DOI: 10.1111/j.1570-7458.2007.00647.x
Blackwell Publishing Ltd
Predatory response of Xylocoris flavipes to bruchid
pests of stored food legumes
Sharlene E. Sing1* & Richard T. Arbogast2
1
Department of Land Resources and Environmental Sciences, Montana State University, PO Box 173120, Bozeman,
MT 59717-3120, USA, 2US Department of Agriculture–Agricultural Research Service (USDA-ARS), Center for Medical,
Agricultural, and Veterinary Entomology, 1700 S.W. 23rd Drive, Gainesville, FL 32608, USA
Accepted: 25 September 2007
Key words: predator, stored products, biological control, Acanthoscelides obtectus,
Callosobruchus analis, Callosobruchus chinensis, Callosobruchus maculatus, Zabrotes subfasciatus,
Heteroptera, Anthocoridae, Coleoptera, Bruchidae
Abstract
Biological control may provide an affordable and sustainable option for reducing losses to pest
Bruchidae in stored food legumes, a crucial source of human dietary protein. Previous investigations
have focused primarily on the role of parasitism in bruchid biological control, while the potential of
generalist predators has been comparatively unexplored. The true bug Xylocoris flavipes (Reuter)
(Heteroptera: Anthocoridae) exhibited a Type II functional response to the majority of cosmopolitan
bruchid species evaluated when data were fit to Holling’s disc equation. A negative correlation was
detected between mean pest species body weight and rate of predation. The rate of attack on adult
prey was quite low but fairly consistent, with the larger-sized female predators generally more effective.
The eggs and neonate larvae of Acanthoscelides obtectus Say (Coleoptera: Bruchidae) were the only
accessible immature stages among all prey species examined; predation on A. obtectus eggs and larvae
was higher than on any adult bruchids. Mean predator kill of A. obtectus immature stages was 40 first
instars or 10–20 eggs per 24-h interval. Further investigation of the biological control potential of
X. flavipes against pest Bruchidae is merited due to the predator’s ability to kill adult stages of all prey
species evaluated.
Introduction
Food legumes (Fabaceae), also known generically as pulses
or dried beans, are an economically significant commodity
(Cardona, 2004) and an important source of affordable
and accessible dietary protein (Smartt, 1990). Bruchids
(Coleoptera: Bruchidae) are responsible for the greatest
post-harvest loss to stored legumes, directly through
consumption of the resource, and secondarily through the
qualitative deterioration of the commodity or reduced seed
stock viability (Southgate, 1979; Salunkhe et al., 1985).
Callosobruchus maculatus (Fabricius), Callosobruchus
chinensis (L.), Callosobruchus analis (Fabricius), Acanthoscelides obtectus Say, and Zabrotes subfasciatus (Boheman)
are representative species of the three key economically
*Correspondence: Sharlene E. Sing, Department of Land Resources
and Environmental Sciences, 334 Leon Johnson Hall, Montana State
University, PO Box 173120, Bozeman, MT 59717-3120, USA.
E-mail: ssing@montana.edu
significant bruchid genera (Rees, 1996, 2004; Keals et al.,
1998). Callosobruchus maculatus, the most widely distributed
of the Callosobruchus species, is thought to be of African
origin, but has now become established in the Americas,
the West Indian and Pacific Islands, the Mediterranean
region, and Australia (Southgate, 1978; Hill, 2002). Callosobruchus chinensis, native to Asia, is similarly widespread
although it has not yet attained pest status in Central and
South America, Europe, or North Africa (Rees, 2004). The
origins of C. analis are unclear, as specimens reported to be
C. analis have routinely been found to be misidentified
C. maculatus (Southgate et al., 1957). Although Rees (2004)
reports its distribution as limited to South and South-East
Asia, Southgate (1978) suggests that C. analis probably arose
in South-East Asia and India and later became established
in Africa.
