• Coccinellidae;
  • cytoplasmic microorganisms;
  • Harmonia axyridis;
  • male-killing;
  • maternal inheritance;
  • sex ratio.

Females from three populations of Harmonia axyridis (Pallas) were assayed for two phenotypical indicators of the presence of male-killing endosymbionts: low egg hatch-rates and strongly female-biased progenic sex ratios. Samples from Sapporo City, Japan, and the Altai Mountains, Mongolia, but not from Novosibirsk, Russia, were found to contain some females displaying both of these traits. Furthermore, there was a profound difference in the prevalence of infection between the Japanese and Mongolian populations. The proportion of females infected from the Japanese sample was approximately 0.49, whereas that from Altai was only 0.02. The trait was inherited by more than 99% of the progeny of infected females. The trait was inherited maternally with the same high efficiency over five generations. Treatment with antibiotics produced a small increase in the production of males from the male-killing lines, suggesting that the male-killer is bacterial in nature. Causes of variation in prevalence are discussed, and the consequences of high levels of infection with an efficiently transmitted male-killing microorganism on host population demography are considered.