Split sex ratios and virginity in a gall-inducing thrips

Split sex ratios have been predicted in haplodiploid populations with high proportions of reproductive virgins, but there has been little empirical su pport. We found such split sex ratios in the gall-inducing thrips, Kladothr ips rugosus. Sex ratios of juveniles from 96 galls were determined using ch aetotaxy over two consecutive summers. The population-wide sex ratio was un biased, but bimodal. Twenty-four per cent of galls only contained male juve niles. These galls were induced by a female that was probably a virgin. The mean sex ratio of all other galls was 0.36 +/- 0.02, which is not signific antly different from the theoretical evolutionarily stable sex ratio of 0.3 4, calculated from a previous model (Godfray, H.C.J. 1990. J. Evol. Biol. 3 : 3-17) for 24% constrained females in a panmictic population. These data p rovide the first empirical support for the constrained sex allocation model of Godfray.