Acanthoscelides obtectus and Z. subfasciatus are New World
species thought to have originated in South and Central
America (Hill, 2002). The geographical distribution of
both species is now considered cosmopolitan (Southgate,
© 2008 The Authors Entomologia Experimentalis et Applicata 126: 107–114, 2008
Journal compilation © 2008 The Netherlands Entomological Society
107
108
Sing & Arbogast
1978; Hill, 2002). Zabrotes subfasciatus has not been reported
from Asia or the Australasian region, and European records
are limited to Italy and Portugal (Southgate, 1978; Rees, 2004).
With the exception of A. obtectus, eggs of the species
listed above are adhered to the outer testa of the host seed
during the oviposition process (Hill, 2002; Rees, 2004).
This ovipositional exudate additionally functions as a
fairly impenetrable protective coating once it becomes
hardened (Southgate, 1979). Hatching larvae do not wander
but bore directly into the host seed where all development
is completed. In contrast, A. obtectus eggs are deposited
loosely throughout the bulk of stored host seeds leaving
eggs and first instars susceptible to mortality by predation
and desiccation until the larva enters a host seed (Howe &
Currie, 1964; Hill, 2002).
Surface and fumigant chemical applications are thought
to be the most effective methods for managing bruchid
infestations, especially for large scale or extended storage
(Rahman, 1990; Visarathanoth et al., 1990; Cardona, 2004;
Mbata, 2004). Prohibitive costs and the risk of adverse
secondary effects from such treatments have necessitated
the exploration of alternative control measures, such as
biological control, and the adoption of a more integrated
management approach (Brower et al., 1996; Abate et al.,
2000; Adebowale & Adedire, 2006).
Xylocoris flavipes (Reuter) (Heteroptera: Anthocoridae)
is a cosmopolitan predator of multiple species and stages of
stored product pests (Arbogast, 1979), although it is most
successful against small-sized, externally developing prey,
particularly accessible eggs and early larval stages that are
neither heavily sclerotized nor overly hirsute (LeCato &
Davis, 1973). Researchers have determined that X. flavipes
is largely ineffective in predating upon the larval stages of
stored product pests, such as bruchids, that typically
develop within seeds (Arbogast, 1979, but see Donnelly &
Phillips, 2001). Records of specific associations between
X. flavipes and bruchid prey are limited. Xylocoris flavipes
has reportedly been collected from legumes infested by
Bruchidius incarnatus Boh. and Bruchus rufimanus Boh.
in Egyptian storage facilities, although both species are
regarded primarily as field crop rather than storage pests
(El-Nahal et al., 1985). Xylocoris flavipes has also been
recovered from imported cowpea, Vigna unguiculata (L.)
Walp., by the US Department of Agriculture Insect
Identification and Parasite Introduction Research Branch,
Beltsville, MD, although no record of the specific prey is
given (Jay et al., 1968).
Functional response characterizes the relationship
between the number of prey consumed by individual
predators and the density of available prey (Solomon,
1949; Holling, 1959a,b). The potential biocontrol efficacy
of candidate agents can be extrapolated by quantifying
functional response, which serves as a predictor of attainable
top-down, density-dependent regulation of a given pest
species (Murdoch & Oaten, 1975). Published accounts of
anthocorid functional response to stored product insect
pests have thus far excluded evaluations of the predator’s
potential to control bruchid prey (LeCato & Arbogast,
1979; Parajulee et al., 1994; Donnelly & Phillips, 2001).
In LeCato & Arbogast (1979), the functional response of
X. flavipes to the eggs and early instars of the Angoumois
grain moth, Sitotroga cerealella (Olivier), indicated a
preference for larval prey; the authors suggested that prey
stage preference was linked to the predator’s stimulation by
the movement and the small size of the larval prey. Adult
bruchids capable of actively avoiding or defending against
attacks might be considered ‘difficult prey’ for no other
reason than their considerable body size advantage.
However, X. flavipes is known to attempt and persist in
attacking non-preferred prey species/stages when faced
with starvation if preferred prey was unavailable (LeCato,
1976). Larval A. obtectus may similarly avoid or defend
against X. flavipes attack, but lacking the armature or
mass of adult bruchids might in fact attract predators as
‘stimulating prey’ through their defensive behavior. Egg
prey provides neither resistance nor stimulation to predators;
although this stage might be categorized as ‘easy prey’, it
does require predator initiative to be recognized as such.
