How to Get Rid of Apple Maggots, Planet Natural

Apple Maggot

This small insect can create big problems in apples and other fruits across the United States. Learn safe, natural and organic solutions to get rid of apple maggots here.

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A destructive pest of commercial and backyard orchards across North America, apple maggots (Rhagoletis pomonella) ​​will also attack plum, apricot, pear, cherry and hawthorn.​ ​Contaminated fruits often ​show​ small pinpricks or pitted areas ​on the apple surface ​​with brown or rotten trails running throughout the flesh. If trees are neglected, 100% of the crop can be wormy rendering the fruit unfit to eat and suitable only for livestock feed.

​Identification​

Slightly smaller than a housefly, adult apple maggots​ ​are 1/5 inch long and have conspicuous black bands — resembling a W — running across their transparent wings. The larvae (1/4 inch long) are white, tapered maggots that tunnel throughout the flesh of fruit.​ ​Sometimes called railroad worm​s​, they ​may be​ found in large numbers and ​will​ quickly reduce a beautiful apple to a brown, pulpy mess.

Life Cycle

Apple maggots overwinter as pupae in the soil. Adult flies emerge in late spring and begin to lay eggs just under the apple skin. The eggs hatch, and the larvae begin to tunnel through the fruit. When mature, the maggot leaves through a small opening made in the side of the fruit and enters the soil. One or two generations per year.

How to Control

  1. Most maggots leave the fruit several days after it has fallen from the tree. As a result, a certain level of control can be achieved by picking up and discarding the dropped apples.
  2. Red Sphere Traps will greatly reduce damage and work well to capture and reduce the number of egg laying adults. Traps should be placed within the canopy just as trees are finished blooming. Hang spheres high in the brightest areas of the tree, 6-7 feet from the ground. Set out one trap for every 150 apples (2 traps per dwarf tree).
  3. Beneficial nematodes are microscopic, worm-like parasites that actively hunt, penetrate and destroy the pupal stage of this pest. For best results, apply in the early spring or fall around the base of trees, out to the drip line. One application will continue working for 18 months.
  4. Surround WP — made from kaolin clay — will suppress a broad range of insects and has shown over 90% control of apple pests. It also has a positive effect on fungal diseases like fire blight, sooty blotch and flyspeck.
  5. Fast-acting botanical insecticides should be used as a last resort. Derived from plants which have insecticidal properties, these natural pesticides have fewer harmful side effects than synthetic chemicals and break down more quickly in the environment.

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Beneficial Nematodes

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Apple Maggot (Reusable)

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Surround WP

Applied as a liquid, Surround WP — kaolin clay — evaporates leaving a protective film.

Orchard Spray

This one-hit product protects against common insect attacks and fungal problems.

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Biology of Centistes delusorius, a parasitoid of adult apple blossom weevil

*Laboratory of Entomology, Agricultural University, Wageningen, The Netherlands and †Plant Research International, P O Box 16, 6700 AA Wageningen, The Netherlands

*Laboratory of Entomology, Agricultural University, Wageningen, The Netherlands and †Plant Research International, P O Box 16, 6700 AA Wageningen, The Netherlands

Dr L. Blommers. Tel.: + 31 317 475 762; e‐mail:

*Laboratory of Entomology, Agricultural University, Wageningen, The Netherlands and †Plant Research International, P O Box 16, 6700 AA Wageningen, The Netherlands

*Laboratory of Entomology, Agricultural University, Wageningen, The Netherlands and †Plant Research International, P O Box 16, 6700 AA Wageningen, The Netherlands

Dr L. Blommers. Tel.: + 31 317 475 762; e‐mail:

Abstract

Natural control of apple blossom weevil, Anthonomus pomorum (L.), deserves attention, as the pest is regaining importance with the declining use of non‐selective pesticides in apple and pear orchards. In this study the biology of Centistes delusorius (Förster), a specific parasitoid of adult apple blossom weevil, is investigated.

