ABSTRACT
Solitary parasitoids of colony-forming hosts may produce quasi-gregarious broods, which favours sibmating on the natal patch and local mate competition (LMC). We examined seasonal variations in brood size and sex ratio in three species of solitary parasitoids of aphids associated with trophobiotic ants. Adialytus arvicola, a parasitoid of Sipha agropyrella on grasses, had the smallest broods (mean=4.2, maximum 19), while Lysiphlebus hirticornis, a parasitoid of Metopeurum fuscoviride on tansy, had the largest broods (mean=32.0, maximum 265). In Pauesia pini, a parasitoid of Cinara piceicola on Norway spruce, broods comprised an average of 5.8 (maximum 41) individuals; brood size increased during early summer when hosts became more available but remained the same later in the season. In all three species the sex ratio at eclosion was female-biased, with broods containing approximately two daughters for each son in both A. arvicola and L. hirticornis; the degree of female-bias was least in P. pini. The sex ratio did not vary with brood size. In A. arvicola, the variance of the number of sons declined with an increase in brood size, consistent with "precise" sex allocation. In contrast, in L. hirticornis, the overall sex-ratio variance was greater than its binomial expectation, while it did not differ from binomial in P. pini. A large proportion of broods contained only sons or only daughters, especially in A. arvicola. An excess of male-only broods is expected if constrained females (which can produce only sons) contribute offspring prior to mating. The number of male-only broods, however, did not differ from the number expected if all females are mated and allocate offspring sexes binomially, except in P. pini. In the latter species, broods with two daughters (as opposed to two sons) exceeded binomial expectations. We propose that P. pini is largely outbreeding, while the strongly female-biased sex ratio in A. arvicola and L. hirticornis is consistent with partial sibmating and LMC. Ant-parasitoid interactions could account for a different population mating structure in the three parasitoid species. Both A. arvicola and L. hirticornis mimic the epicuticular hydrocarbon pattern of their aphid hosts. Eclosing wasps are ignored by honeydew-collecting ants and hence can mate and forage on the natal patch. In contrast, P. pini generally depart the mummy area to avoid attacks by trophobiotic ants and mate off patch.
ABSTRACT
Dendrocerus carpenteri (Curtis) (Hymenoptera: Megaspilidae) is a solitary hyperparasitoid, which attacks prepupal and pupal stages of hymenopteran parasitoids inside mummified aphids. The larva feeds externally on the host, which is envenomed by the female at oviposition. To evaluate the influence of variations in host quality on the growth, development and fitness of D. carpenteri, we varied the size and developmental stage of the primary parasitoid host (Aphidius ervi Haliday), which was reared on different instars of pea aphid [Acyrthosiphon pisum (Harris)] and English grain aphid [Sitobion avenae (F.)]. Within each kind of host, females eclosed from the relatively larger mummies, while males eclosed from the smaller mummies. Host size and hyperparasitoid size were correlated, and females were larger than males. In hyperparasitoids developing on prepupal and pupal hosts, development time from oviposition to adult eclosion was proportional to size; females required more time for development than males. The mean relative growth rate was the same in males and females and increased with host quality, as predicted by the growth model of Mackauer and Sequeira (1993) for idiobiont parasitoids. Larvae developing on late-pupal stages and pharate adults of A. ervi were unable to consume sclerotized host tissues; they were smaller and needed more time for development. The average number of mature eggs at eclosion was six, except in females developing on suboptimal hosts, which contained only one egg or none. Egg volume was correlated with female size, possibly reflecting differences in larval ontogeny. We provide equations describing the relationship between host quality as indexed by hind-tibia length of the mummified aphid and adult body size in terms of dry mass, development time and mean relative growth rate of D. carpenteri. We discuss the usefulness of host size as a proxy of host quality for idiobiont parasitoids, and provide examples of exceptions.
