We examined the influence of offspring mortality caused by hyperparasitism on the secondary sex ratio of Lysiphlebus hirticornis Mackauer, a solitary endoparasitoid of the aphid Metopeurum fuscoviride Stroyan, in the field. Females of L. hirticornis produce pseudo-gregarious broods, which may comprise more than 200 offspring. Hyperparasitoids [mainly Syrphophagus aphidivorus (Mayr)] attacked and killed up to 60% of the primary parasitoids inside mummified aphids, especially late in the season. Hyperparasitized broods were larger than hyperparasitoid-free broods, which suggests that the risk of hyperparasitism increased with mummy density. We tested the hypothesis that mortality caused by hyperparasitism is greater for female than male offspring of L. hirticornis. If mummy quality scales with mummy size, hyperparasitoids should choose the relatively larger over the relatively smaller mummies. In the absence of hyperparasitism, broods of L. hirticornis included approximately two daughters for each son; the sex ratio did not vary with brood size. In hyperparasitized broods, the sex ratio was nearly even. This result indicates that relatively more female offspring (developing in the larger mummies) than male offspring (developing in the smaller mummies) were killed by hyperparasitoids. We propose that sex-differential offspring mortality in L. hirticornis is the result of differences in optimal host choice between the primary parasitoid and the hyperparasitoids.
On the basis of a twenty-year investigation, the life-cycle of Torymus cyanimus Boheman (Hymenoptera: Torymidae), a hyperparasitoid of a gall-forming fly in the Volga-Kama region is described. This parasitoid is the top-consumer in a food chain on Cirsium setosum (Willdenow) Iljin, in which the herbivore is Urophora cardui L. (Diptera: Tephritidae) and the primary parasitoids belong to the genus Eurytoma (Hymenoptera: Eurytomidae). Mating and oviposition behaviour were studied, and the superparasitism and larval cannibalism investigated in the second parasitoid generation. The superparasitism in T. cyanimus evolved in connection with the ovipositor elongation, leading to eggs being laid later when the fly host has already been completely consumed by larvae of Eurytoma serratulae F. Hyperparasitism and larval cannibalism in the second generation of T. cyanimus might account for the evolution of hyperparasitism in this species.