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