predation of aphids in organic prunes: how many predators...
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Predation of Aphids in Organic Prunes: How Many Predators are Enough?
Nick MillsUniversity of California, Berkeley
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Mealy Plum Aphid, Hyalopterus pruni (MPA)in Organic Prune Production in California
• Reduces tree growth • Produces copious honeydew• May interact with irrigation to increase fruit splitting
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Aphid Life Cycle – Mealy Plum Aphid (MPA)Prune
Prunus domestica
Common ReedPhragmites australis
Holocyclic andheteroecious
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Predation in Organic Prunes
• In organic prunes MPA arepreyed upon by a wide range of aphid predators
• How may predators are needed to achieve adequate control of MPA?
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Predator Assemblage of MPA in California Relative Abundance
Chrysopa nigricornis Harmonia axyridis Hippodamia convergens
Chrysoperla plorabunda Leucopis spp. Aphidoletes aphidimyza
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Understanding Predation: Theoretical Basis
dN/dt = rN - gP
For aphids, population size (N ) results from a balance between reproduction and mortality and can be represented by a simple model of exponential growth (r) and predation (gP ):
The integrated form of this model is:
Xia et al. (2003)Van der Heide et al. (2006)Nt+1 = Ntert + gP/r (1-ert)
So for zero growth (Nt+1 = Nt):gP = rNt
the daily predation capacity (gP) must balance the potentialfor aphid reproduction (rNt)
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How Many Predators are Enough?
This is determined by two factors:
r - the seasonal change in aphid population growth
g - the per capita daily predation capacity
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Estimating r : Seasonal Changes inMPA Population Growth
• Estimated from small individual colonies of MPA enclosed in gauze bags (toexclude predators) over a 1 wk period
• Monitored on new shoots in 3 fully-irrigated orchards for each of 2 yrs
• Estimated as r = (1/t)ln(Nt+1 /Nt)
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Popu
lati
on g
rowt
h ra
ter
(95%
CI)
Julian date
May JunApr Jul Aug
Estimating r : Seasonal Changes inMPA Population Growth
Latham & Mills (2010)
-0.5
-0.3
-0.1
0.1
0.3
70 100 130 160 190 220 250
y = 0.127 - (0.00005*exp[0.0354*x])F2,46 = 22.67, R2 = 0.47, P < 0.001
Mar
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Estimating g: Per Capita Predation Capacity(Mills 2005)
• Laboratory estimation - # prey consumed over 24hin artificial arenas by individual predators
• Direct field observation – point observations from manypredator individuals or continuous monitoring ofsingle individuals for feeding events
• Field cage estimation – difference in prey abundanceafter 24h between cages with and w/out predators
• Field gut detection – quantify immunological or molecularmarkers in the gut content of individual predators
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Estimating g: Per Capita Predation Capacity
Per capita daily predation rate gij was estimated by direct observation from 30 trees in each of 2 orchards in June (peak predator abundance):
gij = (ta * fij)/tcij
ta – active period each day, assumedequal to daylight hours (11-16h)
fij - proportion of ta spent feeding bypredator of stage j and species i
tcij – consumption time for single aphidby stage j of species i, h
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0
20
40
60
80
100Pe
r ca
pita
dai
ly c
onsu
mpt
ion
of a
phid
s (1
6 h)
Estimating g: Per Capita Predation Capacity
Latham & Mills (2010)
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10
100
1000
10000
70 90 110 130 150 170 190 210
Julian day
Max
MPA
per
pre
dato
r fo
rno
pop
ulat
ion
grow
th
H. convergensC. nigricornisH. axyridisLeucopis spp.A. aphidimyza
Apr May Jun JulMar
How Many Predators are Needed for MPA?Max MPA per Predator Larva
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MPA and Predator Densities in PrunesSutter Orchard 1997
Mea
n ap
hids
per
100
cm2
0
200
400
600
800
90 110 130 150 170 190 210 230 250Apr May Jun Jul Aug
Julian date
Mea
n pr
edat
ors
per
100c
m2
0
0.1
0.2
0.3
0.4
0.5
90 110 130 150 170 190 210 230 250
Chrysopid larvaeCoccinellid larvaeHarmonia adultsHippodamia adultsLeucopis larvaeAphidoletes larvae
Apr May Jun Jul Aug
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C. n
igri
corn
isla
rvae
per
100
cm
20.0
0.5
1.0
1.5
2.0
70 100 130 160 190 220 250
NeededObserved
Julian day
Aph
ids
kille
d pe
r 10
0 cm
2
0
20
40
60
80
100
70 100 130 160 190 220 250
RequiredObserved
Apr May Jun Jul AugMarApr May Jun Jul AugMar
How Many C. nigricornis Larvae are Needed?Sutter Orchard 1997
Enhancementonly feasibleearly season
1 larva/100
leaves
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Conclusions
• Natural predator densities don’t preventoutbreaks of MPA in organic prunes
• If feasible, predator enhancementfor MPA must occur early-seasonbefore natural populations are active
• Increase of gP through importation of parasitoid, orreduction of r through deficit irrigation may be morefeasible than increasing the number of predators to asufficient level in organic prunes
• Simple models can help to evaluate the feasibility of predator enhancement
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