Functional response is an appropriate way to characterize
the interaction of X. flavipes with a number of different
bruchid prey species and stages in a highly simplified
environment. The objectives of this study were first to
substantiate observational reports that this predator could
kill adult bruchids, and then to determine in the context of
functional response, if male and female X. flavipes predated
at different rates on the adults of the various bruchid
species listed above.
Materials and methods
Bruchid prey
Bruchids used in this study were obtained from continuous
cultures maintained at the Stored-Product Insects Research
and Development Laboratory (SPIRDL), Savannah, GA,
USA. The A. obtectus, Z. subfasciatus, C. analis, and
C. chinensis cultures were started in 1981 from specimens
provided by the Pest Infestation Control Laboratory,
Slough, Bucks, UK, while the C. maculatus culture originated
from US Department of Agriculture–Agricultural Research
Service (USDA-ARS) research laboratories in Fresno, CA,
USA. Food legumes used in all bruchid cultures were
purchased locally and certified as organically produced.
Bulk 11.4 kg bags of black-eyed pea/cowpea (V. unguiculata),
white navy bean (Phaseolus vulgaris L.), and garbanzo
Predatory response of Xylocoris flavipes 109
bean/chickpea (Cicer arietinum L.) were disinfested in cold
storage at –17.8 °C for at least 2 weeks (Johnson &
Valero, 2003), then acclimated before use in a walk-in
environmental growth chamber at 29 ± 2 °C, 65 ± 5% r.h.,
the controlled conditions used for both culture maintenance
and all experiments. Environmental equilibrium was verified
by repeated dry weight determination of grain legume
moisture content.
Adult bruchids of a known age (0–24 h) were obtained
by sifting the contents of culture jars in a US Standard no.
6 sieve 24 h after an initial sifting that had removed all
previously emerged adults. Individual adult bruchids were
sexed according to authoritative keys (Southgate et al.,
1957; Southgate, 1958; Halstead, 1963), then collected and
retained in male:female (1:1) pairs in 18.3-ml plastic shell
vials (7 × 2.5 cm in diameter).
Acanthoscelides obtectus eggs used in the present study
were 0–24 h old, gathered from oviposition jars fitted with
screened platforms that allowed eggs to drop through to
Petri dishes positioned below. Acanthoscelides obtectus
larvae were collected from Petri dishes where newly
collected eggs were retained until hatching occurred,
generally 96–120 h after oviposition.
Predators
Predators used in this study were obtained from a continuous
culture of X. flavipes originating from specimens collected
in 1977 from a population that had self-infested an
experimental warehouse facility at SPIRDL. Predators were
reared under the environmental conditions stated above in
3.78-l glass jars outfitted with Hexcel® (Dublin, CA, USA)
paperboard harborage and provisioned with previously
frozen then thawed Plodia interpunctella (Hübner) eggs.
Culture jars were cleared of all adult predators and
experimental subjects were then collected from the pool
of newly emerged adults 0–6 days after the initial sorting.
Subjects were sexed according to Arbogast et al. (1971)
and retained individually in gelatin capsules.
Predatory response of Xylocoris flavipes to adult bruchid prey
Experimental arenas consisted of glass Petri dishes, 9.0 cm
in diameter. Liquid Teflon® (Wilmington, DE, USA) was
applied to the interior walls of each arena and allowed to
cure for a minimum of 24 h in a fume hood before use; this
was a prophylactic treatment to prevent the potential escape
of experimental subjects. A fine mesh nylon fabric circle
was placed on the bottom of each arena to ensure that the
prey remained accessible to X. flavipes, which experienced
difficulty walking on the smooth glass surfaces of the
arenas. Five replicates of 1, 2, 3, or 4 male:female (1:1)
adult prey pairs were added to each arena. Sixty arenas in
total per bruchid species were exposed to one of three
treatments: control (no predators – to determine naturally
occurring mortality during the experiment), one male
predator, or one female predator. Arenas were inspected at
24-h interval over the 72 h duration of the experiment.
During each inspection, the number of dead prey was
recorded then replaced with 0–24-h-old live prey to maintain
the level of available live prey per observation interval.
