (Mac quart) and Melangyna novaezelandiae (Mac quart)
(Diptera: Syrphidae), the females of which consumed more pollen in summer than they did in
winter (Irvin et al., 1999). Similarly, Silberbauer et al. (2004) observed that one of the
disadvantages of pollen when used as a marker was that pollen availability was influenced by
the time of the year in which the study was conducted. Generally, pollen availability in winter
is lower than that in other seasons (Silberbauer et al., 2004).
There was non-significant difference in the proportion of weevils that contained different
abundance categories of pollen in their gut (Fig. 4). However, there tended to be fewer
weevils in the high pollen abundance categories; most weevils that had pollen in their gut had
1 to 10 grains of pollen.
It is difficult at this stage to conclude if the amount of ingested pollen was a product of
deliberate or incidental consumption by the weevil. In fact, pollen in the gut of field-collected
Argentine stem weevil occurs at least to some extent due to contamination of leaf surfaces on
which the weevils are feeding; however, some Argentine stem weevils feed deliberately on
the grass inflorescences (G.M. Barker, personal communication, September 2004). Further
studies are needed to determine the nature of pollen ingestion (i.e., incidental v. deliberate) by
the weevil. Further research is also needed to identify the nutritional requirements and
behavioural factors involved in pollen consumption by the weevil.
The abundance of pollen in the weevils' gut had a non-significant second-order interactions
with gender. Males and females showed a tendency to eat similar amounts of pollen over all
collection dates (Fig. 5). This agrees with the results for other species, such as the pea weevil,
Bruchus pisorum L. (Coleoptera: Bruchidae), where both genders fed equally on pollen
(Pesho and van Houten, 1982). Similarly, the boll weevil showed no significant difference
between males and females neither in the proportion of individuals that ingested pollen nor in
the mean number of pollen grains consumed per weevil (Jones, 1997).
Effects ofpollen consumption on gonad development
Although throughout all the collection dates more male than female Argentine stem weevil
fed on pollen, there was a tendency in spring that more female than male weevil consumed
pollen (i.e., 77% of the weevils that consumed pollen were females). This could be explained
by the need of overwintering female weevils to consume pollen during spring to mature eggs.
29
Indeed, the addition of bee-collected pollen to the ryegrass diet of caged Argentine stem
weevil significantly increased the cumulative number of eggs laid per female (Evans and
Barratt, 1995). This is consistent with the results found in the present work where there was a
significant and positive correlation between pollen consumption and gonad development (Fig.
6). Even with a low amount of pollen grains consumed (i.e., 1 to 10 grains), the proportion of
sexually mature individuals was 78%. Moreover, the weevils that consumed over 100 pollen
grains always had mature gonads.
Similarly, Annis and O'Keeffe (1984) observed that the quantity of pollen available was very
important and directly related to the proportion of female field-collected pea weevils that
initiated oogenesis. Moreover, female adult aphidophagous hover flies, M fasciatum and M
novaezelandiae, required pollen for egg maturation (Hickman and Wratten, 1996). In fact,
more than 95% of all gravid female hover flies caught in pastoral areas in New Zealand and
wheat fields in Hampshire (UK) had pollen in their gut (Hickman and Wratten, 1996). All this
evidence supports the idea that pollen could be essential for the Argentine stem weevil egg
maturation.
Sexually mature weevils were around 44% of caught weevils over the collection dates, except
in winter (i.e., June), when the proportion of sexually mature individuals decreased to 20%
(Fig. 7). This is known to occur in the Canterbury region of New Zealand, where diapause
takes place from March to August (Goldson and Emberson, 1980; Goldson, 1981b), and that
is accompanied by a progressive decline in gonad development of both males and females
(Barker et al., 1988). There were no significant third-order interactions between collection
date, pollen content, and gonad development. This could be explained by the onset of
diapause, which progressive decline in gonad development of the weevils could have
influenced the gonad status of the individuals to a greater extent than their pollen
consumption.
Gender and gonad development
There was a significant second-order interaction between gender and gonad development,
whereby the proportions of sexually mature and immature females were significantly different
(i.e., 21.8% and 40.5% respectively), whereas the proportions of sexually mature and
immature males were almost the same (i.e., 18.6% and 18.9% respectively). This could be
30
explained by two reasons: different genders contained different amounts of pollen, or the
weevils' gonad development of males and females changed differently over the collection
dates. However, it was observed that both genders ate similar amount of pollen (Fig. 5), so the
more likely explanation is differential responses between genders in gonad condition over
collection dates (Fig. 7).
Indeed, most adult weevils enter into diapause in the autumn (Barker et
at., 1988b). They
exhibit delayed development of their reproductive organs until the following spring (Goldson,
1979 and 1981b; Barker and Pottinger, 1982). The morphological and physiological changes
associated with onset of diapause include atrophy of the gonads, cessation of ovulation,
resorption of terminal oocytes and cessation of mating (Barker et at., 1988). However, males
seemed to be more 'insensitive' to diapause condition than females (Fig. 7). The same trend
was observed in the boll weevil (Jones et
at., 1992), and the pea weevil (Pesho and van
Houten, 1982), where the percentage of reproductive females decreased in winter, whereas the
reproductive status of males changed little during these months.
In addition, a change in sex ratio was observed over the collection dates. However, there was
a non-significant increase in the sex ratio of field-collected Argentine stem weevils from
December (i.e., summer) to June (i.e., winter) and then a non-significant decrease from June
to September (i.e., spring, Fig. 9). These results agree with the sex ratio changes observed in
the laboratory during experiments carried out with caged Argentine stem weevils (see Chapter
3). Similarly, field-collected boll weevils had a sex ratio (i.e., male/female) of 0.79:1 during
early winter, but it increased progressively until late winter months to 1: 1 in Massachusetts,
USA (Jones et
at., 1992). However, the trend found in the present experiment differed from
the Argentine stem weevil sex ratio of 1:1 described by Chapman (1984). Moreover, Barker et
at. (1988b) found in the North Island of New Zealand, a significantly superior number of
males than females, with male/female ratios of 1.26: 1. They found this excess of males in
peaks of reproductive activity, in October - December and February - April (Barker et
at.,
1988b).
Changes in sex ratio could be caused by changes in emergence of adult weevils, mortality
changes in both or either of the genders, or because different trap methods captured a different
proportion of genders. For example, the sex ratios from the literature were based on suctiontrapping, wet-sieving and flotation in magnesium sulphate, and sweep-netting sampling
31
(Barker et al., 1988b); however, the present experiment used only sweep netting for
collection. Further research is needed to identify possible differences in sex ratio captures
with different sampling methods. Behavioural (i.e., one gender tending to climb more than the
other does) or physical (i.e., body weight or differential strength to cling to a ryegrass leaf)
components could vary the sex ratio of the sample.
Types ofpollen
The types of pollen found in the weevils' gut were very diverse in terms of the number of
families found. However, most of the families identified in the pollen assessment seemed to
have been consumed incidentally by the weevils with the consumption of ryegrass leaves; the
same phenomenon was observed in pastures in the North Island of New Zealand (G.M.
Barker, personal communication, 'September 2004). Poorly represented in the insects' gut
were tree/shrub families generally, such as Agavaceae 0.1% (e.g., cabbage tree), Betulaceae
0.2% (e.g., birch), Cupressaceae O. 7% (e.g., macrocarpa), Pinaceae 0.1 % (e.g., pine),
Rosaceae 1.5% (e.g., apple tree), and Salicaceae 0.5% (e.g., willow). If the field-collected
weevils had been collected within a paddock instead of along road sides, probably the
frequencies of these pollen types would have been even lower. Further research is needed to
identify differences in pollen availability and consumption on road sides compared with
within-paddock samples.
On the other hand, annual and biannual weeds and pasture species contributed a variable
amount of pollen in the insects' gut, such as Ranunculaceae 0.1% (e.g., buttercup),
Plantaginaceae 29.8% (e.g., Plantago major L., P. lanceolata L.), Poaceae 8.7% (e.g., ,
Latium spp., Poa annua L., Triticum aestivum L., Zea mays L., etc.), and Fabaceae 0.1 % (e.g.,
clover). Probably Fabaceae and Ranunculaceae pollen was also consumed incidentally along
with ryegrass leaves as explained before. Nevertheless, further research is needed to identify
what pollen frequencies could be considered as incidental or deliberate feeding. Low
percentages of pollen could be a product of deliberate consumption, but affected by pollen
availability, low pollen requirements by the weevil, low detection techniques, etc. On the
other hand, Plantaginaceae and Poaceae pollen frequencies suggest that these types of pollen
were deliberately consumed by the weevil. These results partially agree with those found in
pastures in the North Island of New Zealand, where weevils fed actively on pollen of P.
annua inflorescences (G.M. Barker, personal communication, September 2004). Also, the
32
Argentine stern weevil evolved feeding on Poaceae plants in its native South America
(McNeill et ai., 1996). It is possible that the weevil showed certain degree of preference by
this type of pollen compared with others. However, further research is needed to identify
other plant families which could be consumed by the weevil.
The frequencies of the different pollen types found on the exoskeletons of the weevils were
almost always higher than their respective frequencies inside the weevils' gut. This suggests
that even though certain types of pollen were present in the pasture, they could not be
consumed by the weevil in high amounts due to low access to the pollen source (i.e.,
tree/shrub pollen), or feeding was incidental and not due to active feeding on the plant
flower/inflorescence (i.e., ryegrass leaves contamination). Nevertheless, Plantaginaceae
pollen frequency was lower on the exoskeleton (i.e., 8.6%) compared with the frequency in
the weevils' gut (i.e., 29.8%), which suggests that weevils deliberately fed on inflorescences
of this plant family. Similarly, Poaceae pollen was found in higher frequencies in the gut (i.e.,
8.7%) than on the exoskeleton (i.e., 0%). Besides deliberate pollen consumption on the
inflorescences, it is possible that the weevils groomed their bodies after pollen feeding and
consumed the grains obtained by this process.
The proportion of unidentifiable pollen grains was much higher in the gut (i.e., 58.3%) than
on the exoskeletons (i.e., 12.3%). This could be explained by pollen degradation due to
digestive action in the weevils' gut as explained before.
Studies of other field-collected weevils, have revealed pollen grains of over a dozen plant
families,
including:
Amaranthaceae,
Anarcaridaceae,
Asteraceae,
Brassicaceae,
Chenopodaceae, Euphorbiaceae, Fabaceae, Fagaceae, Malvaceae, Poaceae, Polygonaceae and
Solanaceae (Rummel et ai., 1978; Benedict et ai., 1991; Jones et ai., 1993; Jones, 1997;
Hardee et ai., 1999; Cuadrado and Garralla, 2000; Jones and Hardee, 2000; Cuadrado, 2002).
The most important plant families in terms of amount of pollen grains consumed were
Asteraceae, Poaceae, and Malvaceae, which suggest the importance of Poaceae pollen in the
diet of Curculionidae.
33
Ryegrass particle content
Since the amount of ryegrass consumed by caged Argentine stem weevils decreased to 40%
with the addition of pollen to the weevils' diet (Evans and Barratt, 1995), it was expected that
field-collected Argentine stem weevils that fed on pollen would have decreased their ryegrass
intake. However, there was no significant interaction between pollen intake and ryegrass
content in the gut. This could be due to a lower availability of pollen in the field compared
with a caged conditions were pollen was easily available in terms of quantity and
accessibility. Probably, in the field, pollen was scarce and was eaten sporadically and
-
'~-.-.'-'-'~'~'
.'
alternatively to ryegrass. Furthermore, due to the rapid pollen degradation by gastric activity,
pollen could be digested much more rapidly in weevils that ate nothing but pollen; probably
several weevils that were found with no-pollen and no-ryegrass in their gut had previously fed
only on pollen. Further research in needed to elucidate this, a shorter sampling interval could
determine more accurately the changes in gut contents of weevils.
