SEX ALLOCATION AND SEXUAL ASYMMETRIES IN INTRA-BROOD COMPETITION IN THE PARASITIC WASP BRACON-HEBETOR

1. Bracon hebetor is a gregarious parasitoid with a female-biased sex ratio. While female-biased sex ratios are often indicative of local ma te competition (LMC), B. hebetor outbreeds, making LMC an unlikely exp lanation for biased sex ratios in this species. In this study we exami ned an alternative hypothesis that may explain female-biased sex ratio s: competitive asymmetries between larval sons and daughters. 2. We me asured two types of competitive asymmetries: asymmetric density respon ses (differential responses to clutch size), and asymmetric compositio n responses (responses to change in clutch sex ratio). We measured com petitive asymmetries in terms of their effects on body size, developme nt time, survivorship, longevity, and daily and lifetime fecundity or mating ability. 3. Males and females emerged as smaller adults, develo ped more quickly and experienced higher rates of larval mortality when they developed in larger clutches. The effect of clutch size on body size and survivorship was similar for males and females. Increasing cl utch size decreased male larval development time more than female deve lopment time, although the effect was marginal. 4. Male and female bod y sizes increased as within-clutch sex ratios became more male-biased, indicating that females, not males, are stronger competitors. This sh ould lead to male-biased sex ratios, rather than the female-biased sex ratios that exist in B. hebetor. Sex ratio had no influence on develo pmental time or survivorship of either sex. 5. Larger males and female s lived longer than smaller individuals, whether they fed on hosts or honey. Larger females had higher daily and lifetime fecundities than s maller females. Larger males copulated with more females and sired mor e daughters both per day and per lifetime than did smaller males. Thes e body size effects were only detectable when hosts were abundant. At low host densities, differences between large and small wasps were abs ent. 6. Although we detected asymmetric composition responses in terms of body size, the effects were slight. Therefore, we conclude that co mpetitive asymmetries are unlikely to have important effects on male a nd female fitness or on the sex allocation decisions of B. hebetor.