The parasitoid hibernates as young larva in an adult weevil, and juvenile development is resumed in early spring. The fully grown parasitoid larvae leave their hosts during full bloom at the end of April and early May, to pupate. The adults emerging in May oviposit into the newly emerged weevils, which initially feed on apple leaves.

Centistes delusorius was detected in six out of 15 host‐weevil infested orchards, but was only common in two with larger apple trees standing in grass. There, parasitism levels of around 30% were usual in hosts taken from treebands in winter.

The delicate larva is vulnerable, and the thin cocoon provides little protection against either desiccation or drowning on a weedless orchard floor. Observations indicate that successful pupation of C. delusorius demands stable humid conditions and some shelter, such as that found in grass or woodland soils.

Parasitoid females, provided with honey, lived for a mean of 6.3 ± 2.1 days under outdoor conditions in June. Their life span was similar whether they had access to and oviposited in hosts, or not. The species is pro‐ovigenic, and potential fecundity is about 40 eggs. Oviposition usually takes a few seconds. Parasitized female hosts do not reproduce.

Up to 95% of the parasitoid eggs laid in May develop into a second generation, the adults of which appear in July, when the host has entered aestivation. Older (British) records of C. delusorius outside orchards suggest that some parasitized hosts, like the healthy ones, leave the orchard prior to aestivo‐hibernation, so that the latter do not escape parasitoid attack in July.

A trapping sample in late June, when most non‐parasitized weevils have gone into aestivo‐hibernation, is probably the most efficient method to detect parasitized weevils.

The (near‐)absence of C. delusorius in many orchards is probably due not only to pesticide side‐effects, or scarcity of its host, but also to the absence of suitable pupation sites for the wasp.

Introduction

The apple blossom weevil, Anthonomus pomorum (L.) (Coleoptera: Curculionidae) occurs on apple and pear in the western Palaearctic region ( Miles, 1923 ; Speyer, 1939 ; Zerova et al., 1991). This univoltine species overwinters as adults under the bark of fruit trees, in fallen leaves on the ground, or in other shelters in the direct vicinity of orchards ( Troitzky, 1928 ; Toepfer et al., 2000 ). In early spring, the weevils leave their wintering sites, feed on developing buds and mate, before reproductive maturation is complete ( Čtvtečka & Žd’árek, 1992 ; Duan et al., 1996 ; Toepfer et al., 1999 ). Eggs are laid in flower buds during the period of bud burst, and larvae develop in the characteristic unopened flower, known as the ‘capped blossom’ ( Miles, 1923 ). After pupation, the adult weevils emerge towards the end of May or the beginning of June and feed on leaves and flowers ( Huysmans, 1944 ). Their activity decreases after a few weeks and the weevils move to sheltered places to aestivate, during which they can be active at high temperatures. Winter diapause follows gradually after aestivation ( Koštál & Šimek, 1996 ).

The apple blossom weevil has been a minor pest for many years due to the post‐flowering use of broad‐spectrum (mainly organophosphorous) compounds against other pests, and carbaryl for fruit thinning. Employment of more selective control methods in integrated pest management (IPM) programmes allows it to reach pest status more often ( Gruys, 1982 ; Blommers, 1994 ). Thus, natural control of the species deserves more attention.

Major natural enemies of apple blossom weevil include two parasitic wasps Scambus pomorum (Ratzeburg) (Ichneumonidae, Pimplinae), which attacks the weevil larva, and Centistes (formerly Syrrhizus) delusorius (Förster) (Braconidae, Euphorinae) ( Zijp & Blommers, 1992 ; Cross et al., 1999 ), the lesser known of the two species.