There was a significant second-order interaction between gender and ryegrass content in the
gut (Fig. 12). Males had a near normal distribution of individuals with no, little or full gut
with ryegrass. Conversely, females had a higher proportion of individuals with either little or
full gut with ryegrass; only a few individuals had no ryegrass in their gut. This could be
explained because females needed to mature eggs in spring or because they have a higher
body mass than males (Goldson and Emberson, 1981) and may therefore need to consume
more ryegrass.
There was a significant second-order interaction between gonad development and ryegrass
content (Fig. 13), whereby males and females with mature gonads generally had their gut full
of ryegrass, whereas immature individuals had no or little ryegrass in their gut. This could be
explained because individuals that had immature gonads were often parasitised (i.e., 50%
parasitism rate based on dissection) and parasitised weevils decrease their ryegrass-feeding
rate (Barratt et at., 1996). Alternatively, breaking diapause is characterised by an increase in
gonad development and a resumption of feeding activity (Goldson, 1979 and 1981; Barker
and Pottinger, 1982). Since gut fullness was assessed in September, when diapause is being
broken, probably a higher gonad development associated to a higher ryegrass consumption
was a coincidence, that is, weevils could have had an increased gonad development as they
broke diapause (i.e., controlled by photoperiod) and a higher ryegrass content as they resumed
feeding activity. Further research is needed to explore interactions between gonad
34
development and ryegrass consumption. Ryegrass content, for instance, should be assessed
over all collection dates.
Parasitism
There was a significant and positive correlation between pollen consumption and gonad status
of the Argentine stem weevil in the laboratory (Evans and Barratt, 1995) and in the field (i.e.,
section: Effects of pollen consumption on gonad development, above). Also, the resources
carried over from the host yary with the size and nutritional quality of the host (Beckage and
Riddiford, 1983; Harvey et al., 1995; Jervis et al., 2001), and parasitoids prefer to oviposit on
high quality hosts to satisfy minimum physiological and dietary requirements for larval
development and growth (Mackauer and Michaud, 1996; Tagawa et al., 2000). Then, pollen
consumption could mean high quality hosts and hence, in theory, a high parasitism rate should
be found in those hosts. However, there was non-significant second-order interaction between
pollen consumption and parasitism (Fig. 14). This means that M hyperodae oviposited
without making any distinction between both pollen fed and non-pollen fed weevils. It also
means that both, parasitised and non-parasitised weevils ate the similar amounts of pollen,
independent of their parasitism status.
Although pollen consumption increased gonad development in the weevil, it is not exclusively
needed for gonad maturation. Indeed, as discussed previously, ryegrass consumption could
have influenced gonad development as well, and ryegrass was more frequently consumed by
the weevils, compared with pollen. In theory, M hyperodae could have used hosts' gonad
development as a cue for oviposition, but, due to the low number of individuals that
consumed pollen compared with the vast majority that consumed ryegrass, there was no
statistical interaction between pollen consumption and parasitism rates. On the other hand, M
hyperodae could have looked for quality cues in the weevils other than gonad development.
Other nutritional components of the host could be more important in terms of host choice,
such as fat content or muscle development of the host for instance. Further research is needed
to investigate what cues are used by M hyperodae to select its host for oviposition.
A significant second-order interaction was observed between collection dates and parasitism
rates. Indeed, the parasitism rate varied widely between collection dates (Fig. 15). The
absence of parasitoids in December 2003 agrees with the results of Phillips et al. (1998), who
35
found extremely low parasitism rates on mid December 1996. The parasitism rates in
February and March 2004 of the present experiment also agree with the results of Phillips et
al. (1998), who found parasitism rates around 15 to 20% during the same months in 1996. It is
important to clarify that not every year is the same in terms of parasitism rates, since
environmental conditions, parasitoid and weevil densities vary with time. The absence of
parasitoid eggs or larvae in June 2004 was surprising, since the parasitoid overwinters as
larvae within the host (Goldson et al., 1993a), hence at least a few parasitoid larvae should
have been found in the weevils. Similarly, the high parasitism rate observed in September
2004 was probably explained by a build up of parasitoid populations over the years, since the
parasitism rates found by Phillips et al. (1996) were lower for the same month in 1996. These
last two collection dates of the present experiment (i.e., June and September) could have also
varied compared with the rest of the collection dates because the collection sites varied within
the Lincoln area. Considering also that M hyperodae is a parasitoid of low mobility, about 1.5
to 3 km per year (McNeill et al., 2002a), the parasitism rates could vary between locations a
few kilometres apart. Further research should sample only one paddock and thus record
parasitism rates of a specific location.
Effects ofparasitism on weevil fitness and ryegrass intake
There were significant second-order interactions both between parasitism and gonad
development (Fig. 16), and between parasitism and ryegrass content in the gut (Fig. 17).
These results agree with those of Barratt et al. (1996), who reported that parasitised Argentine
stem weevils experienced reduced feeding activity and rapid castration (i.e., hence reduction
in gonad development) produced by its parasitoid M hyperodae. Indeed, there is a general
tendency for solitary koinobiont parasitoids, such as M hyperodae, to reduce host feeding
activity. In addition, the advantage of host castration is that it frees for the parasitoid's
immediate use those resources that a host would normally invest in gonads (Quicke, 1997).
Also, castration of both male and female weevils was evident during weevil dissection in the
present experiment. These findings are extremely important considering that one of the main
hypotheses (see chapter 1. General Introduction) of this thesis is based on the theory that the
addition of different diets (i.e., pollen) to the host would affect its fitness (i.e., increase in
gonad development). A theoretical increase in host fitness would mean more resources
available for the developing parasitoid larva and thereby increase the fitness of the parasitoid.
Nevertheless, this mechanism worked only with respect to the first part of the theory, where
36
the addition of pollen to the weevil's diet increased the cumulative number of eggs per female
weevil (Evans and Barratt, 1995). However, in the present study, this happened in those
weevils that were not exposed to adult parasitoids. Although there was no significant secondorder interaction between pollen ingestion and parasitism, which means that parasitised and
non-parasitised weevils ate the same amounts of pollen, the presence of parasitoid larvae
within the weevils could have caused host castration, which destroyed the pathway of the
theory described above.
To study further the hypothesis that the effects of the addition of different diets to the host.
would affect its fitness and thereby affect the fitness of the parasitoid, a laboratory experiment
was designed. This will be detailed in the next chapter.
37
CHAPTER 3
EFFECTS OF HOST DIET ON COMPONENTS OF FITNESS
OF Microctonus hyperodae LOAN
INTRODUCTION
Evans and Barratt (1995) reported an increased cumulative number of eggs per female of
Argentine stem weevil after the addition of pellets of bee-collected pollen to the weevils' diet
(Fig. 18). Several researchers have reported increases in the proportion of female weevils with
mature gonads after the addition of pollen to various weevil species' diet (Pesho and van
Houten, 1982; Annis and Q'Keeffe, 1984; Jones et al., 1993). Adult wasp fitness is likely to
be affected by host nutrition, since resources carried over from immature stages to the adult
stage of the parasitoid vary with the size and nutritional quality of the host (Jervis et al.,
2001).
Several authors have observed decreases in the proportion of female Argentine stem weevils
with mature gonads after the provision of ryegrass with different endophyte strains (Bell and
Prestidge, 1991; Popay and Wyatt, 1995; Popay et al., 1995; Barker and Addison, 1996;
Popay, 1997). This could mean fewer resources and possibly indirect sublethal effects, caused
by the endophyte on the developing parasitoid inside the host, and therefore lower fitness and
survival may be expected for parasitoids emerging from these weevils. It is possible
endophyte could also influence the fitness of parasitoids by affecting the nutrition of their
immature stages.
Floral resources are widely distributed in nature; buckwheat, in particular, has a small,
shallow flower where pollen and nectar are easily available to a wide range of insects (Lovei
et al., 1993). Stephens et al. (1998) found from 5 to 33 weevils m-2 on flowers of buckwheat
patches sown as understorey in apple orchards at Lincoln, New Zealand, but did not specify
the weevils' species. The provision of buckwheat flowers (i.e., nectar) increased the longevity
of M hyperodae more than any other flower species tested by Vattala et al. (2004) and
doubled the longevity of the parasitoid compared with water, so is one of the best candidates
38
for habitat manipulation in conservation biological control of its host, Argentine stem weevil
(Vattala et at., 2004). However, given that Argentine stem weevil benefits from pollen
(Chapter 2; Evans and Barratt, 1995), it is possible that floral resources targeted at M
hyperodae could also increase the fitness of the pest. In conservation biological control,
therefore, the relative influences of resource subsidies on pests and their natural enemies (both
direct and indirect) must be weighed up, otherwise such subsidies could inadvertently provide
greater benefits to the pest than the natural enemy.
Weevil fecundity
70
en
C)
C)
Q)
....0
....
Q)
60
50
..c 40
E
::s
c::
.
.,
Q)
;~>'.-. r
>
30
>
..!!! 20
::s
Ryegrass
only
E
::s 10
()
/
0
0
30
60
90
120 150 180 210 240 270 300
Time (days)
Fig. 18. Cumulative number of eggs per female Argentine stem weevil with a diet of rye grassonly, or of ryegrass plus bee-collected pollen (From Evans and Barratt, 1995, used with
permission)
Two experiments were conducted to explore the mechanisms involved in this tri-trophic level
interaction. The first experiment comprised the addition of ryegrass and/or bee-collected
pollen as food sources for the Argentine stem weevil. The second, more elaborate experiment
tested additional diets, including buckwheat pollen and ryegrass with endophyte, to further
examine these interactions.
This chapter investigates these tri-trophic level interactions on components of parasitoid
fitness and survival by manipulating the host diet. The concept of benefiting the third trophic
level with 'resource subsidies' made available to the second level leads to the suggestion of a
new mechanism for enhancing parasitoid fitness, that is 'indirect' conservation biocontrol.
39
MATERIALS AND METHODS
Experiment 1
1. Experiment set up
One thousand Argentine stem weevil adults were collected from the Lincoln area on the 15
August, 2003, using a sweep net. A proportion of these weevils contained immature
parasitoids. The weevils were stored at 10 D C for approximately 4 days until they were
separated into two treatments (control and treatment 1) replicated five times, with 100 weevils
per replicate. Then the weevils randomly assigned to the control were fed with endophyte-free
ryegrass cv. Tama, simply referred to as ryegrass-only from now on. Two ryegrass bunches
were placed in each box and replaced every three days. The weevils in treatment 1 (Tl) were
fed with ryegrass cv. Tama plus approximately 10 pellets of a pollen mixture (bee-collected
pollen pellets; Plate 5) obtained from beehives in Canterbury. Both, rye grass and beecollected pollen were replaced every three days.
Both treatments included several moist dental wicks, which were soaked in water every 48 h,
so the weevils had access to water. The weevils were held in cages following the Phillips et
al. (1996) procedures described in section 3. The parasitoids emerging from weevils in the
different treatments and replicates were labelled and stored at -80°C for subsequent
measurement of the length of the hind tibia and egg load as described in sections 4 and 5,
respectively.
2. Rearing plants for Argentine stem weevil in experiment 1
Ryegrass cv. Tama was planted in a greenhouse a few weeks before the weevils were
collected, and was given as seedlings to the weevils. Ryegrass seeds were sown every two
weeks to maintain a supply of ryegrass. The seeds were sown in paper pots filled with potting
mIX.