Predatory response of Xylocoris flavipes to immature stages of
Acanthoscelides obtectus
Experiments assessing the functional response of X. flavipes
to the egg and larval stages of A. obtectus egg and larval
prey followed the protocol described above, except that
experiments were terminated after 24 h. Prey densities used
were 5, 10, 15, 20, or 50 individual eggs or larvae, subjected
to the same three predator treatments listed above. The
experimental assessment of the functional response of
X. flavipes to all prey species/stages was performed twice in
entirety.
Statistical analyses
Data were fitted to models representing a characteristic
Type I (Na = aTN), II [Na = aTN/1 + (aThN)], or III {Na =
N[1 – e–a(T – NaTh)]} functional response (Holling, 1959a,b)
using PROC NLIN (SAS Institute, 2004).
Models were then parameterized, with Na as the number
of prey killed, T as the duration of prey exposure to the
predator(s), N as the initial density of prey, a as the
instantaneous rate of discovery (‘attack rate’), and Th
indicating the length of time prey were handled by the
predator (handling time). PROC NLIN (SAS Institute,
2004) was used to discriminate contributions of predator
sex and prey density to total variation observed in the
number of bruchids killed.
Results
Preliminary analyses indicated that the coefficients of
determination (r2 values) for X. flavipes’ Type I, II, and III
functional responses to bruchid prey were similar (Table 1),
a trend also reported in Donnelly & Phillips (2001). However,
although similar, the r2 value for Type I functional
response was consistently lower than that reported for
Type II within the same predator sex/bruchid species prey
stage combination. Furthermore, we found that fitting the
data to a Type III functional response equation resulted in
a negative, essentially biologically untenable parameter
value for Th, the length of time prey was handled by the
predator. We therefore concluded that the functional
response of both sexes of X. flavipes to most bruchid prey
species and stages evaluated was generally best described
by the Type II Holling’s disc equation.
110
Sing & Arbogast
Table 1 Coefficients of determination (r2) for fit of Type I, II, and
III functional response curves to male or female Xylocoris flavipes
predation on bruchid prey
Bruchid prey
species
Prey
stage
Predator
sex
A. obtectus
A. obtectus
A. obtectus
A. obtectus
A. obtectus
A. obtectus
C. analis
C. analis
C. chinensis
C. chinensis
C. maculatus
C. maculatus
Z. subfasciatus
Z. subfasciatus
Adult
Adult
Egg
Egg
Larva
Larva
Adult
Adult
Adult
Adult
Adult
Adult
Adult
Adult
Female
Male
Female
Male
Female
Male
Female
Male
Female
Male
Female
Male
Female
Male
Functional
response equation
Type I
Type II
Type III
0.5663
0.5253
0.8973
0.6711
0.9516
0.9716
0.8321
0.6995
0.8244
0.4950
0.7559
0.5772
0.8192
0.5667
0.6376
0.6073
0.9446
0.8524
0.9612
0.9824
0.8730
0.7195
0.8544
0.5426
0.7575
0.5869
0.8433
0.5764
0.8266
0.7708
0.9016
0.7330
0.9531
0.9720
0.9213
0.8807
0.9114
0.8957
0.9430
0.8642
0.9160
0.8771
The attack rates for both predator sexes against bruchid
prey were comparable within prey species/stage groupings
(Table 2). However, the reported parameter estimates
indicate that prey handling time for the larger-sized female
predators was consistently lower than for the smaller male
X. flavipes (Table 2). Adult X. flavipes are sexually dimorphic
in body size, with gravid females averaging 2.55 mm in
length compared to 1.93 mm for males (Arbogast et al.,
1971); previous studies have detected a difference in the
relative contribution of the sexes to prey suppression
(Donnelly & Phillips, 2001). An evaluation of the contribution of prey density, predator sex, and their interaction to
variation observed in the functional response of X. flavipes
to bruchid prey confirmed that the difference was significant
in the number of prey attacked due to predator sex, for all
prey species/stages evaluated, other than for A. obtectus
larval prey (Table 3).
