progress in plum curculio management: a review

Progress in plum curculio management: a review

Charles Vincenta,*, GeÂrald Chouinardb, Stuart. B. Hillc

aHorticultural research and development centre, Agriculture and Agri-food Canada, 430 Gouin BLVD.

Saint-Jean-sur-Richelieu, QC, Canada J3B 3E6bCentre recherche en productions veÂgeÂtales, MinisteÁre de l'Agriculture, des PeÃcheries et de l'Alimentation du QueÂbec,

3300 Sicotte, C.P. 480, Saint-Hyacinthe, QC, Canada J2S 7B8cFaculty of Social Ecology UWS ± Hawkesbury, Richmond NSW 2753, Australia

Abstract

The plum curculio, Conotrachelus nenuphar Herbst (Coleoptera: Curculionidae), is a key pest of apple orchards of North

America. In absence of chemical treatments, up to 85% damage is caused to the fruit at harvest. In spite of its importance,

plum curculio is one of the least known pest of apple orchards. This paper reviews research advances that were made since

1992 to implement sound IPM programmes against this pest. These advances include behavioral studies, trapping methods,

attractants, entomogenous nematodes and treatments of peripheral zones, with special reference to apple orchards. Plum

curculio management achieved through peripheral-zone treatments have been successful in commercial situations. # 1999

Elsevier Science B.V. All rights reserved.

Keywords: Plum curculio; Conotrachelus nenuphar; Curculionidae; Apple orchard; IPM; Peripheral-zone treatments; Behavioral studies;

Monitoring

1. Introduction

Commercial apple orchards typically harbour many

insect species (Croft and Hull, 1983; Vincent and

Bostanian, 1988; Prokopy and Croft, 1995). In Eastern

North America, three species limit the implementation

of effective biological control programs: tarnished

plant bug, (Lygus lineolaris P. de B.: Miridae), plum

curculio (Conotrachelus nenuphar Herbst: Curculio-

nidae) and apple maggot (Rhagoletis pomonella

Walsh: Tephritidae). All three share the following

seven characteristics:

1. they are indigenous to North America;

2. their present distribution is restricted to North

America;

3. their natural enemies provide inefficient controls in

both untreated and treated orchards;

4. they are direct pests;

5. their life cycle is such that adults are the most

convenient target for effective control;

6. they are relatively mobile; and

7. in Quebec orchards, they complete only one gen-

eration per year on apple trees (Vincent and Roy,

1992).

So far, no resistance to synthetic insecticides has been

reported for these pests.

Agriculture, Ecosystems and Environment 73 (1999) 167±175

*Corresponding author. Tel.: +450-346-4494 ext. 202; fax: +450-

346-7740; e-mail: [email protected]

0167-8809/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 7 - 8 8 0 9 ( 9 9 ) 0 0 0 2 5 - 0

As no biocontrol agents have yet been identi®ed that

can effectively control these pests in commercial

orchards (van Driesche et al., 1987), broad-spectrum

insecticides remain as the primary controls. Conse-

quently, the implementation of second-stage inte-

grated pest management (IPM) programs (Prokopy

et al., 1990) is a priority in order to reduce the

dependence of broad-spectrum insecticides. With

the apple maggot, for example, signi®cant reductions

in pesticide use have been achieved by placing sticky-

coated, red spheres on trees along the perimeter of

orchards to intercept immigrating ¯ies (Prokopy and

Croft, 1995).

The ecology and management of the plum curculio

has been reviewed most recently by Racette et al.

(1992). The present paper reviews subsequent

research on plum curculio management, based pri-

marily on the author's work on the univoltine (i.e.,

Northern) strain in southwestern Quebec. Most of the

work has focused on the behaviour and monitoring of

adults in commercial orchards, and on visual and

olfactory cues that would allow the design of reliable

monitoring tools to time more accurately, and there-

fore reduce dependence on, insecticide treatments.