3. Argentine stem weevil cages
The weevils were held in plastic boxes comprising two chambers, one on top of the other,
separated by plastic mesh (holes of 0.5 x 0.5 mm). The plastic mesh was glued to the bottom
of the top chamber and prevented adult weevils from falling into the bottom chamber, but
40
allowed M hyperodae larvae to do so. These boxes were kept at 20 0 e and 14: 10 (L:D) hours
photoperiod in controlled temperature cabinets. Newly emerged M hyperodae larvae crawled
under tissue paper laid on the floor of the lower chamber to spin their cocoons. The cages
were checked for M hyperodae pupae every 24 h.
Microctonus hyperodae cocoons were cut out of the paper and placed in a Petri dish, and the
number and date of collection of the cocoons was recorded. Petri dishes were kept in the same
controlled temperature cabinet at 20 D e and 14:10 (L:D) photoperiod. A small cotton bud
soaked in water was placed in each Petri dish to maintain high relative humidity. Petri dishes
were checked every 24 h, and newly emerged adult parasitoids were transferred to a labelled
plastic vial (500 mm height and 15 mm diameter), with a filter paper (10 x 10 mm) soaked in
50:50 honey and water solution and maintained at 20 D e and 14:10 (L:D) photoperiod. After
24 h, the parasitoids were stored at -80 oe for subsequent measurement of the length of the
third tibia and egg load (sections 4 and 5, respectively).
4. Parasitoid morphological measurement
The length of a hind tibia was measured following the procedures of Phillips and Baird
(2001). The tibia length was measured at SOX magnification using an eyepiece graticule (100
units, 100 subdivisions; Plate 6). To obtain the tibia length in mm, the measurement read in
the graticule was multiplied by a factor of 0.02.
5. Quantifying egg load of the parasitoids
The eggs were counted using the method of Phillips and Baird (2001), which is described
below. The parasitoids obtained from the experiment were either stored at -80oe or dissected
less than two hours after they had been killed. To count the eggs, each parasitoid was
dissected using two pairs of forceps. The reproductive organs were extracted by carefully
pulling the ovipositor posteriorly away from the abdomen. The abdomen was then prised from
the metasomal tergum and torn open longitudinally so that any remaining internal organs
became exposed. The reproductive organs connected to the ovipositor and the severed
abdomen were placed in a watch glass containing a protein stain solution (i.e., 0.1 g nigrosin
(BDH) in 100 ml H20 followed by 3 g trichloroacetic acid).
41
I
Plate 5. Listrollotlls honariensis
feeding on bee-collected pollen.
Plate 6. Tibia from the hi nd leg of
MicroctollllS hyperodae ready for
measurement
Plate 8. Pollen in ListronOIllS
Plate 7. Dissected abdomen of
Microctof/IIS hyperodae ready for
egg counting.
hOl/arief/sis gut.
42
The watch glass was covered with a glass slip and the parasitoid tissue was left to soak in the
stain for 24 h at room temperature. Afterwards, the tissue from each insect was placed on a
separate microscope slide with a drop of Hoyers mountant and covered with a glass slip. The
stained tissue was then gently agitated by rubbing the upper surface of the glass slip in a
circular motion with a pair of fine forceps. This agitation continued, while viewing the stained
material under a binocular microscope at 63X magnification, until the parasitoid eggs, which
were initially tightly bound together within the ovarioles, were sufficiently separated to permit
counting. The eggs were stained dark blue and were readily discriminated from other stained
, • .:,.. • ", ',~ .!"", '0: I
;'*'''-~~·'';'-'''-·i
materials by their size (approximately 321 /lm long and 24 /lm wide) and shape (Plate 7).
Eggs were counted at 20X or 64X magnification. After this, the mounted material was sealed
by applying nail polish to the joints of the cover slip and the slide, and then labelled recording
the treatment, replicate, wasp number, date of mounting and number of eggs .
.',
--~,'
Experiment 2
6. Experiment set up
Two thousand and eighty Argentine stem weevil adults were collected from the Lincoln area
on the 11 February, 2004, using a sweep net. A proportion of these weevils contained
immature parasitoids. The weevils were stored at 10DC for 6 days until they were separated in
four treatments (control, treatment 1 to 3, see Table 1) replicated four times, with 130 weevils
per replicate. Then the weevils were assigned randomly to the treatments and were fed with
the diets listed in Table 1.
Table 1. Combination of treatments of experiment 2
Major treatment
no pollen
bee-collected pollen
buckwheat flowers
ryegrass
Control
Tl
T2
ryegrass with
T3
endophyte
43
Two ryegrass bunches were placed in each box of control, treatment 1 (Tl) and treatment 2
(T2), -and replaced every three days. Two ryegrass bunches with endophyte var. Aries
containing 94% of the strain ARW of the endophytic fungus N lolii, were placed in each box
of the treatment 3 (T3), and replaced every three days. Approximately 10 pellets of a pollen
mixture bee-collected in Canterbury was used in the Tl. It was replaced every three days.
Flowering buckwheat plants cv. Katowase were provided in T2, the apical part of the plant
containing the flowers was introduced into each cage through a circular hole (30 mm in
diameter) and the hole was sealed with a plastic bag and tape.
All treatments included several moist dental wicks, which were soaked in water every 48 h, so
the weevils had access to water. The weevils were held in cages following the Phillips et al.
(1996) procedures described in section 3. After the parasitoids had ceased emerging from the
weevils, a sample of five weevils per replicate treatment was taken to verify that no parasitoid
larvae were still developing in the weevils. At the same time, another sample of 10-15 weevils
per replicate treatment was taken to assess the gonad development following the procedures
of Goldson and Emberson (1981), and Barker (1989) (section 9). Gut fullness was also
assessed (section 10).
7. Methods during and after exposure to parasitoids
The surviving weevils from each of the four replicates were split into two cages and
maintained on their respective diets. In one set of cages, 40 weevils per replicate were
exposed to one adult parasitoid following the method of Phillips et al. (1996) (section 3). In
the second set of cages, the remaining non-exposed weevils were maintained on their
respective diets.
The parasitoids which emerged from the exposed weevils were labelled and stored at -80 DC
for subsequent measurement of the hind tibia length and egg load as described in sections 4
and 5, respectively. One hundred and seventy seven days after initiating the treatments, and
133 days after exposure to parasitoids, the surviving weevils were labelled and stored at -80D C
for subsequent assessment of gonad development following the procedures of Goldson and
Emberson (1981), and Barker (1989) (section 9). Gut fullness (section 10) and the presence or
absence of parasitoid eggs, and whether they were encapsulated, was also assessed.
44
The non-exposed weevil s were maintained in plastic boxes (bottom boxes of Phillips el at. ,
( 1996) procedures, section 3) at 20°C and 14:10 L:D photoperiod. One hundred and forty
seven days after initiati ng the diet treatments, and 103 days after the sp lit from the exposed
weevil s, the surviving non-exposed weevil s were label led and stored at -80°C for subseq uent
assessment of gonad development following the procedures of Go ldson and Emberson ( 198 1)
and Barker ( 1989) (section 9). Gut fullness was also assessed (section 10).
8. Rearillg plallts for Argelltille stem weevil ill experimellt 2
Ryegrass cv. Tama, ryegrass with endophyte var. Ari es, and buckwheat cv. Katowase seeds
were planted in a greenhouse a few weeks before the weevils were coll ected. Pl anting was
repeated every two weeks to ensure a continuous supp ly of food for the weevi Is.
Ryegrass and ryegrass with endophyte were gIven as seed lings to the weev il s, while
buckwheat flowers were grown for their pollen. The buckwheat pollen was provided to the
weevi ls as complete fl owers. Seeds of these plants were sown in a greenhouse using paper
pots filled with pOlli ng mix.
9. GOlllul developmellt ill the Argelltille stem weevil
The weevils were dissected in dark purple paraffin wax plaques moulded in 9 cm diameter
plastic Petri dishcs. Each weevil was sccurcd for dissection by pressing its ventral surface into
a small area of wax which had been mel ted using a hot needle. All work was performed under
a microscope at 20X, 40X, or 63X magnifi cation. The elytra were lifted off, and the folded
wings were removed. The dorsal surface of the abdomen was opened with force ps and pulled
back. This revealed the gut, overlying the gonads. In females, there are two ovari es located
dorsolaterally in the abdominal cavity. Each consists of a pair of ovarioles. In reproductively
mature females, there are as many as 12 developing oocytes in each ovariole and up to 4 eggs
in each calyx, which are enlarged. In males, there is one pair of testes located dorsolaterally in
the abdom inal cavity, which are divided by sutures into 8- I 0 wedge-shaped testicular
follicles. Seminal vesicles and prostate glands are contiguous with the testes and vary in size
according to the level of reproductive activ ity. Sex ually mature males have larger testes,
vesicles and glands than do sex ually immature males. Weevils with en larged calyxes (and
eggs present) or with fu lly-grown testes, semi nal vesicles and prostate glands were considered
45
as sexually 'mature' individuals. The opposite condition was considered as sexually
'immature' individuals.
10. Gut fullness of the Argentine stem weevil
10.1.
Pollen content
To assess the pollen content on the weevils' gut, first the weevils were washed with water and
"',~,-;-
J~
.-".
=' --
,
I
'
-,_, _-__ -._.
-'...-~~-- -~.'
. :.r":.~L ... -,,-,,--,",
detergent for at least one minute, to dislodge pollen grains present on the exoskeleton of the
insects (J. Martin, personal communication, November 2003). Then the weevils were
dissected as described in the previous section. The gut was removed after severing its anterior
and posterior ends and placed on a slide. The method oflrvin et ai. (1999) was used to count
pollen grains and this is described below.
Each gut was placed on a glass slide in alcohol, and then the gut was teased apart with
mounted needles. The gut contents were spread over part of the slide, and then a drop of
saffranin (i.e., 0.1% w/v) and a drop of jelly were added, followed by a coverslip. The jelly
comprised 7 g gelatine, distilled water to 19 ml, 33 g 82% glycerine, and 1 g phenol crystals.
The slides were subsequently scanned under a microscope at 63X magnification (Plate 8).
Pollen numbers were estimated on a semi-quantitative scale without detailed counting at the
higher pollen categories, into one of seven frequency classes (i.e., 1 = no pollen grains, 2 = 110 grains, 3
=
11-100 grains, 4 = 101-1000 grains,S
=
1001-3000 grains, 6 = 3001-5000
grains, 7 > 5000 grains). To prevent pollen contamination between samples, the forceps and
needles were sterilised using alcohol and a Bunsen flame after each dissection (Wratten et ai.,
1995).
10.2.
Ryegrass particle content
The presence or absence of ryegrass in the gut was recorded in both parasitoid-exposed and
non-exposed weevils. At the same time as recording pollen grains, the gut fullness was
recorded as 'full gut', when all the alimentary canal was full of ryegrass particles, 'little grass'
when ryegrass particles were present only in the foregut or midgut, and 'no grass' when no
ryegrass particles were observed.
46
11. Exposure of weevils to adult parasitoids
Sixteen adult M hyperodae were randomly assigned to exposure cages. Every parasitoid was
<48 hours old and was placed with 40 weevils for 24 h, then removed. Filter paper (10
* 10
mm) soaked in 50:50 honey and water solution was placed on the gauze lid of the exposure
cage as food for the parasitoid. One bunch of ryegrass was placed in every cage of the
treatments that used ryegrass (i.e., T1, T2 and Control, see Table 1), and one bunch of
ryegrass with endophyte was placed in every cage of the T3 treatment (i.e., see Table 1).
Every exposure cage included one moist dental wick so the weevils and the parasitoid had
access to water.
12. Statistical analyses
A generalised mixed model was used to analyse the number of surviving weevils, the number
of emerged parasitoids, the time (i.e., days) to 50% parasitoid emergence, and parasitoid tibia
length and egg load. However, for the correlation between tibia length and egg load, Pearson
correlation coefficients were used. In addition, a Pearson correlation coefficient was used to
analyse the correlation between days after weevil exposure and parasitoid egg load.