The immature stages of A. obtectus were readily attacked
by both sexes of X. flavipes. Handling times of egg prey
were considerably different for the two predator sexes, but
not for larval prey (Table 2). Acanthoscelides obtectus eggs
were more heavily predated upon by female X. flavipes at
the two highest densities evaluated (Figure 1, Table 4). The
active neonate larvae were attacked by both predator sexes
at a more equivalent (Table 3) and higher rate (Table 4)
than A. obtectus eggs. When the difference in predation by
the two sexes was evaluated at each adult prey density level,
variation in the mean number of kills was found to be
statistically and consistently significant only for Z. subfasciatus
and C. chinensis, the two smallest bruchid species (Table 5).
Discussion
Although most predators attack the largest available
individuals of their prey species, those species are
generally smaller in body size than the predator. Predatory
arthropods are known to be an exception to these limiting
predator:prey relative body size ratios, because maximum
prey size can be increased through the use of venoms, traps,
or group hunting (Sabelis, 1992). The results of the present
Table 2 Parameter estimates for the functional response of Xylocoris flavipes to bruchid prey according to predator sex and prey
species/stage
Bruchid prey species/stage
Predator sex
Attack rate (a)
Handling time (Th)
r2
A. obtectus eggs
A. obtectus eggs
A. obtectus larvae
A. obtectus larvae
A. obtectus adults
A. obtectus adults
C. analis adults
C. analis adults
C. chinensis adults
C. chinensis adults
C. maculatus adults
C. maculatus adults
Z. subfasciatus adults
Z. subfasciatus adults
Female
Male
Female
Male
Female
Male
Female
Male
Female
Male
Female
Male
Female
Male
0.0457 ± 0.0046
0.0597 ± 0.0147
0.0464 ± 0.0037
0.0467 ± 0.0025
0.0457 ± 0.0709
0.0219 ± 0.0318
0.0326 ± 0.0117
0.0214 ± 0.0130
0.0270 ± 0.0104
0.0244 ± 0.0330
0.0080 ± 0.0007
0.0157 ± 0.0143
0.0224 ± 0.0084
0.0077 ± 0.0055
0.5318 ± 0.0696
1.8877 ± 0.2332
0.1775 ± 0.0434
0.1885 ± 0.0290
27.3858 ± 8.1259
48.7082 ± 15.9215
7.4750 ± 2.0196
13.0739 ± 5.3581
8.8910 ± 2.6229
31.2443 ± 12.5168
n/a1
24.4738 ± 11.5153
7.8998 ± 2.9303
19.8166 ± 16.1709
0.9446
0.8524
0.9612
0.9824
0.6376
0.6073
0.8730
0.7195
0.8544
0.5462
0.7559
0.5869
0.8433
0.5764
1
Data for female predator’s functional response to adult Callosobruchus maculatus fitted to Type II model yielded a negative handling time;
data were therefore fit to Type I model.
Predatory response of Xylocoris flavipes 111
Table 3 Contribution of prey density, predator sex, and the
interaction of prey density and predator sex to total variation
observed in functional response of Xylocoris flavipes to
bruchid prey
Source
A. obtectus – egg stage
Prey density
Predator sex
Prey density*predator sex
A. obtectus – larval stage
Prey density
Predator sex
Prey density*predator sex
A. obtectus – adults
Prey density
Predator sex
Prey density*predator sex
C. analis – adults
Prey density
Predator sex
Prey density*predator sex
C. chinensis – adults
Prey density
Predator sex
Prey density*predator sex
C. maculatus – adults
Prey density
Predator sex
Prey density*predator sex
Z. subfasciatus – adults
Prey density
Predator sex
Prey density*predator sex
d.f.
F
P>F
5,108
1,108
5,108
61.62
52.09
13.96
<0.0001
<0.0001
<0.0001
5,108
1,108
5,108
304.71
0.00
0.04
<0.0001
0.9580
0.9990
3,72
1,72
3,72
0.71
9.88
0.12
0.5505
0.0024
0.9509
3,72
1,72
3,72
6.76
20.17
4.34
0.0004
<0.0001
0.0072
3,72
1,72
3,72
3.21
37.22
1.41
0.0280
<0.0001
0.2477
3,72
1,72
3,72
9.84
7.35
2.42
<0.0001
0.0084
0.0726
3,72
1,72
3,72
6.67
48.81
2.89
0.0005
<0.0001
0.0413
study indicate that X. flavipes is capable of low level but fairly
consistent success in killing its larger adult bruchid prey.
