Daisuke KAGEYAMA, Gen NISHIMURA, Sugihiko HOSHIZAKI and Yukio ISHIKAWA
Laboratory of Applied Entomology, Graduate School of Agricultural and Life Sciences,
The University of Tokyo, Tokyo 113-8657, JAPAN

Female-biased sex ratio has been observed in the Japanese populations of the Asian corn borer, Ostrinia furnacalis, and feminization of genetic males caused by bacterial infection is considered to be an underlying mechanism for this phenomenon (Kageyama et at, 1998).However, frequencies of female-biased sex ratio in the wild populations, whether female-biased sex ratio occurs also in other species of the genus Ostrinia or not, and identification of the causal agent (bacterium) of feminization have remained to be studied. In four geographical populations of 0. furnacalis and 0. scapulalis, females that produce offspring with significantly female-biased sex ratio were found with frequencies 10% in both species.

Female-biased sex ratio was maternally inherited from most of the the lygenic females. In most of the maternal lines with female-biased sex ratio, treatment with tetracycline for two generations resulted in production of only male progeny. This result indicated that the genetic males were feminized in these maternal lines. PCR diagnosis assay revealed that all of the maternal lines that were considered as "feminized" were infected with Wolbachia, and that all except two families with sex ratio not significantly different from 1:1 were not infected with Wolbachia. It was thus suggested that Wolbachia underlies the feminization of genetic males in 0. furnacalis and 0. scapulalis.