Centistes delusorius has been found in several European countries where apple blossom weevil is common: Great Britain ( Graham, 1955 ; Stelfox, 1955 ), Germany ( Speyer, 1939 ), Holland ( Zijp & Blommers, 1992 ), Trentino, Italy (L. Blommers, unpublished data) and the Ukraine ( Zerova et al., 1992 ). Speyer (1939 ) first recorded C. delusorius as an endoparasitoid of adult A. pomorum. He found adults parasitizing young weevils in June, and fully grown larvae leaving the host in the following spring, during the oviposition period of apple blossom weevil. Pupation occurs outside the host weevil in a thinly woven, white cocoon, but fails if the emerging larva does not find suitable conditions, in particular elevated humidity and shelter, within 24 h (L. Blommers, unpublished data). The adult wasp emerges after about 3 weeks. In Holland, unpublished research in 1966–1967 showed the occurrence of a partial second generation with adult flight in early August ( Zijp & Blommers, 1992 ). Centistes delusorius is not known to attack species other than A. pomorum, but a few similar larvae were found in field‐collected pear bud weevil Anthonomus piri Kollar (J. P. Zijp, unpublished data).

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This paper reports results from a study of natural control of apple blossom weevil, recording observations on C. delusorius, its occurrence in Dutch apple orchards, and its phenology and reproductive capacity, and discusses the species’ potential contribution as natural enemy.

Materials and methods

Sampling and rearing

Beating samples and treebands were used to collect adult weevils, to determine the presence of C. delusorius, and to provide a source of the parasitoid. One hundred branches were beaten above a 0.5 m 2 rectangular net to collect weevils during their active period prior to and during oviposition in early spring, and for a few weeks after adult emergence. Treebands consisting of corrugated cardboard, protected by layers of jute and greaseproof paper, were fixed to the trunks of trees in various plantings in early summer. They were collected and checked for blossom weevils, either in late summer or, usually, in winter. Aestivating or hibernating weevils were also collected from behind tree bark. Samples in private orchards were preferably taken from apple cv Elstar, and often from one additional variety.

Parasitism by C. delusorius was determined by either rearing adult wasps from samples of 25 weevils kept in rearing boxes and fed with apple leaves (see below), or by careful dissection of weevils under a stereomicroscope. Teratocytes develop in the host’s haemocoel as the parasitoid larva grows. Wasps developing within overwintered larvae are called ‘winter generation’ to distinguish them from the partial second ‘summer generation’.

The presence of apple blossom weevil and C. delusorius was determined, by means of beating samples, in 30 orchards between 13 March and 11 April 1991. The weevils were dissected within a few days after collection, and the presence of parasitoid larvae scored. The sampling was repeated in 13 orchards in June 1991, i.e. after the first flight period of adult parasitoids. Tree bands were deployed in the following three winters. Integrated pest management ( Blommers, 1994 ; van den Ende et al., 1996 ) was used in all orchards, except on four ‘organic’ farms where no synthetic pesticides were applied.

In order to rear parasitoids, groups of about 25 field‐collected weevils were put into transparent plastic boxes (10 × 10 × 8 cm), with ventilation openings (diameter 25 mm) covered with fine mesh in the sides and with filter paper on the base. The weevils were fed with apple buds or leaves put onto a grid of iron wire, and attached halfway up each box. Initially, the boxes were inspected for emerged parasitoid larvae daily, and during the emergence period up to four times a day. Emerged larvae were transferred, using a soft brush, to similar boxes containing a medium for pupation. Pupation media included potting soil for ferns/Anthurium (ASEF ™ ), a mixture of potting soil and hydro‐granules (ASEF ™ ), vermiculite and peat moss (Sphagnum sp.). From 1992 onward, field‐collected weevils were released directly into boxes with a layer of Sphagnum, as this substrate was easily handled. Rearing boxes were kept in an outdoor insectary, unless stated otherwise. Cages and other devices of confinement in field experiments were always placed in the shadow between adjacent trees, and were filled simultaneously with wasps (adults or larvae) in every experiment and observational series.

