Strongly biased sex ratio in cuckoo wasp Chrysura hirsuta (Hymenoptera: Chrysididae), a parasitoid of the mason bee Osmia orientalis

We examined the rate of parasitism and sex ratio of the cuckoo wasp Chrysura hirsuta (Gerstaecker) (Hymenoptera: Chrysididae) that emerged from nests of the mason bee Osmia orientalis Benoist (Hymenoptera: Megachilidae) in Nara, Japan. Nests of O. orientalis were found in empty shells of two snail species, Satsuma japonica (Pfeiffer) and Euhadra amaliae (Kobelt). The percentage of parasitism by cuckoo wasps per all collected cocoons tended to be high (20–50%) even though interannual variation and the average number of cocoons per nest did not differ across snail shell species within each year. Our results from three years of observation, combined with previous reports, showed that the adult sex ratio of C. hirsuta was strongly female-biased, which suggests that the species reproduces by thelytokous parthenogenesis.