A log-linear model was used to analyse the statistical significance of the interactions between
categorical variables measured in the present experiment (P < 0.05). The variables were
analysed for second-order interactions between each other, and then for third-order
interactions where significant second-order interactions were observed. Also, a Chi-squared
(X2) analysis was used to look for further statistical differences between pollen categories.
RESULTS
Experiment 1
Parasitoid tibia length and egg load
Microctonus hyperodae from the ryegrass-only treatment had a mean tibia length of 0.769
±0.006 mm (±standard error) (n = 46), which was similar to those from the ryegrass plus beecollected pollen treatment (F 1,109
=
1.01, P > 0.05; Fig. 19), which had a mean tibia length of
0.761 ±0.01O (n = 59). The mean egg load of M hyperodae from the ryegrass-only treatment
47
was 48.4 ±1.7 eggs (n = 44), and was not significantly different (F 1,109 = 1.01, P > 0.05) from
the egg load of the ryegrass plus bee-collected pollen treatment, with a mean of 45.9 ±3.2
eggs (n = 58; Fig. 20).
Overall there was a positive, though non-significant correlation between tibia length and egg
load (rp = 0.176, P > 0.05; Fig. 21), described by the equation: y = 48.598x + 10.089. A
similar situation was observed in the ryegrass-only (rp
=
0.195, P> 0.05) and in ryegrass plus
bee-collected pollen treatment (rp = 0.164, P > 0.05).
a
0,8
a
0,7
E 0,6
505
.c
g>
'
0,4
(ij
0,3
Q)
:.c
F
02
'
0,1
0,0+--Ryegrass + bee-collected pollen
Ryegrass only
Treatment
Fig. 19. Mean ± SE hind tibia length (mm) of Microctonus hyperodae from hosts fed either
ryegrass-only or ryegrass plus bee-collected pollen. Bars sharing the same letter do not differ
atP = 0.05
48
Fig. 20. Mean ± SE egg load of Microctonus hyperodae from hosts fed either ryegrass-only or
ryegrass plus bee-collected pollen. Bars sharing the same letter do not differ at P = 0.05
o
Ryegrass only
•
Ryegrass + bee-collected pollen --Linear all treatments
60
50
"'C
!II
40
.Q 30
Cl
Cl
UJ
20
10
•
0+-----,----,-----,-----,----,-----,----,
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
Tibia length (mm)
Fig. 21. Correlation between egg load and tibia length (mm) of M hyperodae parasitoids
emerged from hosts fed ryegrass-only or ryegrass plus bee-collected pollen. There was a nonsignificant difference at P = 0.05.
49
Experiment 2
Diet consumption by the weevil
Pollen intake
In the ryegrass plus bee-collected pollen treatment, there was a significant second-order
interaction between pollen consumption and parasitoid exposure (i.e., the unparasitised
portion of exposed weevils, and non-exposed weevils to adult parasitoids had more pollen in
their gut; FI < 0.001, P < 0.05; Fig. 22). The unparasitised portion of the exposed weevils will
be called 'exposed' from now on. Forty four percent of non-exposed weevils had pollen in
their gut; in contrast, 18% of exposed weevils had pollen in their gut. In addition, there was a
third-order interaction between pollen consumption, parasitoid exposure and the fieldcollected weevils that were used at the beginning of the experiment. At the end of the
experiment, weevils exposed and non-exposed to adult parasitoids had significantly higher
amounts of pollen in their gut compared with the weevils at the beginning of the experiment
(F2 < 0.001, P < 0.05; Fig. 22). There was non-significant difference between the number of
weevils which had consumed pollen in exposed weevils fed ryegrass plus bee-collected pollen
or ryegrass plus buckwheat pollen (X 2 = 3.818, P
=
0.282). As expected, no weevils from the
ryegrass-only, and ryegrass with endophyte treatments had pollen in their gut.
/a
<=
45
40
.J::
~ 35
.!!l 30
.~ 25
8
~
8.
Q)
;: 20
~
15
~ 10
5jo 5
ffi
c..
0
Start of experiment
Non-exposed weevils Exposed weevils at the
at the end of
end of experiment
experiment
Fig. 22. Percentage of Argentine stem weevils with pollen in their gut at the beginning of the
experiment (A), and at the end of experiment (B), which comprises non-exposed and exposed
weevils to adult parasitoids. Bars sharing the same letter do not differ at P = 0.05.
50
Ryegrass intake
Exposed and non-exposed weevils consumed similar amounts of ryegrass (F 1 = 0.879, P >
0.05). However, there was a significant second-order interaction between ryegrass
consumption and treatment (F8 < 0.001, P < 0.05, Fig. 23). For instance, 35% of weevils in
the ryegrass-only treatment had their gut full of ryegrass, while this figure was 2% in the
ryegrass plus bee-collected pollen treatment, and 21 % in ryegrass plus buckwheat pollen and
ryegrass with endophyte treatments. There was also a significant second-order interaction
between rye grass and pollen consumption (F4 < 0.001, P < 0.05, Fig. 24); 9.6% of weevils
had ryegrass in their gut, and no pollen; in contrast, 1.5% of weevils had ryegrass and pollen
in their gut.
o No grass m Little grass • Full grass
C
2c
0
u
"5
0.8
C)
Q)
::c 0.6
ro
..:;:
ro
>
0.4
0
c
0
t
0
a.
e
CL
0.2
0
Ryegrass only
Ryegrass +
bee-collected
pollen
Ryegrass +
buckw heat
pollen
Ryegrass +
endophyte
Treatment
Fig. 23. Percentage of Argentine stem weevils with different gut fullness from four
treatments. There were significant differences at P < 0.05. Analysis was a log-linear model of
categorical data. Pair-wise comparisons of dates/categories, etc., were not possible.
51
o No grass
~
os:
Q.)
Q.)
~
0
Q.)
C)
.£9
c
Q.)
(.J
Q)
a..
l'!!iiI
Little grass • Full grass
20
18
16
14
12
10
8
6
4
2
0
No pollen
With pollen
Fig. 24. Percentage of Argentine stem weevils with different ryegrass and pollen gut fullness.
There were significant differences at P < 0.05. Analysis was a log-linear model of categorical
data. Pair-wise comparisons of dates/categories, etc., were not possible.
Survival andfitness-related data ofArgentine stem weevil
Survival
The number of surviving weevils was significantly different (F 3 ,12 = 0.018, P < 0.05; Fig. 25)
among the four treatments. The mean number of weevils was 13.3 ±1.1 (n
=
53) in the
ryegrass-only treatment; 15.8 ±2.3 weevils (n = 63) in the ryegrass plus bee-collected pollen
treatment; 10.3 ±2.4 weevils (n = 43) in the ryegrass plus buckwheat pollen treatment; and 6.5
±0.9 weevils (n = 26) in the ryegrass with endophyte treatment. The ryegrass-only, ryegrass
plus bee-collected pollen and ryegrass plus buckwheat pollen treatments were not
significantly different (P > 0.05). The same pattern was observed between the ryegrass plus
buckwheat pollen and ryegrass with endophyte (P > 0.05). However, the ryegrass with
endophyte treatment was significantly different from ryegrass-only and rye grass plus beecollected pollen treatments (P < 0.05).
52
20
a
~
'>
Q)
Q)
;=
15
-
10
E
5
Q)
.~
0
L-
Q)
..c
:J
Z
0
Ryegrass only Ryegrass + bee- Ryegrass +
collected pollen buckwheat pollen
Ryegrass +
endophyte
Treatment
Fig. 25. Mean ± SE number of Argentine stem weevils exposed to adult Microctonus
hyperodae after the termination of parasitoid emergence, from different treatments. Bars
sharing the same letter do not differ at P = 0.05.
Weevil fitness-related data (i.e.) gonad development)
There was a non-significant second-order interaction between gonad development and
exposure status of the weevils (Fl = 0.183, P > 0.05). However, there was a significant
second-order interaction between gonad development and the diet treatments (F3 < 0.001, P <
0.05, Fig. 26). The percentage of weevils with mature gonads was 40.5% in the ryegrass-only
treatment, 51% in the ryegrass plus bee-collected pollen, 40.5% in the ryegrass plus
buckwheat pollen, and 16.5% for the ryegrass with endophyte treatment.
In addition, there was a significant second-order interaction between gonad development and
pollen consumption (F2 < 0.001, P < 0.05, Fig. 27); of those weevils that had no pollen in
their gut, the percentage of sexually mature weevils was 31.3%. However, when weevils had
more than 11 pollen grains in their gut, the percentage of sexually mature weevils increased to
97.1%.
53
50
~
Q)
40
~
'0 30
Q)
OJ
.l!!
<::
~
20
Q)
a.
10
o +-_--L----'----,-_ _
Start of
experiment
Ryegrass only Ryegrass +
bee-collected
pollen
Ryegrass +
buckwheat
pollen
Ryegrass +
endophyte
Fig. 26. Percentage of Argentine stem weevils with mature gonads at the beginning of the
experiment, and from four different treatments at the end of the experiment. There were
significant differences at P < 0.05. Analysis was a log-linear model of categorical data. Pairwise comparisons of dates/categories, etc., were not possible .
• Sexually mature gonads 0 Sexually immature gonads
.!!!.
.>
Q)
Q)
3:
"0
Q)
C)
ro
c:
Q)
~
Q)
a..
100
80
60
40
20
0
No pollen
1 -10 pollen
grains
> 11 pollen
grains
Fig. 27. Percentage of Argentine stem weevils with mature and immature gonads in relation to
different pollen abundance categories. There were significant differences at P < 0.05.
Analysis was a log-linear model of categorical data. Pair-wise comparisons of
dates/categories, etc., were not possible.
54
Sex ratio
There was a significant second-order interaction between gender and gonad development (F 1
< 0.001, P < 0.05, Fig. 28). Twenty percent of females were sexually mature over all
treatments, but 61 % of males were mature over all treatments. However, there was a nonsignificant interaction between gender and treatments (F3
=
0.222, P > 0.05, Fig. 29). In
addition, there was a non-significant third-order interaction between gender, sex ratio at the
beginning of the experiment, and sex ratio 177 days later (F 1 = 0.252, P > 0.05, Fig. 29).
Furthermore, there was a non-significant second-order interaction between weevil's gender
and pollen consumption (F2 = 0.093, P > 0.05) .
• Sexually mature DSexual1y immature
100
.!!!.
.:;
Q)
Q)
3:
0
-...
80
60
Q)
·-"';_· .... _.. c....,>:;
Cl
III
t:
40
Q)
0
Q)
a..
20
0
Male
Female
Fig. 28. Percentage of male and female Argentine stem weevils with mature and immature
gonads. There were significant differences at P < 0.05. Analysis was a log-linear model of
categorical data. Pair-wise comparisons of dates/categories, etc., were not possible.
55
o Ryegrass only
• Ryegrass + bee-collected pollen
ll\! Ryegrass plus buckwheat pollen
FJJ Ryegrass + endophyte
1.5
o 1.25
~
OJ
'iii
E 0.75
~
~
0.5
~
0.25
III
o
o
28
147
177
Days after experiment set up
Fig. 29. Sex ratio of Argentine stem weevil from feeding different treatments over time. There
were no significant differences between treatments or days at P = 0.05.
Components offitness of the parasitoid
Body size
Microctonus hyperodae mean tibia length did not vary significantly between treatments (F 3,12
= 1.08, P > 0.05; Fig. 30). Parasitoids from the ryegrass-only treatment had the longest tibia at
0.856 ±0.010 mm (n = 38). Parasitoids from the ryegrass plus bee-collected pollen treatment
had a mean tibia length of 0.848 ±0.012 mm (n
...... -: ...-:...~--
~.-:
=
33), and those from the ryegrass plus
buckwheat pollen treatment had a mean tibia length of 0.844 ±0.012 mm (n = 31). Finally,
parasitoids from the ryegrass with endophyte treatment had the shortest tibia length with
0.806 ±0.016 mm (n = 22).