Phenology and parasitism level

In order to establish the emergence periods of fully grown larvae and adult wasps, apple blossom weevils were collected in an organic orchard in Asch on 15 April 1992, 16 and 23 April 1993 and 22 April 1994 for winter generation emergence, and on 15, 11 and 21–22 June, respectively, for emergence of the summer generation. Simultaneously, the emergence of adult blossom weevils was scored daily from capped blossoms collected in the same orchard on 22 May 1992, 10 May 1993 and 20 May 1994, and stored individually in glass tubes in the insectary.

The development of parasitism was determined by immediate dissection of weevils collected by beating branches, in the same organic orchard on 22 May, and 1 and 7 June 1993. On 1 June 1993, blossom weevils collected from behind the bark of the trees in the same planting were dissected for comparison. This comparison was repeated in the same orchard on 11 June 1993, and in Experimental Orchard De Schuilenburg, Kesteren, on 17 June, where the parasitoids were reared from the weevils collected. Levels of parasitism were compared by the Model II Contingency test ( Sokal & Rohlf, 1969 ).

The head capsule widths of C. delusorius larvae were determined four times, using a stereomicroscope with ocular micrometer. The beating sample on 7 June 1993 and the bark sample of 4 days later were combined for measurements prior to emergence of the summer generation larvae. The next measurements were of larvae found in weevils used for parasitoid rearing, on 12 July 1993, i.e. 5 days after emergence of the summer generation larva. For measurements thereafter, only weevils collected from behind bark on 17 August 1993 were used, as sampling by beating branches was not possible. Thirty‐nine of these weevils were dissected immediately. Another hundred were stored in rearing boxes in the insectary until their dissection on 4 January 1994.

Pupation in the field

As pupation of C. delusorius occurs outside the host ( Speyer, 1939 ; Zijp & Blommers, 1992 ), two orienting experiments were devoted to the preferred habitat for pupation. First, 69 field‐collected blossom weevils were released in each of two emergence traps (numbers 1 and 2) on 24 April 1992. Each trap (‘Bodenphotoeklektor’, Ecotech™, Bonn, Germany) consisted of a 25 cm high ring, enclosing a ground surface of 1 m 2 , covered by a cone‐shaped tent of black cloth, with an opening at the top leading to a transparent collection box. The traps were placed between trees in a herbicide‐free planting and were filled with a layer of apple tree prunings, so as to offer the descending parasitoid larvae the choice of pupating on branches, or soil. To determine the choice, the prunings were transferred to two of four newly installed traps (numbers 3 and 4) on 25 May, when the emergence of parasitoid larvae had ceased, at least in the insectary. The other two traps (numbers 5 and 6) remained empty, so as to serve as a negative control. Emerged adult wasps were counted daily in the collection boxes on top of the traps.

To see where C. delusorius pupates in the soil, two plastic cylinders (diameter 11 cm) were pushed 20 cm deep into the ground; one in bare soil and the other in grass. Twelve laboratory‐reared parasitoid larvae were dropped into each cylinder on 13 June 1992. The cylinders were covered with a transparent Petri dish to keep predators out. Three days later, the cylinders were taken into the laboratory and the enclosed soil was examined carefully for the presence of parasitoid cocoons. The bare soil area was created using the herbicide diuron, applied in early April.

The survival of pupae was determined in the field with three pairs of similar cylinders, one of each pair placed in grass and the other in bare soil. Equal numbers of freshly emerged parasitoid larvae were released simultaneously in each pair of cylinders: 10–38 per cylinder between 18 and 26 June 1992. The ground around the cylinder was covered with sand and a gauze cage was placed over each to retain emerging wasps. Honey water was provided, and the cages were checked daily for the emergence of adults. The cylinders were searched for cocoons on 23 July.