Stimulated X. flavipes direct a scent-gland exudate
thought to be defensive in nature over a wide area (Remold,
1963). Phillips et al. (1995) determined that the terpene
constituents of this secretion, particularly α-terpineol
and linalool, have significant toxic activity against adult
Tribolium castaneum (Herbst) and Oryzaephilus surinamensis
(L.), with the potential mode of toxicity being competitive
inhibition of acetylcholinesterase. Furthermore, the
consistent rapid liquefaction of large adult bruchid prey
after attack by X. flavipes (SE Sing, unpubl.) suggests that
the predator utilizes an enzymatic salivary venom for
extraoral digestion, a common strategy of predaceous
arthropods preying on large prey with intractable cuticles
(Cohen, 1995). Finally, the low level of X. flavipes predation
on adult bruchids can be explained not only by the greater
challenge to subdue large prey, but is also correlated
with daily ingestion rate and gut capacity; the nutritional
resources of large prey generally exceed the daily food
requirements of small predators (Peters, 1983). Predation
of additional adult bruchids in experimental arenas may
also have been reduced due to return feeding on the readily
apparent previous kills.
Xylocoris flavipes killed significantly more ‘stimulating’
larval prey than ‘easy’ egg prey, reiterating earlier results
(LeCato & Arbogast, 1979; Russo et al., 2004). The functional
response of the predator to both the egg and larval stages
of A. obtectus indicates that X. flavipes has potential as a
biological control agent of this particular bruchid species.
Predation on adult A. obtectus may have been confounded
by the presence of eggs freshly oviposited by the experimental
subjects.
The potential of a ‘knock down’ or disorientation effect
from the scent-gland secretion (Phillips et al., 1995) combined with the catastrophic disruption of neuromuscular
function (Blum, 1981) caused by the injection of salivary
venom could account for the predator’s ability to kill the
comparatively more ‘difficult’ adult bruchid prey. Howe &
Currie (1964) list the mean body weights of all bruchid
species discussed here; results of the current study indicate
that highest rate of predation occurred with the lightest
weight species, Z. subfasciatus and C. chinensis, and decreased
with increasing mean body weight of the heaviest prey
species, A. obtectus, C. analis, and C. maculatus. Additionally,
observation of predator interaction with mated pairs of
bruchid prey indicated a high level of mating and oviposition disruption, and opportunistic predation on bruchids
engaged in copulation (SE Sing, pers. obs.) indicates that
presence of X. flavipes in the bruchid–grain legume
complex may significantly impact prey populations.
Figure 1 Predation on Acanthoscelides obtectus eggs within
a 24-h period by Xylocoris flavipes adults as a function of prey
density, -, female predator, -, male predator. Lines plotted
are Holling’s disc equation fitted to the data.
112
Sing & Arbogast
Table 4 Female or male Xylocoris flavipes mean kill by density of Acanthoscelides obtectus eggs and larvae per 24-h exposure period
Prey density
A. obtectus – eggs
2
5
10
15
20
50
A. obtectus – larvae
2
5
10
15
20
50
Kills by female X. flavipes
Kills by male X. flavipes
Prob>|T|
1.6500 ± 0.2115
4.4800 ± 0.3384
8.7800 ± 0.5546
10.8800 ± 1.7308
16.2900 ± 1.4534
24.5000 ± 1.0462
1.7500 ± 0.1951
4.4800 ± 0.2286
6.8800 ± 0.0171
7.3800 ± 0.8475
10.1900 ± 1.2543
10.2000 ± 1.6111
0.7322
1.0000
0.1183
0.0860
0.0052
<0.0001
1.6000 ± 0.2000
4.5000 ± 0.4000
9.9200 ± 0.0067
14.8500 ± 0.1012
19.9600 ± 0.0000
39.3000 ± 3.1342
1.8000 ± 0.0000
4.9000 ± 0.0000
9.8200 ± 0.1024
14.8500 ± 0.0989
19.7600 ± 0.2000
38.8000 ± 2.0645
0.3306
0.3306
0.3306
1.0000
0.3306
0.8955
Prob>|T|, probability of a greater absolute value of t when the null hypothesis that the means of the two groups are equal.