1.0
E
0.8
.s:::.
0.6
g
0,
c
~......-:::.....~.....
';::'-=-~J.:-.o'::.J_~:'
.---.~- ,~
~
'iii
:.c
a
a
a
a
0.4
i= 0.2
0.0
Ryegrass only Ryegrass + beecollected pollen
Ryegrass +
buckwheat
pollen
Ryegrass +
endophyte
Treatment
Fig. 30. Mean ± SE hind tibia length (mm) of M hyperodae parasitoids emerging from
different treatments. Bars sharing the same letter do not differ at P = 0.05
56
Egg load
Parasitoids mean egg load did not vary significantly between treatments (F3,12 = 1.08, P >
0.05; Fig. 31). The mean egg load in the ryegrass-only treatment was 46.1 ±1.4 eggs (n = 38),
what was almost identical to the mean egg load of the parasitoids from the ryegrass plus
buckwheat treatment, with 46.2 ±1.8 eggs (n
=
31). Parasitoids from the ryegrass plus bee-
collected pollen treatment had the greatest egg load with 48.9 ±2.0 eggs (n = 33); parasitoids
from the ryegrass with endophyte treatment had the lowest mean egg load, of 43.0 ±2.4 eggs
(n = 22).
~--.r_
2_-_'_-_-,
~...r_-_._-_-.-_~_;
60
50
VI
-
40
§
20
8l
Q)
a
~ 30
Q)
.c
z
10
0
Ryegrass only Ryegrass + beecollected pollen
Ryegrass +
buckwheat
pollen
Ryegrass +
endophyte
Treatment
Fig. 31. Mean ± SE egg load of M hyperodae parasitoids emerged from different treatments.
Bars sharing the same letter do not differ at P = 0.05.
Correlation between body size and egg load
Overall there was a positive and significant correlation between tibia length and egg load (rp =
0.293, P < 0.05; Fig. 32) over the treatments, described by the equation: y
=
42.761x + 10.5.
The same was observed in the ryegrass-only treatment (rp = 0.329, P < 0.05). There was also a
positive, though non-significant, correlation between tibia length and egg load in the
treatments ryegrass plus bee-collected pollen (rp
=
0.341, P > 0.05), ryegrass plus buckwheat
pollen (rp = 0.095, P > 0.05), and ryegrass with endophyte (rp = 0.432, P > 0.05) treatments.
The percentage of variation in egg load accounted for variation in body size, and P values are
detailed in Table 2.
57
o
•
X
Ryegrass plus bee-collected pollen
Ryegrass with endophyte
~ Linear ryegrass only
Ryegrass only
" Ryegrass plus buckwheat
-Linear all treatments
10
0+-----,------,-----,------,-----,-----,
0.70
0.75
0.80
0.85
0.90
0.95
1.00
Tibial length (mm)
Fig. 32. Correlation between egg load and tibia length of Microctonus hyperodae from
different treatments. There were significant correlations in the ryegrass-only treatment, and
with all the treatments at P < 0.05. Analysis was a log-linear model of categorical data. Pairwise comparisons of dates/categories, etc., were not possible.
Table 2. The relationship between tibia length and egg load for parasitoids emerged from
weevils fed with four treatments
% egg load variation
P - value
Treatment
accounted for tibia length
-
all treatments
8.6
0.001
ryegrass-only
10.8
0.044
ryegrass plus bee-collected pollen
11.6
0.053
ryegrass plus buckwheat pollen
0.9
0.608
ryegrass with endophyte
18.6
0.051
.... :-
Development time
Overall there was a positive, though non-significant, correlation between days after weevil
exposure to adult parasitoids, and the egg load of the parasitoids emerging from weevils fed
different treatments (rp = 0.039, P > 0.05; Fig. 33), described by the equation: y = 0.0193x +
44.786. This correlation was negative and non-significant in the ryegrass-only treatment (rp
=
-0.132, P > 0.05) and ryegrass with endophyte (rp = -0.254, P > 0.05). However, the
correlation was positive and non-significant in the ryegrass plus bee-collected pollen (rp =
0.175, P > 0.05) and ryegrass plus buckwheat pollen (rp = 0.092, P > 0.05).
58
There were non-significant differences in the time to SO% emergence of the parasitoids over
the treatments (F3 ,1l
=
0.46, P > O.OS; Fig 34). The lowest mean time to SO% of parasitoid
emergence was 72.0 ±9.S days (n = 38) in the ryegrass-only treatment. This was similar to the
mean of 73.0 ±7.9 days (n = 22) in the ryegrass with endophyte treatment. The time taken
SO% of parasitoid emergence was highest in the ryegrass plus buckwheat pollen, at 80.S ± 7.3
days (n
=
31), followed by the ryegrass plus bee-collected pollen treatment, with 80.3 ±S.9
days (n = 33).
o
o
Ryegrass only
6. Ryegrass plus buckwheat
- - Linear all treatments
X
Ryegrass plus bee-collected pollen
Ryegrass with endophyte
80
:;~-:,.':-:<::!'
"0
C\l
0
70
60
50
s!~
oX~
-;, 40
Cl
W 30
~
0 0
6.&
0
~ ~
1&
g
~
6.
20
10
0
60
70
80
0
90
0
X
100
~
0
0
0
6.
~~H
~ ~S ~0
06.
0
~ ~O
~
6.
110
6.
0
0
120
0
~
130
140
150
Days after exposure
Fig. 33. Correlation between egg load and days after Microctonus hyperodae emergence.
There were no significant differences at P = O.OS.
90
a
a
75
60
VI
~ 45
D
30
15
0
Ryegrass only Ryegrass + beeRyegrass +
collected pollen buckwheat pollen
Ryegrass +
endophyte
Treatment
Fig. 34. Mean ± SE days to SO% of parasitoid emergence from weevils feeding on different
treatments. Bars sharing the same letter do not differ at P = O.OS
59
Number of adult parasitoids emerged
Overall, there were non-significant differences in the number of adult parasitoids emerging
from the treatments (F3,12 = 0.69, P > 0.05; Fig 35). The highest mean number of parasitoids
was 9.5 ±2.5 (n
=
38) in the ryegrass-only treatment. This was followed by 8.3 ±0.8 (n = 33)
in the ryegrass plus bee-collected pollen treatment. Then followed by 7.8 ±2.4 (n = 31) in the
ryegrass plus buckwheat pollen treatment, and the lowest mean was 5.5 ±1.8 (n
=
22) in the
rye grass with endophyte treatment.
a
12
II)
"C
B
·iii
~
10
co
8
"5
"C
co
6
0
4
-....
Co
....
Q)
.0
E
:::J
2
Z
0
Ryegrass
only
Ryegrass + Ryegrass +
bee-collected buckwheat
pollen
pollen
Ryegrass +
endophyte
Treatment
Fig. 35. Mean ± SE number of adult Microctonus hyperodae emerged from different
treatments. Bars sharing the same letter do not differ at P = 0.05
DISCUSSION
It was hypothesised that the provision of different diets to the host would affect its fitness and
thereby affect the fitness of the parasitoid (Fig. 20). After providing different diets to hosts,
the following types of effects are possible: 1) affects on both host and parasitoid; 2) affects
only on one species "and not on the other; 3) a effects neither host or parasitoid. These effects
could be positive or negative depending on whether they increase or decrease fitness. These
combinations of positive, negative or no effects produced in each animal, are used to frame
this discussion.
60
HYPOTHESES
M. hyperodae
L. bonariensis
Ryegrass + pollen I endophyte
t/t
gonad development
i l l adult fitness
(i.e., egg load,
body size)
r
---+---+ t / tnutrients for
M. hyperodae larva
Fig. 36. Hypotheses about how the diet manipulation of the host could affect its parasitoid.
Possible effects on number of individual s and/or their fitne ss are represented by: "i" =
positive effect, "1" = negative effect
Experiment J
Parasitoid body size alld egg load
It was hypothesised that bee-co llected pollen would increase the fitne ss of the host, perhaps
via an increase in gonad development, and thereby enhance fitness of the parasitoid.
However, the tibia length and egg load of parasitoids from the ryegrass-only and ryegrass plus
bee-collected pollen treatments did not differ significantly (Fig. 19 and 20). Moreover, the
correlation between tibia length and egg load was non-significant in experiment I (Fig. 2 1).
Phillips and Baird (2001) found that M hyperodae from east of the Andes mountains showed
no significant correlation between egg load and hind tibia length, yet parasitoids from Chile
showed a signi fican t, though weak, positive correlation. The data obtained from experiment I
are, therefore, consistent with the occurrence of a high frequency of eastern M. hyperodae in
the present study. Indeed, eastern M hyperodae has been shown to be the more successful of
the two strains throughout New Zealand (Phillips el ai, 1997; Phillips el ai, 2004). Although
61
the addition of bee-collected pollen to the host diet had no significant effect on the
parasitoids' body size, egg load, or the relationship between these two variables, there are
some factors that could have partly obscured a positive correlation between parasitoid fitness
and pollen consumption by the weevils, and these are discussed below.
Experimental set up
Since some of the weevils used for the experiment 1 had been parasitised in the field, the
parasitoids which emerged had completed part of their development before to the beginning
of the experiment, and hence before to the provision of different diets to the weevils. As a
result, the potentially better nutrition from the ryegrass plus bee-collected pollen treatment
may have been provided too late to detect any difference between treatments. For example,
when larvae of Drosophila melanogaster L. (Diptera: Drosophilidae) were fed yeast
immediately after parasitisation, they were able to encapsulate the eggs/larvae of its
parasitoid, Leptopilina boulardi (Barbotin, Carton & Kelner-Pillault). However, when D.
melanogaster larvae were fed yeast 24 h after parasitisation, the diet had no effect on the
immune response (Vass and Nappi, 1998).
To account for this possibility, experiment 2 provided the diets to unparasitised weevils
before they were exposed to adult parasitoids in the laboratory. In this way, the weevils could
experience diet-related effects before to parasitism, and hence the parasitoid larva completed
its entire development within a host subjected to one dietary regime.
Weevil physiology
1. Weevil age
Since the weevils used for the experiment 1 were captured in August, it is likely that they
belonged to the second generation from the previous summer (Barker et al., 1988b). Mortality
of the overwintering generation reaches a peak in early December (Phillips et al., 1998).
Therefore, the weevils captured in August that were used in experiment 1 were old weevils.
This could have decreased their response to the pollen diet and obscured any changes in
parasitoid fitness. For example, Sequeira and Mackauer (1992) observed that old hosts
produced parasitoids with decreased fitness. Furthermore, Tagawa et al. (2000) demonstrated
that certain components of fitness of the hyperparasitoid, Eurytoma goidanichi Boucek, were
reduced when they emerged from old hosts compared with those from young hosts, and they
62
argued that old hosts provided low quality resources. In experiment 2, younger weevils
belonging to the early second generation (i.e., captured in February) were used.
2. Immune system
It is possible weevils fed ryegrass plus bee-collected pollen increased their immune response
and were able to encapsulate more parasitoid eggs or larvae than the rest of the weevils fed
ryegrass-only. This mechanism has been demonstrated in the vinegar fly, D. melanogaster.
When the fly larvae were fed with yeast immediately after parasitism by the wasp L. boulardi,
the host larvae were able to encapsulate a significantly higher percentage of the parasitoid's
eggs than did hosts deprived of yeast (Vass and Nappi, 1998). However, no weevil dissection
was performed in experiment 1 to assess the number of eggs/larvae encapsulated within hosts,
but such dissections were performed in experiment 2.