Longevity and reproductive capacity

To determine the longevity of C. delusorius, 15 unmated females were placed singly into plastic pots (height 12 cm, diameter 5.5 cm) in the insectary, within 24 h of their emergence, between 31 May and 5 June 1994. All females were provided with diluted honey, and five were maintained without hosts. To each of five other females, young unparasitized weevils were offered sequentially until 10 had been accepted, i.e. stung with the ovipositor. This procedure was repeated daily. Numbers of successful ovipositions by two females were determined by dissection of accepted weevils. The last five females received 50 weevils and fresh apple leaves every 2 days. The females were observed daily until they died. The average ambient temperature during the experiment was 14.2 °C, varying between 6.6 and 22.0 °C. Differences in longevity between treatments were analysed by means of two‐way anova .

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Four female wasps were dissected within 24 h after emergence in the insectary in May 1992, and developed eggs were counted.

To examine the reproductive performance of the female parasitoid, 10 branch sleeves made of fine netting (diameter 30 cm, length 50 cm) were attached to apple trees, after careful examination of the branches and removal of spiders and other predators. Thirty unparasitized apple blossom weevils were released into each sleeve in July 1992, and a single mated C. delusorius female was added within 24 h of emergence. When the female could be found in the sleeve, she was transferred to a new sleeve after 24 h. All sleeves were examined and weevils dissected in early September.

Results

Sampling and rearing

Apple blossom weevils were found in 15 of the 30 farms where beating samples were taken in one or more plantings during early spring 1991. As no weevils were recorded in all eight farms examined in the region South of Rotterdam (Numansdorp, Dinteloord), no further work was undertaken in this area. Densities in infested orchards were usually lower than 10 weevils per 100 branches beaten, but exceeded 40 specimens per 100 branches in two cases, and 60 once. Dissection of 767 weevils in 25 samples yielded 53 parasitized specimens, and showed that C. delusorius was present on six farms, including one organic farm: four of them in the province of Gelderland, and one in both Utrecht and Limburg.

Neither beating samples in June, nor samples from tree bands in summer 1991 or the following winter, involving a total of 1415 dissected weevils, identified any new orchard with C. delusorius. Hence, it was decided to limit the monitoring of parasitism in private orchards to an inspection of overwintering weevils in treebands. These observations (Table 1) showed that C. delusorius almost disappeared from all but one private orchard in following years. The remaining orchard, in Asch (Gelderland), consisted of old standard trees standing in grass, and was subject to organic crop protection. Here, 109 out 366 weevils from treebands (29.8%), were found to be parasitized in early 1993, 94 out of 265 weevils (35.4%) one year later, and 30 out of 122 (24.6%) on 28 November 1994. Consequently, this orchard served as major source of parasitized weevils, in addition to experimental orchard De Schuilenburg.

1991–1992 1992–1993 1993–1994
Blossom weevils
Number of farms 11 (11) 15 (15) 14 (17)
Number of weevils/treeband 6.11 (652) 5.83 (964) 1.59 (1609)
Number of weevils dissected 615 1165 880
Number of weevils dissected/planting 44 (14) 65 (18) 42 (21)
Centistes delusorius
Number of farms with parasitoids 3 2 1
Number of specimens found 21 110 94

In rearings during 1992–1994, up to 84% of emerged larvae (n = 160) made a cocoon, and up to 66% developed to the adult stage (n = 85). Male and female numbers were about equal (Table 2).

Year Winter generation Summer generation
1992 54 (115) 35 (20)
1993 61 (67) 65 (26)
1994 48 (56) 33 (3)
Mean 54.6 51.0

Phenology and parasitism level

Overwintered larvae of C. delusorius emerged from their host in the last week of April and the first half of May (Table 3). Emergence coincided with full bloom (Fleckinger stage F2) of cvs. Elstar and Jonagold. Fifty percent cumulative emergence of the adult wasps occurred 3–4 days before 50% emergence of apple blossom weevils from the capped blossoms.