2. Pest status and control of plum curculio

The plum curculio commonly causes severe

damage to fruit in North American apple orchards

(Croft and Hull, 1983). In Quebec, up to 85% of

harvested apples may be damaged by this pest in

unsprayed orchards (Vincent and Bostanian, 1988;

Vincent and Roy, 1992). In commercial orchards,

plum curculio populations return to levels of economic

importance within one to three years after cessation of

pesticide spraying (Glass and Lienk, 1971; Hall, 1974;

Hagley et al., 1977). Plum curculio causes two types of

damage. In spring, females oviposit in young fruit,

marking them with their half-moon shaped scars; and

in summer, adults puncture the fruit with their round

(2±3 mm diameter) feeding scars. The appearance of

plum curculio damage is highly variable and, among

all fruit damage rated by IPM specialists, damage of

plum curculio had the lowest average agreement level,

i.e. 71.8% (Vincent and Hanley, 1997).

Quaintance and Jenne (1912) is a valuable source of

information on the plum curculio's synonymy, distri-

bution, developmental stages, life history, habits, host

plants, natural enemies, and control measures. Other

key papers include Chapman (1938) and Smith and

Flessel (1968). Holloway (1977) provides a list of

papers published between 1911 and 1972, and Le

Blanc (1982) reviewed the literature up to 1982. A

review on the ecology and management of the plum

curculio has been published by Racette et al. (1992).

The present review examines advances on plum cur-

culio management, with special reference to the uni-

voltine, i.e. Northern strain. Since the review of

Racette et al. (1992), most of the work has focused

on the behaviour and monitoring of adults in orchards,

along with research on visual and olfactory cues that

would allow the design of reliable monitoring tools

that would improve the timing of insecticide treat-

ments.

3. Management of the plum curculio

The small number of predator and parasite species

of the plum curculio are unable to provide an effective

alternative to chemical insecticides in commercial

orchards (van Driesche et al., 1987; Racette et al.,

1992). Consequently, insecticide treatments are

usually recommended against the adults at petal fall

and once or twice thereafter at 10±14 day intervals

(Chouinard et al., 1998). No resistance to synthetic

insecticides has yet been recorded for this pest.

4. Behavioural studies

Observations on the behaviour of adults, both in the

®eld and in cages, has led to the design of better IPM

programs for plum curculio. Using Zn65 as a marker, it

has been shown that, in autumn, most adults move

from orchards to surrounding woodlots (La¯eur et al.,

1987). After overwintering, preferably under maple

leaves (La¯eur et al., 1987), the returning plum cur-

culios gradually invade adjacent apple orchards

between pink and petal fall (La¯eur and Hill,

1987), after spending several days on the ground under

the perimeter rows of trees, which most of them then

climb (Racette et al., 1990; Chouinard et al., 1993;

Chouinard et al., 1994). From full bloom to nine days

after fruit set, plum curculio adults were found to be

168 C. Vincent et al. / Agriculture, Ecosystems and Environment 73 (1999) 167±175

active mainly during the night (Racette et al., 1991). In

®eld cages, adults labelled with Zn65 also showed a

similar diel periodicity while foraging on dwarf apple

trees (Chouinard et al., 1992a). Because adults are

most active in the trees during the night, Chouinard et

al. (1992a) have recommended that insecticide treat-

ments are likely to be most effective if applied during

the ®rst part of the night.

There have been several attempts to relate adult

activity to ambient temperature to optimize the timing

of insecticide treatments (reviewed by Racette et al.,

1992). Two approaches have been investigated: the

development of a trap to evaluate adult populations

and relate population levels to risks (Prokopy and

Wright, 1998), and the development of day-degree

models to predict the appearance of damage in the

orchards (Reissig and Nyrop, 1995). So far, both

methods have not been used as the sole method to

manage plum curculio populations.

5. Trapping methods

The best available monitoring method for plum

curculio in commercial orchards still remains careful

examination of thousands of small fruit to detect fresh

egg-laying scars (Hoyt et al., 1983; Le Blanc et al.,

1984). In Quebec, Vincent et al. (1997) successfully

used a threshold of 1% damaged fruit, based on careful

monitoring of fruit three times a week, for managing

localized peripheral treatments following full-block

treatment at petal fall. A lower threshold and daily

monitoring would be required, however, in areas

where the pest pressure is very high. Limb tapping

as a monitoring technique is not popular with growers

because beating sticks damage the trees, and accuracy

varies with cultivar, tree shape, time of the day and

scout experience (Racette et al., 1990).