Experiment 2
Diet consumption by the weevil
Pollen intake
The Argentine stem weevils used for the present experiment had pollen in their gut before
collection (i.e., pollen consumed in the field), and after collection (i.e., pollen consumed in the
laboratory). However, in the laboratory, pollen consumption changed with parasitoid
exposure. Weevils exposed to adult parasitoids ate significantly less pollen than did the nonexposed weevils (Fig. 22). Similarly Barratt et al. (1996) and Gerard (2000) observed that the
unparasitised portion of exposed weevils to adult parasitoids exhibited decreased feeding
activity, compared with non-exposed weevils. Parasitoid presence had a detrimental effect on
pollen consumption even in the unparasitised portion of weevils exposed to adult parasitoids,
because physical disturbance of the weevils by the parasitoids resulted in a reduction of
weevil feeding activity during the period of exposure (Barratt et al., 1996; Gerard, 2000).
Even though parasitoids reduced weevil feeding, exposed and non-exposed weevils that were
provided pollen diets in the laboratory had significantly higher amounts of pollen in their gut
compared with the weevils in the beginning of the experiment (Fig. 22). This suggests pollen
availability may limit pollen consumption by Argentine stem weevil in the field (Chapter 2).
63
Although the number of exposed weevils taking pollen did not differ significantly between the
ryegrass plus bee-collected pollen and ryegrass plus buckwheat pollen, the numbers of pollen
grains consumed were different. Seven percent of the exposed weevils fed ryegrass plus
buckwheat pollen had from 1 to 10 grains of pollen in their gut, and none had more than 10
pollen grains in their gut. In contrast, of the exposed weevils fed ryegrass plus bee-collected
pollen, 7% had between 1 to 10 pollen grains in their gut, 12% had more than 10 pollen grains
in their gut, and 2% of the weevils had up to 3000 pollen grains in their gut. The lower
number of pollen grains consumed in the ryegrass plus buckwheat pollen treatment could be
explained by more difficult access to buckwheat pollen in this experiment. In the ryegrass
plus buckwheat treatment, weevils had to climb the buckwheat stems to reach the flowers and
eat the pollen, but in the ryegrass plus bee-collected pollen treatment, bee-collected pollen
was more readily accessible because it was provided as pollen-pellets in the bottom of the
cage. However, incidental pollen consumption via rye grass leaves contaminated with
buckwheat pollen could also explain this observation. Buckwheat flowers were placed on top
or beside the ryegrass, so pollen from buckwheat anthers could have been shed on top of the
ryegrass leaves. In contrast, bee-collected pollen was placed in the bottom of the cage, so
contamination of rye grass leaves by gravity was unlikely.
Ryegrass intake
Several researchers have reported that endophyte reduces feeding activity and damage caused
by Argentine stem weevil (Bell and Prestidge, 1991; Popay and Wyatt, 1995; Popay et al.,
1995; Barker and Addison, 1996; Popay, 1997) and other insect pests (Bultman and Bell,
2003; Bultman et al., 2004). In the present study, the ryegrass intake of the weevils fed
ryegrass with endophyte was lower compared with weevils fed ryegrass-only (Fig. 23).
The weevils' ryegrass intake in the ryegrass plus bee-collected pollen treatment was
significantly lower compared with the ryegrass-only treatment (Fig. 23). This agrees with the
results of Evans and Barrat (1995), who observed that the amount of ryegrass consumed by
weevils provided with pollen and ryegrass was approximately 45% lower than those provided
with ryegrass alone. It also agrees with the result that there was a significant and negative
second-order interaction between ryegrass and pollen consumption (Fig. 24). Weevils fed
ryegrass plus buckwheat also had a lower ryegrass intake compared with weevils fed
ryegrass-only, which could be explained the same way. Although weevils fed ryegrass plus
64
buckwheat had a higher rye grass intake compared with those fed ryegrass plus bee-collected
pollen (Fig. 23), this could be explained due to the lower pollen consumption of weevils fed
ryegrass plus buckwheat compared with weevils fed rye grass plus bee-collected pollen.
Survival and fitness ofArgentine stem weevil
Survival
Weevil survival was influenced by the rates of parasitism that occurred in each treatment, and
may also have been influenced by weevil diet. In these experiments, there was no significant
difference between treatments in parasitism rates (see section 'Number of adult parasitoids
emerged' below). However, the survival of weevils fed ryegrass with endophyte was
significantly lower than weevils from ryegrass-only and ryegrass plus bee-collected pollen
treatments (Fig. 25). Barker and Addison (1996) also reported a decrease in weevil survival
when fed ryegrass with endophyte compared with weevils fed ryegrass-only. This decrease in
survival could have been produced by starvation of the weevils due to deterrence by the
endophyte, or a toxic effect of the endophyte after consumption. In contrast, the survival of
weevils fed ryegrass-only, ryegrass plus bee-collected pollen and ryegrass plus buckwheat
pollen was not significantly different (Fig. 25). These results agree with those found by Evans
and Barratt (1995) who observed no difference in survival of weevils fed ryegrass-only or
rye grass plus bee-collected pollen.
Weevil fitness-related data (i. e., gonad development)
Non-exposed weevils and the unparasitised portion of exposed weevils exhibited similar
proportions of individuals with mature gonads. These results were different from those of
Barratt et al. (1996), who observed that the fecundity of Argentine stem weevil was
progressively reduced as exposure to adult parasitoids increased. However, these researchers
assessed the gonad development of the weevils based on counts of eggs laid per female, not
on dissections of both genders as was done in the present experiment. It is possible in the
experiment of Barratt et al. (1996) that adult parasitoid presence could have affected the
oviposition behaviour of the female weevils, but not their gonad development.
It was hypothesised that the addition of pollen to the weevils' diet would increase their gonad
development. Conversely, the addition of ryegrass with endophyte would decrease the gonad
65
development. Indeed, weevils fed ryegrass with endophyte experienced a drastic decrease in
gonad development (Fig. 26) which could be explained either by toxic effects of the
endophyte on the weevils, or by starvation. Several authors have also observed that Argentine
stem weevil had a lower oviposition rate (Bell and Prestidge, 1991; Popay and Wyatt, 1995;
Popay et ai., 1995; Popay, 1997) and decreased gonad development (i.e., oocyte resorption)
(Barker and Addison, 1996) when fed ryegrass with endophyte compared with ryegrass-only.
Oosorption occurs at a lower level in female weevils fed non-endophyte rye grass (Barker et
ai., 1988b). However, the ryegrass plus buckwheat pollen treatment did not increase gonad
development. Weevils that were fed ryegrass plus buckwheat pollen tended to have slightly
lower gonad development than weevils fed ryegrass-only (Fig 26). Although consumption of
just 1-10 pollen grains increased weevil gonad development (i.e., considering bee-collected
and buckwheat pollen altogether. Fig. 27), buckwheat pollen alone did not increase gonad
development (Fig. 26). This could be explained by a lower nutritional quality of buckwheat
pollen. In pea weevil, for example, the proportion of females developing eggs appeared to be
due to the low nutritional quality of pea pollen (Annis and O'Keeffe, 1984). In contrast, the
higher gonad development of weevils fed rye grass plus bee-collected pollen (Fig. 26) could
be due to better nutritional quality of bee-collected pollen. Pesho and van Houten (1982)
demonstrated that oocyte growth of female pea weevils was enhanced by the addition of pea
pollen to the weevils' diet of alfalfa, Medicago sativa L., leaves. Similarly, ovariole width in
boll weevils was promoted after pollen intake (Jones et ai., 1993).
It has been demonstrated that the addition of pollen to the weevils' diet increased their gonad
development compared with weevils fed ryegrass-only (Evans and Barratt, 1995). However,
over all treatments, gonad development declined between the start and end of the experiment
(Fig. 26). These results differ from those of Evans and Barratt (1995), who observed an
increase in the cumulative number of eggs per female weevil over time in those weevils fed
ryegrass plus bee-collected pollen, compared with weevils fed ryegrass-only. One or more
factors could have produced this difference including the onset of diapause in the weevils,
weevil density, or sex ratio changes.
1. Diapause effect
The lights of the laboratory that would produce a 14:10 L:D photoperiod (i.e., weevil critical
day-length 13.3:10.7 L:D; Goldson et ai., 1993a) were turned off by mistake from when the
66
experiment was set up until 68 days later when they were turned on. The weevils therefore
probably entered diapause soon after the start of the experiment. In addition, some weevils
could already have been in diapause before the experiment began since these weevils
belonged to the second generation, which enters directly into diapause in autumn (Barker et
at., 1988), independently of temperature (Goldson and Emberson, 1980). In addition, secondgeneration adult weevils do not become reproductively mature before winter, probably to
preserve resources until the following spring (Goldson, 1979 and 1981b; Barker and
Pottinger, 1982). The morphological and physiological changes associated with onset of
diapause include atrophy of the gonad, cessation of ovulation, resorption of terminal oocytes
and cessation of mating (Barker et at., 1988b). The onset of diapause is likely to have reduced
the response of gonad development to pollen.
2. Weevil density
It is possible that the weevil density used in the present experiment was too high (140 weevils
m-2 ). Barker et
at. (1989) reported that weevil densities above 75 adults m-2 produced rapid
decline in ovarian condition and oviposition effort per female weevil. This was also suggested
by McNeill et
at. (1998) to explain 40% of non-reproductive female weevils in their
experiment. Similarly, Goldson (1979) discovered that caging fully reproductive weevils led
to a dramatic decline in the proportion of females with eggs from about 90 to 20% in only 10
days. However, Evans and Barratt (1995) used 178 field-collected weevils m-2 in their
experiment, and found an increase in the cumulative number of eggs per female in weevils fed
ryegrass-only over time. Further research is needed to elucidate interactions between weevil
density and gonad development.
3. Sex ratio
There was a significant second-order interaction between gender and gonad development
(Fig. 28). Female Argentine stem weevils had a significantly higher proportion of sexually
immature individuals compared with male weevils (Fig. 28). This could be explained by the
onset of diapause, which affects mainly female weevils' gonad development (Barker et
at.,
1988b). A similar situation was observed in the pea weevil, where males were sexually
mature and females were sexually immature when leaving diapause (Pesho and van Houten,
1982). A decrease in male Argentine stem weevil population over time could have influenced
the decrease in gonad development observed in the present experiment. However, the sex
67
ratio did not change significantly over time, or between treatments. Furthermore, pollen had
no effect on male or female weevil survival, since the sex ratios in the treatments were not
significantly different (Fig. 29). This agrees with Evans and Barratt (1995) who observed no
significant increase in survival of female weevils fed ryegrass plus bee-collected pollen
compared to female weevils fed ryegrass-only.
Fitness 0/ the parasitoid
Body size
The length of the hind tibia of parasitoids that emerged from hosts fed different diets did not
significantly differ between treatments (Fig 30). It is possible that the general decrease in
gonad development of the weevils over time (independent of treatment) could have decreased
the resources made available for the developing parasitoid. This could have obscured any
potential increase in parasitoid size in the present experiment. However, the length of the hind
tibia of parasitoids that emerged from the ryegrass with endophyte treatment did not differ
from those emerging in the other treatments, even though weevils fed ryegrass with
endophyte had a much lower proportion of weevils with mature gonads. These results are
consistent with research on other plant resistance strategies such as transgenics. For example,
the parasitoid wasps Eulophus pennicornis (Nees) and Aphelinus abdominalis (Dalman)
emerging from hosts fed transgenic (i.e., GNA gene) plants of tomato and potato respectively,
did not differ in body size compared with wasps emerged from hosts fed the non-transgenic
version of the crops (Bell et al., 1999; Couty et al., 2001).