Winter generation Summer generation
Year Larvae Adults A. pomorum Larvae Adults
1992: 30.04–13.05 23.05–2.06 25.05–16.06 18.06–2.07 2.07–28.07
50% 6.05 (27) 25.05 (115 * ) 29.05 (87) 25.06 (72) 8.07 (20)
1993: 25.04–1.05 17.05–25.05 19.05–3.06 16.06–7.07 7.07–24.07
50% 28.04 (110) 21.05 (67) 24.05 (88) 26.06 (73) 10.07 (26)
1994: 30.04–17.05 27.05–9.06 29.05–15.06 1.07–16.07 21.07
50% 4.05 (85) 1.06 (56) 4.06 (78) 4.07 (31) 21.07 (3)
  • * Including adults from the experiment on pupation in different substrates.

Parasitism of A. pomorum in beating samples rose from 0 to 82% during the 3 weeks from the start of wasp emergence on 17 May 1993 (Fig. 1). Three out of four parasitoids (n = 12) were at the egg stage on 1 June, falling to 40% (n = 15) one week later. In 1994, 66% (n = 15) and 43% (n = 21) of the parasitoids were in the egg stage in weevils sampled by beating on 14 and 21 June, respectively. Blossom weevils collected from behind tree bark appeared to be parasitized less often than those sampled by beating (Table 4).

Cumulative percent emergence of Centistes delusorius larvae and adults of winter (total number = nW) and summer (nS) generation, and of Anthonomus pomorum, collected at an organic orchard in The Netherlands in 1993 and reared in an outdoor insectary, and the parasitism levels (par%) of the adult weevils in beating samples taken in the same orchard on six dates.

Percentage parasitism (n)
Date Orchard Beating samples Bark P (χ 2 .05[1])
1 June Organic 20 (59) 4 (23) 0.075 (3.17)
11 June Organic 82 (75) 33 (75)

Head capsule widths of Centistes delusorius larvae, measured during dissection of Anthonomus pomorum collected in an organic orchard in 1993. Weevils were dissected from (A) bark and beating samples before emergence of summer generation larvae in June, (B) parasitoid rearings after emergence of summer generation larvae in July, and (C) specimens in aestivo‐hibernation from beneath bark in August and the following January.

Pupation in the field

Three and 12 wasps were recorded in emergence trap numbers 1 and 2, respectively, and no wasps in trap numbers 3 and 4, indicating that few, if any, cocoons had been transferred with the twigs, but that the emerging larvae had pupated in or upon the grass‐covered ground. A single, presumably wild, wasp emerged in the negative control (traps 5 and 6). Apparently, no cocoons were transferred with the twigs, so emerged larvae must have pupated in, or on, the ground in trap numbers 1 and 2. As the wasps appeared between 26 May and 2 June, coinciding with the emergence period in the insectary (Table 3), the rearing method does not appear to have delayed pupation and emergence of adult C. delusorius.

In the cylinder sunk into the grass, four cocoons were found at c. 0.5 cm depth in the upper part of the root system, and one in a grass sheath. The cylinder from bare soil contained one cocoon at the soil surface, and five in the upper 1.5 cm. These observations corroborate the experience with the results of pupal rearing work: the descending parasitoid larva searches an easily accessible, humid hiding place to spin its cocoon.

A mean of 25% survival of the larvae released in the six cylinders was recorded, the 20 adults emerging between 2 and 10 July (Table 5). Data did not allow comparison between treatments. Spiders and staphylinid predators were present in cages on the grass strip, but not on bare soil.

Cylinder no. Bare soil Grass
1 3 5 2 4 6
Number of larvae released 38 10 11 37 10 11
Number of emerged wasps 4 3 2 2 3 6
Number of cocoons found 9 6 2 16 10 1
empty 5 1 8 1
dead wasp 8 1 8 2
brown contents 1 1 8

Longevity and reproductive capacity

Female parasitoids lived for a mean of 6.3 ± 2.1 days under outdoor conditions in early June 1994. There was no significant difference in longevity between treatments (F[2,12] = 1.16, P = 0.35). Attack of an offered host weevil usually took a few seconds, and rarely more than 15 s. Dissections of the weevils attacked showed that one female had successfully oviposited in 46 out of 82 weevils, and the other in 26 out of 53, during 9 and 7 days, respectively. Four freshly emerged females contained a mean of 41.5 ± 4.9 fully grown eggs.

