Biology of Centistes delusorius, a parasitoid of adult apple blossom weevil
Abstract:1 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.
2 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.
3 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.
4 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.
5 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.
6 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.
7 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.
8 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.