Key words: apple blossom weevil, Anthonomus pomorum, diflubenzuron, side effect, insect growth regulator, ... spring, the weevils start to feed on the buds and.
Entomologia Experimentalis et Applicata 98: 115–118, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.
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Short communication
The effect of diflubenzuron on parasitism of Anthonomus pomorum by Centistes delusorius J. P. Zijp1 & L. H. M. Blommers2 1 Laboratory 2 Plant
of Entomology, Wageningen University, P.O. Box 8031, 6700 EH Wageningen, The Netherlands; Research International, P.O. Box 16, 6700 AA Wageningen, The Netherlands
Accepted: September 7, 2000
Key words: apple blossom weevil, Anthonomus pomorum, diflubenzuron, side effect, insect growth regulator, parasitoid Introduction The apple blossom weevil, Anthonomus pomorum (L.) (Coleoptera: Curculionidae), is a specific pest of apple and pear in Europe (Miles, 1923; Speyer, 1939). This univoltine species overwinters as adult. In early spring, the weevils start to feed on the buds and mate. The mature females oviposit into the developing flower buds. After egg eclosion, the larva feeds on the interior of the bud. As a result, the blossom colors brown and does not open. In this ‘capped blossom’ the larva pupates. The adult weevil emerges in late May and early June. Centistes (Syrrhizus) delusorius (Förster) (Hymenoptera: Braconidae: Euphorinae) is an endoparasitoid of adult apple blossom weevil. The parasitoid overwinters as an early instar larva in diapause (Zijp & Blommers, 1992). In early May, the full grown parasitoid larva leaves the host, and spins a cocoon on the ground, where pupation takes place. The adult wasp emerges in late May or early June, when a new host generation appears in the orchard (Zijp & Blommers, 1992). In parasitised hosts, some parasitoid larvae develop into mature larvae, leaving the host between mid June and mid July and emerging in July as adult of the partial second parasitoid generation, and other parasitoid larvae enter diapause and appear next spring. As C. delusorius is a specific parasitoid of A. pomorum, spends most of its life-time in the adult weevil and has a partial second generation while its host has only one, it may be a good candidate for natural control (Gruys, 1982; Blommers, 1994). However, parasitism levels in the orchards are usually low.
Therefore, we investigated the possibility of improving the relative impact of the parasitoid. We tried to achieve this effect by a mild, i.e. partially effective, ovicidal treatment of its host, to obtain a reduction of the host population without affecting the parasitoid population. Chemical control of the apple blossom weevil is usually directed at the adults, prior to egg laying in spring, as both eggs and larvae are well protected inside the infested flower buds (capped blossoms). However, diflubenzuron applied when the weevils start feeding on the apple buds in spring, is known to interfere with subsequent egg eclosion; mainly because the residue ingested by the parent female prevents emergence of the young larva from the egg (Gruys, 1982; Grosscurt & Jongsma, 1987; Olszak, 1988). A similar indirect ovicidal effect of diflubenzuron is known from studies on the related Anthonomus grandis Boheman, the cotton boll weevil (Baronio, 1984). In this paper, we examine the effect of diflubenzuron on the progeny of unparasitised adult weevils fed with treated buds, and on the parasitoid larvae inside parasitised weevils.
Materials and methods Diflubenzuron (Dimilin 480 g/l) was sprayed in concentrations of 0.015%, 0.03% and 0.06% active ingredient respectively with a handlance to run-off on three blocks of ten 13-year old spindle bush trees cultivar Alkmene on 11 March 1994. Mineral oil (0.3%) was added to enhance the effect of diflubenzuron. A fourth block was left untreated. During spraying it was sunny
116 and 10 ◦ C. Immediately after spraying, sleeves of fine netting were attached around branches, while avoiding to touch buds as much as possible. Buds were in Fleckinger stage C-C3 (Bovey, 1979), and the first eggs of the natural population were not found until 14 March. Tree bands with overwintering adult apple blossom weevils, without parasitism (dissection sample of 50 weevils) by C. delusorius, were collected in a commercial orchard on 25 February 1994. The adults were stored in an outdoor insectary, with corrugated cardboard as shelter, but without any food. On 14 March they were placed in a cage under a lamp in the laboratory to activate them. Mating couples were transferred to glass tubes for release in the field. Two couples were transferred to each of ten branch sleeves in the blocks sprayed with 0.015% and 0.03% diflubenzuron and in the untreated block. Twelve other females were dissected on 15 March to determine the developmental stage of the ovaries. The sleeves were removed from the labeled branches on 22 April, at bud stage F, when oviposition was no longer possible. On 12 May, the capped blossoms, that had formed by larval feeding in the buds, were counted on each branch. Also the uninfested flowers were counted. The numbers of capped blossoms were analyzed by ANOVA (Statgraphics 6.0). Bark with overwintering adult apple blossom weevils was collected on 14 and 15 March 1994 from the stems of old apple trees in an orchard with high natural parasitism by C. delusorius. By activating them under a lamp, 219 weevils were collected. Iron wire was placed at half height in each of 32 desinfected transparent boxes of 10 × 10 × 8 cm with a lid and tiny ventilation openings. Filter paper on the bottom enabled weevils to turn when lying on the back, and water was provided on cotton wool during the experiment. Apple buds stage C-C3 were picked from untreated trees and from trees sprayed with 0.015%, 0.03% and 0.06% diflubenzuron on 15 March, and in this order to prevent any contamination. On the same day, five buds were placed on the wire in each of eight boxes per concentration. Six weevils per each of the 32 boxes (i.e. four treatments, eight replicates and six weevils per replicate) were distributed at random, regardless of sex. The remaining 27 weevils were dissected to determine the level of parasitism. After 46 h the boxes were transferred from the insectary to a climate room of 15 ◦ C and 18 h photoperiod. Every 2–4 days the buds were replaced by fresh ones from the treated trees. From 5 April onwards the boxes
were inspected once or twice a day for the presence of emerged parasitoid larvae, which were counted and removed. Dead weevils were removed and dissected to determine the presence of parasitoid larvae. On 29 April the remaining weevils were dissected. Student t-tests were used to analyse differences in number of parasitoid larvae per box between treatments (n = eight boxes per treatment). The experiment was repeated in 1995 according to the described methods, but with buds of cultivar Golden Delicious, treated during bud stage D3 with 0.0075%, 0.015% and 0.03% diflubenzuron on 5 April 1995, during sunny weather. On the same date, weevils were collected by beating in the organic orchard, and put in boxes with buds as described. A beating sample (n=24) was taken on 10 April to determine the level of parasitism. The boxes were inspected for emerged larvae until 29 April 1995, when weevils were counted and dissected.
Results The starved apple blossom weevils started to feed on the buds at once after their release in the branch sleeves on 14 March 1994. Their ovaries were undeveloped, with at most a few slightly enlarged oocytes still in the previtellogenic stage (stages I–III, cf., ˇ Ctvrteˇ cka, R. & J. Ždárek, 1992). The number of capped blossoms that developed per branch (n=10 branches per treatment) did not differ significantly (ANOVA F=0.078, P=0.92) between the treatments (0.015% and 0.03% diflubenzuron and untreated). The overall mean (± std) was 19.5 ± 17.9 capped blossoms per two females per branch sleeve. On average 43% of the flowers was infested, indicating that the number of buds was not limiting oviposition. In the experiments on parasitoid mortality, the parasitism level of the sample of the weevils being fed with treated and untreated buds was 56% (n=27 dissected weevils) in 1994 and 25% (n=24) in 1995. The number of parasitoid larvae emerging from the weevils fed with untreated buds was significantly higher (P