Four wasps were located in the sleeves after 24 h. One was dead, but the other three (numbers 3, 8 and 10; Table 6) were transferred to a second sleeve. On average, 79% of weevils were located in the sleeves at the end of the experiment. Two wasps parasitized at least 20 out of 30 weevils, but superparasitism occurred in most cases, with up to eight larvae in one host. The oviposition capacity of C. delusorius exceeded 30 eggs, but the experimental design probably reduced reproductive success.

Female wasp no. Apple blossom weevils
Unparasitized Parasitized C. delusorius larvae
1 4 23 25
2 5 20 31
3 10 + 11 13 + 3 21 + 3
4 13 15 22
5 9 13 21
6 8 13 15
7 16 10 14
8 24 + 19 1 + 6 1 + 6
9 21 6 6
10 21 + 25 0 + 1 0 + 1

Discussion

The current study represents the first report on the biology of C. delusorius since the species was reported as a parasitoid of apple blossom weevil ( Speyer, 1939 ), and a short study conducted during the 1960s (cf. Zijp & Blommers, 1992 ).

Centistes delusorius overwinters as young, probably second stage larva. The fully grown larva leaves its host at the end of April and in early May around full bloom of apple, spinning a cocoon just below the surface of the ground. The adult wasps emerge in the second half of May and the first week of June. The emergence period is well synchronized with that of the adult apple blossom weevils, which began 2 days later in each year between 1992 and 1994, whereas median emergence dates differed by only 3–4 days. This supports the supposition of Speyer (1939 ) that the wasps mainly attack young apple blossom weevils. The young parasitoid larva either enters diapause to overwinter or, as most appear to do, develop into a fully grown larva, which leaves the host between mid‐June and mid‐July, and becomes adult during July. At this time of year, the female wasps may have difficulty finding hosts, as all apple blossom weevils are aestivating under treebark or fallen leaves in or outside the orchard ( Miles, 1923 ; Troitzky, 1928 ; Koštál & Šimek, 1996 ; Toepfer et al., 2000 ), but records of adult C. delusorius from beneath beeches ( Stelfox, 1955 ; Graham, 1955 ) indicate that the parasitoid may follow its host outside the orchard. The wasps are most likely to be carried there as larvae inside weevils. Attempts to attract adult C. delusorius to confined apple blossom weevils in delta traps failed (L. Blommers, unpublished data).

The potential impact of C. delusorius as a natural enemy of apple blossom weevil was difficult to quantify for various reasons. Estimates of parasitism levels in weevils from treebands in winter neglect any effect of the emigration of weevils from the planting. Levels in blossom weevils from beating samples taken in late April should be more representative, but the return of some weevils to the planting might be delayed ( Toepfer et al., 1999 ).

Two difficulties were also encountered when the percentage of parasitoid larvae that developed into summer generation was assessed by rearing the larvae from field‐collected weevils; one of timing, and the other of sampling. Although parasitism should be determined between emergence of the last host and emergence of the first parasitoid larva ( van Driesche, 1983 ), this period appears to be both short and variable between years, and the start of emergence of the wasp larvae is not known in advance. Moreover, beating samples produced significantly higher figures than samples from beneath bark. Apparently, parasitized apple blossom weevils are more active, as 90–96% of the parasitoid larvae in beating samples (1992–1994) developed into summer generation adults, whereas only 65% (1993) did so in samples from beneath bark. This explains why parasitism in beating samples rose to levels as high as 80%; most unparasitized weevils had eventually disappeared. Thus, it is doubtful whether parasitism levels for wasps of the second generation can ever be established, as migration out of the orchard ( Toepfer et al., 2000 ) represents another complicating factor.

