Reproductive mode and split sex ratios in the facultatively ovoviviparous thrips, Bactrothrips brevitubus

Researching the evolution and maintenance of reproductive mode (oviparity, ovoviviparity and viviparity) is important for identifying the intrinsic and environmental pressures associated with maternal care. We studied the life history, reproductive mode and sex allocation of Bactrothrips brevitubus, a haplodiploid, facultatively ovoviviparous Japanese thrips. Individual females can produce a mass of eggs oviparously and single eggs ovoviviparously. In our study, about 30% of the females reproduced exclusively by oviparity. Nearly 60% of the oviparous females also reproduced ovoviviparously. Local resource competition between reproductive females for food and egg-laying sites may select for ovoviviparity, as females can retain the eggs until a better rearing site is located. We also have preliminary evidence that predatory mites promote ovoviviparity, as they consume the eggs but are ectoparasitic on larvae and adults. Another factor that may influence reproductive mode is that females can produce both sexes oviparously but only males ovoviviparously. Consequently, sex allocation and reproductive mode strategies act in tandem. The population-wide sex ratio was unbiased, whereas the sex ratio of broods that were produced at least partly oviparously was female biased. This femalebiased sex ratio was not significantly different from that predicted by Godfray’s split sex ratio model for a panmictic population. Even so, our evidence for local mate competition and local resource competition suggests that the sex ratios were not Fisherian. We predict that females should reproduce oviparously when constrained by local mate competition, ovoviviparously when constrained by mite predation, with or without local resource competition, and use a mixed strategy when constrained by mite predation and local mate competition.