The results of our analyses indicate that the percentage
of adult bruchid prey attacked by X. flavipes decreased as
prey availability increased, typifying a Type II densityindependent functional response. We would therefore
expect that an equilibrium in predator:prey population
dynamics, the theoretical hallmark of pest population
regulation through biological control, would not be attained
following the release of X. flavipes in food legume storage
facilities pest. However, Schenk & Bacher (2002) state that
evaluations performed under restrictive conditions (cages;
Table 5 Female or male Xylocoris flavipes mean kill by density of adult bruchids per 24-h exposure period
Prey density
A. obtectus – adults
2
4
6
8
C. analis – adults
2
4
6
8
C. chinensis – adults
2
4
6
8
C. maculatus – adults
2
4
6
8
Z. subfasciatus – adults
2
4
6
8
Kills by female X. flavipes
Kills by male X. flavipes
Prob>|T|
0.6333 ± 0.1356
0.7000 ± 0.2015
0.8333 ± 0.2003
0.7667 ± 0.1928
0.3000 ± 0.1048
0.4333 ± 0.1000
0.5333 ± 0.1133
0.3333 ± 0.0994
0.0676
0.2513
0.2088
0.0611
0.9333 ± 0.1556
1.5333 ± 0.1587
2.3333 ± 0.1405
1.8333 ± 0.2992
0.7667 ± 0.1795
0.9667 ± 0.2191
0.8333 ± 0.1667
1.5000 ± 0.2447
0.4919
0.0507
<0.0001
0.3998
0.9000 ± 0.1117
1.3000 ± 0.1606
1.5667 ± 0.2724
1.8000 ± 0.2061
0.4667 ± 0.1237
0.5333 ± 0.1423
0.7333 ± 0.2096
0.6000 ± 0.2155
0.0181
0.0022
0.0261
0.0008
0.4000 ± 0.1089
0.7667 ± 0.1795
0.9667 ± 0.1160
1.6667 ± 0.2981
0.5667 ± 0.1495
0.5333 ± 0.1133
0.3333 ± 0.1721
1.0333 ± 0.1753
0.3794
0.2862
0.0069
0.0837
0.6333 ± 0.1444
1.5333 ± 0.1507
1.4000 ± 0.1778
1.8000 ± 0.2685
0.3667 ± 0.1356
0.3667 ± 0.1356
0.5667 ± 0.1575
0.7000 ± 0.1528
0.1951
<0.0001
0.0025
0.0022
Predatory response of Xylocoris flavipes 113
single prey species) routinely indicate a Type II functional
response in generalist insect predators. Van Lenteren &
Bakker (1976) and van Alebeek et al. (1996) suggest that
the constraints of experimental design might actually
obfuscate the true nature of the functional response curve
in the context of invertebrate predators, specifically citing
how in a confined arena the increased chance of prey
discovery might exaggerate the steepness of the response
curve at the lowest prey densities. Finally, significant
discrepancies in the outcome of laboratory vs. field
evaluations of functional response have been reported
(O’Neil, 1997; Schenk & Bacher, 2002). Although the
predator’s response to all stages of A. obtectus is particularly
encouraging, the results presented here suggest that
further evaluations of the predatory response of X. flavipes
to pest bruchids under more complex experimental
conditions are merited.
Acknowledgements
The authors gratefully acknowledge Dr. Robin K. Stewart
for facilitating the completion of this MSc research project
through the Department of Natural Resource Sciences,
Macdonald College of McGill University; Dr. Kevin M.
O’Neill, Montana State University, for his role in the
conceptualization of this project; and the employees of
the USDA-ARS Stored-Product Insects Research and
Development Laboratory, Savannah, GA, for their practical
contributions to its realization. We also thank all reviewers
of this manuscript, especially Dr. Peter A. Edde, for
comments and suggestions that significantly improved the
final version.
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