A beating sample in the second half of June, when most unparasitized apple blossom weevils have gone into summer diapause, is probably the most efficient detection method for C. delusorius.

Centistes delusorius was absent in most orchards infested by apple blossom weevil. This explains the low numbers of insects in various experiments of this study. The species apparently has difficulty surviving common horticultural practice. It was only continuously present in appreciable numbers in two old‐fashioned plantings: standard trees in an organic orchard, and 40‐year‐old bush‐type trees at De Schuilenburg, both standing in permanent grass and rarely treated with pesticides. The absence of the parasitoid in IPM orchards might be due to various factors, including temporal and/or local absence of suitable conditions for pupation in conventional apple plantings. Pupation is a critical event because the thin wall of the cocoon provides little defence against either drowning or desiccation of the pupa, as was found in initial rearing attempts (L. Blommers, unpublished data). Thus, pupation on the bare mud of the weed‐free strip underneath the trees in modern orchards may be more hazardous than between grass, both in times of rain, and during dry periods. Harrowing, or flaming, applied for weed control in organic orchards, is also expected to kill C. delusorius pupae. Chemical weed control, in the IPM orchards usually coinciding with pupation of the parasitoid in May, mostly involved three herbicides: simazin, glyphosate and diuron. The first two should be harmless to parasitic wasps ( Hassan et al., 1987, 1988 ), but diuron may show some activity ( Stadler et al., 1996 ).

The application of carbaryl for fruit thinning almost coincides with peak emergence of C. delusorius adults at the end of May, damaging populations of the apple blossom weevil (L. Blommers, unpublished data) and its antagonist. Moreover, the parasitoid larvae die when parasitized apple blossom weevils feed on leaves treated with diflubenzuron ( Zijp & Blommers, 2001 ), a compound used against winter moth (Operophtera brumata (L.), codling moth (Cydia pomonella (L.) and other noxious caterpillars ( Van den Ende et al., 1996 ). Additional side‐effects on C. delusorius might be expected because parasitized blossom weevils stay more active, and exposed to pesticides, than healthy individuals. The apparent absence of C. delusorius in so many orchards might mean that the species is easily exterminated, and does not return quickly.

Centistes delusorius is essentially pro‐ovigenic and koinobiont like other euphorine braconids ( Shaw & Huddleston, 1991 ). A female carries over 40 more‐or‐less full sized eggs at emergence, and is able to lay 25 or more eggs in the first day of adult life ( Zijp & Blommers, 1992 ). Maximum oviposition capacity of the parasitoid was 46 under experimental conditions, but the average number of eggs laid per female was only half this number. Average longevity did not exceed one week. The late rise in parasitism level in June 1993, and the rare observations of both eggs and almost fully grown larvae as late as August, suggests that the female wasps may live longer under field conditions. An alternative explanation, i.e. an occasional third generation, could not be substantiated (J. Zijp, unpublished data). The apple blossom weevil is able to lay up to 50 eggs/female ( Schulz, 1924 ; Dicker, 1946 ), while the sex ratio is about 50% in both host and parasitoid. Hence, C. delusorius, with two generations compared to the one of its host, shows promise as an antagonist if relevant mortality factors could be mitigated.

Mortality during pupation is an evident target, because although the parasitoid resides in the host most of the time, as many as 75% were seen to die during pupation. Unfortunately, an experiment to improve pupal survival with a layer wood chips on the weed‐free strip underneath the trees failed, but led us to the discovery of the indirect side‐effect of diflubenzuron, applied against codling moth, on C. delusorius ( Zijp & Blommers, 2001 ). With alternate, more selective means to control codling moth, such as granulosis virus preparations and mating disruption techniques, now available, the parasitoid should have better chances to survive.

Acknowledgements

W. Stange, F. Vaal, H. Helsen, E. Bruins and W. Reerink assisted with this study.

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