Variable sex ratios and ovicide in an outbreeding parasitic wasp

Abstract

Bracon hebetor Say (Hymenoptera: Braconidae) is a gregarious parasitoid of phycitine moth larvae that infest stored grain. It has been hypothesized that B. hebetor females produce proportionately more female offspring under conditions of superparasitism (when laying eggs on previously parasitized hosts) because daughters are reproductively more valuable than sons when resources are limiting and adult body sizes are reduced. This hypothesis was re-examined by measuring the effects of body size on male and female performance and by monitoring the sex ratios and clutch sizes of individual females. The results of this study provide only weak evidence that small size differentially affects the reproductive success of male and female B. hebetor. Sex ratios were more female-biased on superparasitized hosts, but the difference arose as a consequence of two aspects of oviposition behaviour. First, male eggs were laid later within ovipositional sequences, and second, females laid smaller clutches when superparasitizing. A larger sex-ratio shift towards male progeny was seen, however, in females that committed ovicide (i.e. killed some of another's eggs by piercing them with the ovipositor). The offspring sex ratios of ovicidal females were much less female-biased because these females laid male eggs earlier in the ovipositional sequence. Ovicidal females shifted their sex ratios whether superparasitizing or ovipositing alone. None of the females killed their own eggs, even though they were observed probing among them with their ovipositors. It is hypothesized that oviposition behaviour and sex ratio in B. hebetor may be grouped into two syndromes: ovicidal and non-ovicidal. The variation in sex ratio and ovicide may be a consequence of density-dependent selection, favouring non-ovicidal behaviour when population density is low and ovicidal behaviour when the density is high and competition for larval resources is acute.