The role of natural enemies in regulating aphid populations on vegetable
Brassica crops
Liam Harvey (Liam.harvey@warwick.ac.uk) Supervisor: Dr. Dave Chandler
Warwick Crop Centre, University of Warwick, Wellesbourne, Warwick, CV35 9EF
2013 Field Results
Monitoring B.brassicae (1.a) and associated natural enemy populations (1.b. all transplant
dates grouped) on sequentially planted Brussels sprouts (May, June, July & August).
May
• If a forecast model were developed
growers could be alerted of imminent
crashes, allowing them to withhold
insecticide sprays.
4
August
(1.a) B.brassicae populations
6
5
4
3
2
1
0
-1
-2
06-May
20-May
03-Jun
no. of Parasitoid mummies
5
•
•
•
17-Jun
01-Jul
15-Jul
no. of Syrphid larvae
29-Jul
12-Aug
no. of Coccinellidae
26-Aug
09-Sep
23-Sep
no. of Anthocorids
07-Oct
21-Oct
no. of Fungi cadavers
(1.b) Natural enemy community
4
3
2
1
0
-1
06-May
•
• Biological control for
free!
July
7
Mean no. of natural enemies per plant
(ln(+0.5))
Introduction
• Aphid population dynamics are
characterised by a mid-season
population ‘crash’.
• Many factors have been suggested for
the population crash:
• Population processes such as
increased death rate, decreased birth
rate and increased emigration.
• Ecological factors such as increased
natural enemy pressure, plant age &
adverse weather conditions.
• Of the natural enemies, entomopathogenic
fungi have been implicated in the crash but little
is known of their biology.
• Pandora neoaphidis is the most common
entomopathogen causing epizootics in natural
aphid populations.
Benefits to industry
Mean no. of aphids plant (ln(+0.5))
8
June
20-May
03-Jun
17-Jun
01-Jul
15-Jul
29-Jul
12-Aug
26-Aug
09-Sep
23-Sep
07-Oct
21-Oct
Conclusions
Aphid populations on field brassicas increased until mid to late July when they crashed
regardless of plant age.
A fungal epizootic was identified which significantly reduced aphid populations.
Morphological data suggests that Pandora neoaphidis was the cause of the epizootic.
The composition of the guild of natural enemies changed throughout the course of the
field season with parasitoid mummies being the most common.
Aphid instantaneous rate of increase (ri) negatively correlated with numbers of natural
enemies recorded over the course of the field season.
Aphid instantaneous rate of increase:
Mean no. of natural enmies (ln+(0.5))
3.5
3
Nf= final number of aphids
NO= initial number of aphids
ΔT= Time elapsed (days)
2.5
2
1.5
R² = 0.6811
Future work…
1
0.5
0
-0.15
-0.1
-0.05
Coccinellidae
Brevicoryne brassicae infected
with P.neoaphidis.
0
0.05
0.1
0.15
-0.5
• Model the effects of temperature on growth, germination and
virulence
. of various commercially available ‘biopesticides’ and Pandora
neoaphidis.
• Investigate the impact rainfall has on Brevicoryne brassicae populations.
-1
-1.5
Aphid population instantaneous rate of increase (ri)
Figure.2. Aphid instantaneous rate of increase and number of natural
enemies. All transplants grouped for each date. (p = 0.0002)
• Determine the effects of various fungicides on Panodra
neoaphidis.
Warwick Crop Centre
www.warwick.ac.uk/go/wcc