VARIABLE SEX-RATIOS AND OVICIDE IN AN OUTBREEDING PARASITIC WASP

Bracon hebetor Say (Hymenoptera: Braconidae) is a gregarious parasitoi d of phycitine moth larvae that infest stored grain. It has been hypot hesized that B. hebetor females produce proportionately more female of fspring under conditions of superparasitism (when laying eggs on previ ously parasitized hosts) because daughters are reproductively more val uable than sons when resources are limiting and adult body sizes are r educed. This hypothesis was reexamined by measuring the effects of bod y size on male and female performance and by monitoring the sex ratios and clutch sizes of individual females. The results of this study pro vide only weak evidence that small size differentially affects the rep roductive 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 l aid smaller clutches when superparasitizing. A larger sex-ratio shift towards male progeny was seen, however, in females that committed ovic ide (i.e. killed some of another's eggs by piercing them with the ovip ositor). The offspring sex ratios of ovicidal females were much less f emale-biased because these females laid male eggs earlier in the ovipo sitional sequence. Ovicidal females shifted their sex ratios whether s uperparasitizing or ovipositing alone. None of the females killed thei r own eggs, even though they were observed probing among them with the ir ovipositors. It is hypothesized that oviposition behaviour and sex ratio in B. hebetor may be grouped into two syndromes: ovicidal and no n-ovicidal. The variation in sex ratio and ovicide may be a consequenc e of density-dependent selection, favouring non-ovicidal behaviour whe n population density is low and ovicidal behaviour when the density is high and competition for larval resources is acute.