Also, based on the fact that parasitised weevils are rapidly sterilised after parasitism (Barratt
et al., 1996), it is possible that M hyperodae's body size was based on resources other than
weevils' gonads (e.g., flight muscles, body fat, etc.). Hymenopteran parasitoids require
approximately 30 chemicals to fulfil their nutritional requirements, including proteins and/or
amino-acids, B-vitamin complex, energy (i.e., carbohydrates or lipids), fat soluble vitamins,
cholesterol, and polyunsaturated fatty acids (Tp.ompson and Hagen, 1999). The parasitoid
food is of animal origin, thus being generally high in protein content and low in carbohydrate
and fat (Thompson and Hagen, 1999). Theoretically, other compounds such as fatty acids
could be limiting factors for M hyperodae larvae. Indeed, Argentine stem weevils in the field
have 16% of dry weight in fat during winter, which decreases to 8% with the beginning of egg
maturation (Goldson, 1981b). This decrease in fat could play an important role in the nutrition
68
of the parasitoid. Further research is needed to identify nutritional requirements associated to
the larval development of M hyperodae. Protein, lipid and carbohydrate foods could be tested
as possible candidates to increase the parasitoid's body size.
It is possible that other components of fitness of M hyperodae were affected by the nutrition
of its host (e.g., longevity or searching efficiency). Further research is needed to identify other
components of fitness of the parasitoid that could be increased with the addition of pollen to
the host diet.
Features of parasitoid development such as teratocytes and the storage of nutrients in the
parasitoid egg may help to compensate for hosts of low nutritional quality and thus reduce any
effects of host diet on parasitoid fitness. Teratocytes are cells derived from the embryonic
membrane of parasitoid eggs, which are released into the host haemocoel at the time of egg
hatching. They appear to playa role in parasitoid nutrition (Thompson and Hagen, 1999).
Sluss (1968) demonstrated that teratocytes of the parasitoid wasp Perilitus coccinellae
(Schrank) (Hymenoptera: Braconidae) increased in volume several fold in the coccinellid host
and were subsequently eaten by the developing parasitoid larvae. Furthermore, stored
nutrients within the parasitoid egg can support limited development of the parasitoid larva
(Thompson and Hagen, 1999). For example, due to nutrients stored within the parasitoid egg,
partial larval development of the parasitoids Itoplectis conquisitor (Say) (Yazgan, 1972) and
Exeristes roborator (Fabricius) (Thompson, 1981) was achieved with diets lacking various
essential amino-acids and B-complex vitamins. During the dissection of Argentine stem
weevils, teratocytes were frequently observed in parasitised individuals. Further research is
needed to understand the mechanisms which maintain parasitoid fitness in variable quality
hosts.
Egg load
Egg loads of parasitoids that emerged from hosts fed different diets did not differ significantly
between treatments (Fig. 31). It is possible that M hyperodae 's egg load was based on other
resources (e.g., flight muscles or body fat) carried from the host as discussed previously in the
body size section. Further research is needed to explore other nutritional factors associated
with the weevil that could affect parasitoid egg load.
69
It seems likely that diapausing weevils provided parasitoids with similar nutrition to non-
diapausing weevils since the average egg load of the ryegrass-only treatment of 46.1 eggs
recorded in the present experiment corresponded well to the mean egg load of 47 eggs
recorded from non-diapausing weevils fed ryegrass-only by Phillips and Baird (2001).
It was expected that M hyperodae egg loads would be lower in the ryegrass with endophyte
treatment, as other authors have reported for other insect resistant crops. For instance, adults
of the endoparasitoid Cotesia marginiventris (Cresson) (Hymenoptera: Braconidae) which
emerged from hosts fed with transgenic cotton plants expressing the gene Cry1A(c) had lower
egg loads than those which emerged from hosts fed the non-transgenic version (Baur and
Boethel, 2003). However, M hyperodae egg load did not decrease with the addition of
endophyte to the weevils' diet. Similar results were reported for the parasitoid wasps (E.
pennicornis and A. abdominalis) that emerged from hosts fed transgenic plants of tomato and
potato, respectively, expressing the GNA gene. These parasitoids had similar egg loads to
parasitoids that emerged from hosts fed with the non-transgenic version of the crops (Bell et
al., 1999; Couty et al., 2001). This suggests that biological control and plant resistance may
often work in a complementary, or even synergistic, way in integrated pest management
programmes (van Emden, 1990).
Correlation between body size and egg load
The carry-over of resources from the host is often positively correlated with wasp body size
(Harvey et al., 1995; Jervis et al., 2001), and adult body size is generally correlated with egg
load at emergence (Jervis et al., 2001). However, M hyperodae is an unusual case where
some geographic populations (e.g., those from Chile and from San Carlos de Bariloche in the
Argentinean Andes) exhibit relatively strong correlations between body size and egg load,
while other geographic populations from east of the Andes do not (Phillips and Baird, 2001).
In experiment 2, the correlation between tibia length and egg load of the parasitoid over all
treatments was positive and significant, though weak (Table 2, Fig. 32). A significant, though
weak, correlation was also found in the ryegrass-only treatment. However, in the ryegrass
plus bee-collected pollen, ryegrass plus buckwheat pollen, and ryegrass with endophyte
treatments, the result was nearly significant.
70
These results differ from experiment 1, where the correlation was non-significant. It is
possible that a mixture of parasitoid lineages from the east and west of the Andes were
_C'_-_
'.:-'-'1
present in the experiment 2 (Phillips and Baird, 1996; Phillips et al. 2004; Iline and Phillips,
2004). Indeed, the percentage of egg load variation accounted for by the tibia length of
parasitoids that emerged from weevils fed ryegrass-only was 10.8% (Table 2), which
corresponded well with the 10.6% obtained by Phillips and Baird (2001) using a mixture of
parasitoid lineages that had also emerged from weevils fed ryegrass-only. Furthermore, the
........;.. ..
-
percentage of egg load variation accounted for the tibia length, considering all the treatments,
was generally low (i.e., 8.6%; Table 2) and this corresponded well with the results obtained
by Phillips and Baird (2001) for the parasitoid population coming from east of the Andes in
South America (i.e., 8.5% on average for parasitoids from Argentina, Uruguay, and Brazil).
Based on these results, it is probable that the parasitoids obtained in the present experiment
were mainly from populations derived from material from east of the Andes. Further research
should include parasitoid morphological identification, DNA and enzyme to determine its
South American origin.
Developmental time
The onset of weevil diapause probably affected the development time of M hyperodae since
it enters photoperiodically-induced diapause as a first instar larva (Goldson and McNeill,
1992), with a critical photoperiod of 12.3: 11.7 L:D, irrespective of the physiological
condition of the host (Goldson et al., 1993a). Diapause would explain why the emergence of
parasitoid prepupae began 40 days after the exposure of the weevils to adult parasitoids,
instead of after approximately 14 days at 20°C as would normally be expected (Loan and
Holdaway, 1961; C.B. Phillips, personal communication, July 2004), and ceased 129 days
after exposure.
Barker and Addison (1996) demonstrated that the development of M hyperodae larvae was
••
;.:'._.~_e
___ •
retarded when they developed inside Argentine stem weevils fed ryegrass with endophyte
compared with those developing inside weevils fed endophyte-free ryegrass. Similar results
were reported by Bultman et al. (2003), who found that Argentine stem weevils fed some
strains of endophyte affected the development rate of M hyperodae. Similarly, the larval
development rate of the braconid endoparasitoid C. marginiventris was significantly retarded
when developing inside hosts fed transgenic cotton expressing the Cry lA(c) gene compared
71
with parasitoids that had emerged from hosts fed the non-transgenic version of the cultivar
(Baur and Boethel, 2003). However, there was no significant difference in parasitoid
development rates between treatments in the current experiment. This suggests diapause,
rather than toxic effects from endophytes, is the most likely explanation for the observed
protracted development of M hyperodae in the current experiment. It is notable that the
endophyte strain ARW (i.e., the same used in the present experiment), had no retardant effects
on the development of M hyperodae (Bultman et al., 2003).
The long development time of the parasitoid larva could have influenced parasitoid egg load.
For example, parasitoids that spent more time in the host may have had more time to
accumulate resources. However, this did not appear to be so because there was only a weak
non-significant correlation between days after exposure and parasitoid egg load (Fig. 33), and
the time to 50% ofparasitoid emergence was similar for all the treatments (Fig. 34).
The developmental delay caused by diapuase probably did not obscure any treatment effects
since the larval parasitoids and their hosts probably responded to the lights being turned on
simultaneously across treatments. Such phenological synchrony between parasitoids and their
hosts has been widely reported. For example, larvae of Microctonus aethiops (Nees)
(Hymenoptera: Braconidae) developed quickly in alfalfa weevils, Hypera postica (Gyllenhal),
that were sexually mature, but entered diapause in sexually immature weevils (Day 1971).
This dormant period served to synchronise the parasite's life cycle with that of its host (Day,
1971).
Number of adult parasitoids emerged
In the present experiment, the number of adult parasitoids that emerged from the different
treatments did not significantly differ (Fig. 35). These results agree with those of Bultman et
al. (2003) who did not find significant differences in the number of parasitoids that emerged
from laboratory reared weevils fed ryegrass-only or ryegrass with ARW endophyte. However,
in the present experiment the sample variance was very high, probably due to the small
number of wasps that emerged from every replicate. Further research would need to use more
weevils, to increase parasitoid numbers and lower the variance.
72
There was a trend where the number of parasitoids that emerged from the ryegrass plus beecollected pollen and ryegrass plus buckwheat pollen treatments were lower than the ryegrassonly treatment. It is possible that the provision of pollen increased the weevils' immune
response so they could encapsulate more parasitoid eggs or larvae than weevils fed other
diets. However, based on weevil dissections performed after parasitoid exposure, only one out
of 65 weevils in the ryegrass plus bee-collected pollen treatment had an encapsulated
parasitoid egg, and no encapsulated parasitoids were found in the other treatments. Enhanced
weevil immune responses therefore cannot explain the trend for reduced numbers of adult
parasitoids from the ryegrass plus bee-collected pollen treatment.
There was also a trend where the number of parasitoids that emerged from the ryegrass with
endophyte treatment was lower than the ryegrass-only treatment. It is possible that secondary
poisoning of M hyperodae larvae occurred. Plant resistance relying on toxic allelochemicals
may affect parasitoids, and thereby indirectly change parasitoid-host interactions (Bultman et
al. 1997). These chemicals can produce toxicity to parasitoids within hosts or otherwise
reduce parasitoid emergence from them (van Emden, 1990). In fact, artificial diets containing
the diterpenes lolitrem B and u-paxitriol of endophyte origin, reduced the survival of M
hyperodae larvae within hosts at concentrations of 2 Ilg g-l (Barker and Addison, 1996).
Indeed, Lolitrem B was present in the ARW endophyte strain used for the present experiment.
Further research should include dissection of alive parasitised weevils at regular time intervals
to elucidate the within-host mortality of M hyperodae larvae.