Several traps designed for monitoring of other

curculionids (Mitchell and Hardee, 1974; Oehlschla-

ger et al., 1993; Tedders and Wood, 1994; Hardee

et al., 1996) have been tested for monitoring plum

curculio (Table 1). The pyramid trap (Tedders and

Wood, 1994), which has the silhouette of a small tree,

is based on the tendency of plum curculio to move in

spring towards tree silhouettes (La¯eur and Hill,

1987).

In Massachusetts, Prokopy and Wright (1997a, b)

found that pyramid traps, without attractants or baits,

were most effective when positioned near tree trunks,

and when temperatures were below 188C. This latter

restriction limits their value in this State as most

damage is done at temperatures above 188C. These

authors suggest that the addition of an attractant to this

trap should be tested as the next step in its develop-

ment.

5.1. Attractants

Early laboratory work with a McIndoo olfactometer

found that adult plum curculios are attracted to sali-

cycl-aldehyde in the early part of the season, and that

gallic acid was the best attractant tested in the late part

of the season (Snapp and Swingle, 1929a). Further

work showed that extracts of peach distillates exerted

different effects at different concentrations (Snapp and

Swingle, 1929b).

Over the past 20 years, attractants have been tested

for several curculionid species (Hardee et al., 1971;

Hedin et al., 1979; Booth et al., 1983). Hardee et al.

(1971) found that plant extracts signi®cantly increased

the attractive power of the boll weevil pheromone; and

Table 1

Traps tested for monitoring C. nenuphar and closely related species

Species Trap type Reference

C. nenuphar plastic funnel under trees Le Blanc et al., 1981

C. nenuphar sticky apple Yonce et al., 1995

C. nenuphar sticky plastic apple Yonce et al., 1995

C. nenuphar sticky band trunk trap Yonce et al., 1995

C. nenuphar pitfall trap under trees Yonce et al., 1995

C. nenuphar elevated funnel under trees Yonce et al., 1995

C. nenuphar funnel within tree canopy Eller and Bartelt, 1996

Curculio caryae pyramidal trap under trees Tedders and Wood, 1994

C. Vincent et al. / Agriculture, Ecosystems and Environment 73 (1999) 167±175 169

Dickens (1989) found that certain green leaf volatiles

(Visser and Ave, 1978) are also attractive to the boll

weevil.

So far attempts to identify potential attractants for

adult plum curculios in stored apples and aluminium

carbonate (Prokopy and Leskey, 1997), fresh apple

juice and synthetic apple blossom fragrance (Le

Blanc, 1982) have been unsuccessful. Leskey et al.

(1996) reported the attractiveness of a chemically

uncharacterized host odour produced by apples at

petal fall, and of water extracts of small apples and

apple twigs. Hexane extracts of various host parts were

only slightly attractive. Butkewich and Prokopy

(1993) found that because odours of host fruit were

signi®cantly less attractive at 4 and 8 cm than at 2 cm

from plum curculios, fruit odor-based traps are unli-

kely to be useful in commercial orchards.

Alm and Hall (1986) have described antennal sen-

sory structures on plum curculio that are similar to

pheromone receptors found on related curculionids.

Eller and Bartelt (1996) isolated and subsequently

synthesized an aggregation pheromone from male

plum curculios: (�)-(1R,2S)-Methyl-2-(1-methyl-

ethenyl) cyclobutaneacetic acid. This pheromone,

which they named grandisoõÈc acid, is attractive to

both sexes. Attempts to use this by placing live adults

in Tedder's pyramid traps were unsuccessful, baited

traps being no more attractive than unbaited traps,

possibly because of repulsive distress signals emitted

by the curculios (Prokopy and Leskey, 1997). How-

ever, lures impregnated with a racemic mixture of

grandisoõÈc acid have been reported by Johnson et al.

(1997) and Coombs et al. (1997) to increase signi®-

cantly the number of plum curculios trapped in Ted-

der's pyramid traps. Chouinard et al. (unpublished

data) demonstrated a two-fold increase in attractive-

ness when the lure was used in conjunction with small

amounts of green leaf volatiles; high amounts showed

a repulsive effect. Butkewich and Prokopy (1997)

found that both visual and olfactory lures are involved.

