Predation of Aphids in Organic Prunes: How Many Predators...
Transcript of Predation of Aphids in Organic Prunes: How Many Predators...
Predation of Aphids in Organic Prunes: How Many Predators are Enough?
Nick MillsUniversity of California, Berkeley
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
Aphid Life Cycle – Mealy Plum Aphid (MPA)Prune
Prunus domestica
Common ReedPhragmites australis
Holocyclic andheteroecious
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?
Predator Assemblage of MPA in California Relative Abundance
Chrysopa nigricornis Harmonia axyridis Hippodamia convergens
Chrysoperla plorabunda Leucopis spp. Aphidoletes aphidimyza
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)
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
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)
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
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
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
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)
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
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
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
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