Host Discrimination by Unmated Individuals of the Gregarious Parasitoid Wasp, Cotesia (—Apanteles) glomerata (Hymenoptera: Braconidae)

Abstract

Cotesia (=Apanteles) glomerata L. (Hymenoptera: Braconidae) is a well-known gregarious braconid endoparasitoid wasp which, in Japan, mainly parasitizes larvae of the cabbage white butterfly, Pieris rapae crucivora Boisduval (Lepidoptera: Pieridae). For the female parasitoid, host quality might be a key factor affecting her decision of oviposition (Sato, 1976; Satoand Ohsaki, 1987). Among the many potential characteristics of host quality, the presence or absence of prior parasitization must be a major one (Charnov And Skinner, 1985; Van Alphen And Vet, 1986). C. glomerata can discriminate between parasitized and unparasitized hosts, and prefers unparasitized ones (Matsuzawa, 1958; Kusanoand Kitano, 1974; Ikawaand Suzuki, 1982). Recently, Le Masurier (1990) pointed out the importance of differences between host species, which might affect the degree of host rejection by a second female attacking an already-parasitized host, i.e., a C. glomerata female completely rejecting parasitized larvae of P. brassicae in contrast with the occurrence of superparasitism of P. rapae crucivora. This suggests that, in some cases of host-parasitoid interactions, superparasitism may be adaptive for foraging females (Van Alphen And Visser, 1990). However, these works considered mated females only. A haplo-diploid reproductive system of hymenopterans allows unmated females to produce male offspring, and the degree of host preference by unmated females seems worthy of investigation from this perspective. For C. glomerata, the number of eggs laid in a host by unmated females was fewer than that laid by mated females (Tagawa Et al., 1987; Tagawa, 1987). According to the theory of superparasitism for gregarious parasitoids in a local mate-competitive state (Suzuki And Iwasa, 1980; Werren, 1980), a parasitoid wasp ovipositing in an already-parasitized host would be better rewarded if she laid fewer eggs with a higher proportion of males than did the first parasitoid. In C. glomerata, the rate of inbreeding was proved to be high (Tagawa And Kitano, 1981). Under such condition, the fitness of unmated females might be higher if they laid their eggs in parasitized hosts rather than in unparasitized ones, because their male progeny would have a better chance to mate near or on the cocoon clusters without dispersing after emergence. Therefore, unmated C. glomerata may prefer parasitized hosts. The present study investigates the ability of host discrimination and the degree of host preference by unmated females. © 1992, JAPANESE SOCIETY OF APPLIED ENTOMOLOGY AND ZOOLOGY. All rights reserved.