6. Entomogenous nematodes

Entomogenous nematodes are promising biocontrol

agents, especially for soil inhabiting pests. They can

enter their hosts by any ori®ces (i.e. spiracles, mouth

or anus) or through the cuticle (Tanada and Kaya,

1993). Tedders et al. (1982) tested several species of

nematodes for the control of plum curculio larvae and

concluded that they were ineffective under the con-

ditions tested. In laboratory studies the highest larval

mortality, i.e. 95.1%, was caused at a concentration of

400 Steinernema carpocapsae All nematodes per

larvae (Olthof and Hagley, 1993). There was no sig-

ni®cant increase in larval mortality from 200 to 400

nematodes per larvae. At these concentrations, 73.4%

larval mortality was achieved in natural sods. Nema-

tode treatments would also effect larvae of the apple

saw¯y, Hoplocampa testudinea Klug (Hymenoptera:

Tenthredinidae) (Vincent and BeÂlair, 1992). Nema-

tode treatments applied to the soil would not prevent

damage to apples. However, it could lower popula-

tions of both pests for the subsequent growing season.

To prevent damage to fruit, the effectiveness of

repeated applications of Steinernema carpocapsae

to the foliage or areal parts of apple trees was tested

(Chouinard et al., 1996). Although nematodes

remained infective on foliage from 24 to 48 h after

application, persistence was lower on ¯owers and

twigs (10±70 h). Some ®eld ef®cacy of canopy and

ground applications was detected in two years of ®eld

trials (5±55% plum curculio damage vs. 80±85% in

the control), but a replicated test performed in the third

year showed no reduction of fruit damage under

climatic conditions favourable to plum curculio activ-

ity, even after four weekly applications of S. carpo-

capsae, beginning at petal fall (28% damage vs. 31%

in the control). In caged trees, localized applications of

nematodes at the base of tree trunks signi®cantly

reduced adult populations maintained there (82±

100% mortality vs. 0±18% in the control).

7. Treatments of peripheral zones of appleorchards

The strategy to treat 20-m wide peripheral zones of

apple orchards (when needed) in spring was based on

the ®nding that plum curculio damage is frequently

more abundant at this time in peripheral zones (Le

Blanc et al., 1984), and that during the tight cluster

stage most plum curculio adults move only 1 to 4 m

per day when returning to the orchards from their

overwintering sites in adjacent woodlots (La¯eur and

Hill, 1987). During this 5±20 day re-invasion period in


southern Quebec, petal fall was selected as the most

appropriate time for this peripheral-zone treatment.

Using this approach in a 1.7 ha experimental orch-

ard (with standard-size trees) in Frelighsburg, Quebec,

fruit damage at harvest was reduced from 57% to 2.4%

(Chouinard et al., 1992b), while reducing the amount

of insecticide used by 70%, and the plum curculio

adult population by 83%. These results were consis-

tent over a two-year period in one locality. The

mortality data were based on recaptures of plum

Fig. 1. Field layout of an experiment on peripheral-zone versus full plot treatment (i.e. reference plot) in a commercial apple orchard.


curculio adults, radio-labelled with Zn65, that had

been released in a woodlot adjacent to an orchard

in which the peripheral zones had been treated with

insecticides.

In 1991 and 1992, peripheral-zone spraying was

compared for effectiveness with full-block spraying in

four commercial apple orchards in southern Quebec

(Fig. 1). All plots were subdivided into ®ve zones, i.e.,

peripheral zones (North, South, East and West) and a

central zone. To avoid economic losses, additional

sprays were applied if more than 1% of the fruit

examined in a given zone showed fresh curculio

oviposition scars. This was estimated by recording

oviposition scars on 20 apples, randomly selected,

from one side only of each apple tree in the outermost

row of the peripheral zones; and by examining 20

apples per tree on 25, randomly chosen, early cultivar

trees in the central zones.

Plum curculio damage at harvest varied from 0.0%

to 0.7% and from 0.0% to 0.8% fruit in plots receiving

peripheral sprays (experimental) and full-plot sprays

(reference) respectively; and most damaged apples

(95%) were found in peripheral zones. Total insect

damage on fruit at harvest varied from 1.3% to 3.8% in

experimental plots, and from 0.4% to 5.0% in refer-

ence plots.

