Female-biased sex ratio in the Asian corn borer, Ostrinia furnacalis:: evidence for the occurrence of feminizing bacteria in an insect

The maternally inherited, female-biased sex ratio in many arthropods has been attributed to infection with micro-organisms. Male killing, thelytoky and feminization are recognized as the mechanisms of the bacteria-induced sex ratio distortion in arthropods. A sex ratio distortion towards the female has been found in the Japanese population of the Asian corn borer Ostrinia furnacalis (Lepidoptera). In the present study, inheritance of the sex ratio distortion in the Asian corn borer and its underlying mechanism were investigated. Two of the 13 field-collected females produced all-female offspring. The female-biased sex ratio was maternally inherited for three generations. There was no difference in egg hatchability between thelygenic and normal crosses. Treatment with tetracycline for two generations resulted in production of only male offspring. Considering the sex determination system in lepidopteran insects, it was concluded that chromosomal males are feminized by a cytoplasmic agent(s), most probably parasitic bacteria. This is the first report of the occurrence of feminizing bacteria in insects. The sex ratio distorter was considered to occur at low frequency in the Asian corn borer population.