Social biology and sex ratios of the eusocial gall-inducing thrips Kladothrips hamiltoni

1. Thrips comprise the only order besides Hymenoptera where females are dip loid and males are haploid. This makes them useful insects for studying the roles of kin selection and ecology in social evolution. 2. Kladothrips hamiltoni is one of six species of Australian gall-inducing thrips that have been identified as eusocial. Galls are initiated by a sing le foundress, who rears her brood and remains within the enclosed gall for life. The adults of both sexes of her first brood cohort are morghologicall y distinct from the second generation, comprising a nondispersing soldier c aste. The foundress and sib-mated soldiers jointly produce a second, disper sing generation, approximately 60-80% of which are produced by the soldiers . Mean per capita egg production of female soldiers is less than 33% that o f the foundress. 3. Adult eclosion of soldiers is protandrous but the overall sex ratio of t he soldiers lacks bias (52% male). Protandry of soldiers increases the prob ability that female soldiers will be inseminated soon after their eclosion and therefore lay fertilised, female eggs. The lack of bias could be due to a balance between local resource competition and local mate competition. G ender-specific defensive behaviour of soldiers with their enemies may also be important in explaining this unexpected sex ratio. 4. The dispersing generation has an overall extreme female bias (5.6% male) . Soldier incest increases relatedness between females more than between ma les, such that the foundress is more related to her granddaughters than her daughters, and female soldiers are more related to their daughters than th eir sons (assuming within-gall relatedness <1). A female bias in the offspr ing of soldiers should be preferred by both the foundress and soldiers as t hey are more related to soldier-produced dispersing females than any other thrips in the gall. Female bias in the dispersing generation will also redu ce local mate competition between males. Both soldier incest and local mate competition may therefore contribute to the extreme female bias in the dis persing generation. 5. Selection pressures for sociality in gall-inducing thrips appear to be m ore similar to those in gall-inducing aphids and naked mole rats than to th ose in Hymenoptera.