8. Effects of peripheral treatments on non-targetarthropods

In 1991, there were no signi®cant differences

between population abundance estimates in peripheral

versus central zones of spotted tentiform leafminer

(Phyllonorycter blancardella Fab.: Gracillariidae),

apple aphids (Aphis pomi (DeGeer) and Aphis citri-

cola (Van der Goot): Aphididae), leafrollers (includ-

ing Choristoneura rosaceana Harris, Archips

argyrospilus Wlk., Argyrotaenia velutinana (Wlk.):

Tortricidae) and phytophagous mites (Tetranychus

urticae Koch: Tetranychidae, and Aculus schlechten-

dali (Nal.): Eriophyidae).

In 1991, in the experimental plots, populations of

woolly apple aphid, two-spotted spider mite and apple

rust mite were signi®cantly higher in peripheral zones

than in the central zones. In 1992, there were no

signi®cant differences between population estimates

of peripheral versus central zones for both experi-

mental and reference plots, for all species examined

and in all orchards. When the results from all localities

and years were pooled, peripheral zones of experi-

mental orchards received 1.22 insecticide treatments,

compared with 1.29 for reference orchards. However,

the central zones of experimental orchards received

0.38 insecticide treatments, versus 1.13 treatments for

reference plots.

The expected savings in pesticides are a function of

orchard size, the larger the area the larger the savings

(Fig. 2). Assuming a 20 m peripheral zone, depending

on plot size, the percentage saving of insecticide could

range from 20% to 85%.

Peripheral-zone treatments for plum curculio man-

agement is compatible with the agronomic complex-

ities of commercial orchards, e.g., multi-cultivar

plantations and odd-shaped orchards. It does require,

however, increased scouting efforts. From 1992 to

1996, implementation of peripheral-zone treatments

for plum curculio proved to be reliable under all

situations encountered by Quebec commercial apple

growers relying on scouting services: if plum curculio

was found in large numbers in all zones, these were all

treated (equivalent to a full-block treatment) (Y.

Morin, personal communication 1997).

Peripheral-zone treatments have also been used

successfully in Ontario against apple maggot, Rhago-

letis pomonella Walsh, and the codling moth, Cydia

pomonellla (L.) (Trimble and Solymar, 1997). As the

life cycle of each of these pests is quite different from

that of plum curculio (Table 2), they require different

Fig. 2. Expected savings of insecticides as a function of orchard

surface and geometry of the plot.


control strategies. The net impact of the plum curculio,

codling moth and apple maggot managed with per-

ipheral-zone treatments used sequentially is yet to be

evaluated on a short (within a season) and long (over

several seasons) term basis. On a long term basis,

concerns pertaining to the long term stability (sensu

Wildbolz, 1988) of partially treated ecosystems are to

be addressed.

Acknowledgements

The authors thank Y. Morin (Agrilus Inc., St-Alex-

andre, Quebec) for his participation in ®eld work.

Jean-Pierre R. Le Blanc, Nova Scotia Agricultural

College, Truro, N.S., N.J. Bostanian and Gaetan Rac-

ette (HRDC-AAAC/Saint-Jean-sur-Richelieu) kindly

commented an early version of the manuscript. C.

Brodeur (MAPAQ, St-Hyacinthe, Qc) assisted in writ-

ing the ®rst draft of the sections on trapping and

attractants.

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Table 2

Differences among the tarnished plant bug, the plum curculio and the apple maggot, three insects that are entomological limits to biological

control programs in apple orchards of Eastern North America (Modified after Vincent and Roy, 1992)

Tarnished plant bug Plum curculio Apple maggot

Order Hemiptera Coleoptera Diptera

Family Miridae Curculionidae Tephritidae

Host plants ca. 100 Rosacae: apple, plum

blueberry, pear

apple, Crataegus spp.

Window of treatment pink petal fall summer

Larvae in fruit no yes yes

Target stage adult or Nymph adult adult

Activity day night and day day

Maximum damage to fruit in

unsprayed orchard*

15±20% 85% 50±60%

Monitoring white sticky trap Tedders traps (pyramidal) red spheres or AM1 traps

Known attractive compound none Grandisoic acid ammonium sulfate and

apple volatiles


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progress in plum curculio management: a review

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