It is also possible that endophyte reduced the parasitism rate. For example, Barker and
Addison (1996, 1997) reported a significantly lower parasitism rate by M hyperodae on
weevils fed on ryegrass with endophyte (i.e., 66.3% parasitism), than in weevils fed on
ryegrass-only (i.e., 78.2% parasitism). Based on weevil collection, Goldson et al. (2000)
observed 34.1 % parasitism in rye grass with endophyte and 63.6% parasitism on ryegrassonly. There was a significant inverse relationship between parasitism rate and peramine levels
in the field. They concluded that adult M hyperodae parasitoids probably were less successful
attacking hosts on high endophyte plots in the field, because of the reduced feeding activity of
the weevils (Goldson et al., 2000; Phillips, 2002). Indeed, several authors have reported that
Argentine stem weevil had a lower feeding activity in ryegrass with endophyte, compared
with those feeding on ryegrass-only (Bell and Prestidge, 1991; Popay and Wyatt, 1995; Popay
et al., 1995; Popay, 1997). Also, adult weevils were more susceptible to parasitism when they
73
were moving, particularly when feeding (Phillips, 2002). Moreover, Gerard (2000) observed
that about 90% of the adult weevils on ryegrass with endophyte were in positions
unfavourable (i.e., crouching and off plant) for parasitoid oviposition and less time feeding,
compared with weevils on ryegrass-only. Based on these ideas, it is possible that, in the
present experiment, the weevils fed ryegrass with endophyte experienced a lower parasitism
rate compared with the rest of the treatments. However, due to the low number of parasitoids
that emerged, the variability was too high and the differences were not significant. However,
Exner and Vidal (1996) found that endophyte in tomatoes (Lycopersicum esculentum L.) had
no effect on the attack rate of Encarsia formosa Gahan (Hymenoptera: Trichogrammatidae)
on second instar larvae of the whitefly Trialeurodes vaporariorum (Westwood) (Homoptera:
Aleyrodidae).
It is possible that diets with deterrent effects such as endophytes can also cause host starvation
(Bultman et al., 1997) and could also prevent parasitoids from emerging. For example, the
parasitoid Cotesia congregata Say (Hymenoptera: Braconidae) failed to emerge if its host
larva, Manduca sexta (L.) (Lepidoptera: Sphingidae), was starved (Beckage and Riddiford,
1983). However, as discussed in the ryegrass intake section, starvation was unlikely to have
occurred in the present experiment. Further research is needed to understand the interactions
between M hyperodae and endophytes.
74
CHAPTER 4
GENERAL DISCUSSION
THE HOST'S MARK
" ......... far from being a purely passive victim, obliterated without a trace, the host is often able
to impress its mark ...... upon the insect parasitoid that destroys it" (Salt, 1941). In the present
work, the number of parasitoids emerged and their fitness as a product of different diets given
to the host has been the "mark" investigated. The literature in this respect is rather scarce
concerning those "impressed marks" left by the host on its parasite (Salt, 1941; Beckage and
Riddiford, 1983; Harvey et al., 1995; Bernal et al., 1999; Tagawa et al., 2000; Honda and
Luck, 2001). Furthermore, as will be discussed below, even plants or parts of them such as
pollen (i.e., the first trophic level) can shape this mark and influence the interactions of the
trophic levels that follow.
Pollen has evolved for its role in sexual reproduction in plants, but only a small fraction
arrives on another flower, while the remaining pollen has other ecological values (van Rijn et
al., 2002). Pollen can be utilised by several groups of predatory (e.g., Wratten et al., 1995),
parasitic (Zhang et al., 2004) and herbivorous (e.g., Jones et al., 1993) arthropods. Due to the
high nutritional value of pollen (Thompson and Hagen, 1999), and its importance in egg
maturation in some species of predatory (e.g., adult syrphid flies; Wratten et al., 1995) and
herbivorous (Annis and Q'Keeffe, 1984) arthropods, it was hypothesised that the addition of
pollen to the diet of Argentine stem weevil would increase the fitness of the weevil (i.e., the
second trophic level), and thereby enhance the fitness of the parasitoid, M hyperodae (i.e., the
third trophic level). Conversely, for a deleterious food such as ryegrass with endophyte given
to the weevil, the opposite effect was hypothesised.
Interactions between herbivores and their host plants and with their natural enemies can only
be understood when considered within a tri-trophic context (Price et al., 1980). The addition
of different diets to the second trophic level can produce a theoretical combination of no,
positive or negative effects in both the second and the third trophic level, via changes in
75
fitness and/or survival that could affect the host's and subsequently the parasitoid's fitness
and/or survival (Table 3).
Table 3. Combination of possible effects on the survival and/or fitness of host and parasitoid
produced by changes in the host's diet (i.e., -- = no effect, t = positive effect, t = negative
effect)
Host (weevil)
Parasitoid wasp
,-,'.:'--'
t/t
Observed in this study
~
t/t
t /t
Chapter 2 provided evidence that Argentine stem weevil feeds deliberately on specific types
of pollen in the field, and this was associated with a higher rate of gonad development (i.e.,
the number of individuals with sexually-mature gonads; positive effect). In the manipulative
experiment described in Chapter 3, no effect of host diet on parasitoid fitness was detected.
This may have been because the increase in weevil gonad development previously found to be
associated with pollen was countered by a decrease in gonad development due to diapause.
Argentine stem weevil diapause is associated with atrophy of the gonads, cessation of
ovulation and resorption of terminal oocytes (Barker et ai., 1988b). Diapause may therefore
have obscured the effect of diet on weevil fitness and/or survival. Moreover, by causing
female weevils to become reproductively immature during diapause (independent of their
dietary treatment), possible interactions between weevil reproductive status, weevil diet and
parasitoid fitness could have been suppressed.
The negative effects of the endophyte diet on the host's gonad development were apparently
compounded by diapause. A 'false' negative effect in the gonad development condition is
discounted though, since the ryegrass-only treatment enabled the effect of endophyte on
gonad development to be isolated. Also the survival of weevils fed ryegrass with endophyte
was significantly decreased in this treatment, as was expected. However, no negative effects
were observed on fitness of the parasitoids that emerged from weevils fed ryegrass with
endophyte. This is encouraging from an integrated pest management point of view, since the
potential fecundity (i.e., egg load) and body size (i.e., tibia length of hind legs) of the
parasitoid are not affected by the addition of ryegrass with endophyte to the weevil's diet.
76
This means that biological control and plant resistance strategies can work in a
complementary way, supporting the ideas of van Emden (1990). However, it also means that,
based on the results of the present manipulative experiment (Chapter 3), the addition of
'resource subsidies' (Gurr and Wratten, 2000) such as pollen, available to the second trophic
level appears to have little scope for manipulating M hyperodae fitness via the host diet.
Although the concept of benefiting the third trophic level with resource subsidies made
available to the second level remains unproven, it is potentially very worthwhile and warrants
further research.
IMPLICATIONS FOR BIOLOGICAL CONTROL
Conservation biological control involves the manipulation of the environment (i.e., resource
subsidies) to enhance the survival, fecundity, longevity, and change the behaviour of natural
enemies to increase their effectiveness, and to provide shelter from adverse environmental
conditions (i.e., "top - down" effects; Root, 1973; Landis et
at., 2000). The mechanism
studied in the present experiments, however, involves "bottom - up - top - down" effects,
where 'top - down' are a consequence of "bottom - up" effects. An enhancement of
fitness/survival of the third trophic level is theoretically made via an enhanced fitness/survival
of the second trophic level. However, the benefit to the pest due to an increase in fitness of the
second trophic level should be more than counteracted by the effects on the parasitoid
population. In this way, the net benefit would be in favour of the third trophic level.
Parasitoids with higher egg loads would be potentially good candidates to achieve higher
parasitism rates because, based on mathematical models, the degree of manipulation needed
to achieve a given reduction in prey populations is largely determined by the enemy's
potential reproductive rate (Kean et
at., 2003). Consequently, the present mechanism has the
potential to lead to the suggestion of a new way for enhancing parasitoid fitness, that is,
'indirect' conservation biocontrol.
77
Selectivity issues
The manipulation of crop and non-crop plants, without a detailed understanding of the
interactions of the trophic levels involved can produce counterproductive or unexpected
results (Barbosa and Wratten, 1998). Undesirable positive effects on the second trophic level
may result only if a feature benefits the target pest or another herbivore (Gurr et al., 2000) and
not the parasitoid. For example, flowers of coriander enhanced fecundity and longevity of C.
koehleri, the parasitoid of the potato moth, P. operculella. However, the moths were also able
to feed on these flowers, which enhanced the second trophic level to a degree where no
advantage was gained by the addition of "resource subsidies" (Baggen and Gurr, 1998).
Undesirable effects on the second trophic level may be avoided by the use of "selective food
plants" (Baggen et al., 1999; Gurr et al., 2000b).
Buckwheat is being investigated for conservation biological control of Argentine stem weevil
because it increases the fitness of M hyperodae adults (Vattala et al., 2004). The low
consumption of buckwheat pollen by Argentine stem weevils in the present laboratory
experiment (Chapter 3), together with the lower buckwheat pollen availability in the field
compared with that in the laboratory, suggests that buckwheat could be well suited for use in
the Argentine stem weevil- M hyperodae system. However, further laboratory and fieldwork
is needed to investigate the possibility of using buckwheat as a selective plant in conservation
biological control of this pest. Given that consumption of just a few pollen grains (i.e., 1 to 10
grains) can significantly increase the proportion of Argentine stem weevils with mature
gonads, it is essential to be cautious in screening resource subsidies for conservation
biological control in this pest - parasitoid system.
IS THERE ROOM FOR IMPROVEMENT?
Host nutrition is likely to have an effect on the reproductive success of their parasitoids
(Beckage and Riddiford, 1983; Harvey et al., 1995; Jervis et al., 2001). However, based on
the results obtained in the present manipulative experiment (Chapter 3), the diet of the
Argentine stem weevil did not influence the body size or the egg load of M hyperodae, and
several possible explanations for this were discussed in Chapter 3.
If it were possible to increase the egg load of M hyperodae, either in the laboratory or in the
field, could potential fecundity (i.e., egg load) be translated into realised fecundity (i.e.,
78
parasitism rates)? Alternatively, would it be better to try to manipulate other components of
M hyperodae fitness?
It has been speculated that, in its native South America, M hyperodae spends a considerable
amount of time searching for sparsely distributed weevils (McNeill et al., 1996). It may be
expected that M hyperodae became adapted in South America to live just long enough as an
adult to lay all, or nearly all, of its eggs (i.e., so that its realised fecundity is similar to its
potential fecundity). However, under New Zealand conditions, M hyperodae females become
egg limited relatively frequently (Phillips et al., 1998; Phillips, personal communication,
February 2005). This is because M hyperodae is a pro-ovigenic species (Goldson et al., 1995;
Phillips, 1998), so its lifetime complement of eggs is set at emergence from the host. This can
limit parasitoid fecundity if hosts are abundant (Heimpel and Rosenheim, 1997), because the
total set of eggs can be laid before death (Phillips et al., 1998). In fact, approximately 41 % of
the parasitoid's lifespan occurred after cessation of oviposition under laboratory conditions
(Goldson et al., 1995). This (including larviposition) can also occur in herbivorous insects
such as aphids, and there have been a number of ecological reasons proposed for this (Dixon
and Wratten, 1971).
Several researchers have developed models that have considered the importance and risk of
egg limitation in pro-ovigenic parasitoids in the field (Getz and Mills, 1996; Heimpel and
Rosenheim, 1997; Ellers et al., 2000; Ellers and Jervis, 2003; Kean et al., 2003). A consensus
seems to be emerging that parasitoid fitness can be limited by either egg supply or the time
available for locating hosts (Heimpel and Rosenheim, 1997). In fact, models have predicted
that increasing the longevity or fecundity of a parasitoid that becomes egg-limited in the field
would have a little effect on how many hosts are attacked; however, they would have a higher
effect on the parasitoid's searching efficiency (Kean et al., 2003). In addition, parasitoid
searching efficiency has a higher influence on host density than does its fecundity; however,
fecundity may be far more amenable to enhancement compared with searching efficiency
(Kean et al., 2003). In this way, influencing potential fecundity may not be the only way to
enhance the fitness of a parasitoid, but certainly it is on the 'right track'. However, as the
philosopher G.S. Yule said: 'Logic and mathematics are fine as long as they are on the right
track'. So more research is certainly needed on the value of enhancing parasitoid quality in
biological control.
79
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