Mating rates and the prevalence of male-killing Spiroplasma in Harmonia axyridis (Coleoptera: Coccinellidae)

摘要

Maternally inherited bacteria that kill male but not female hosts during embryogenesis have been widely reported in invertebrates. Harmonia axyridis is one of the species infected by male-killing Spiroplasma. The presence of male-killers in host populations can lead to the occurrence of extremely female-biased sex ratios. Furthermore, infected females may have fewer chances to mate if males can discriminate between infected and uninfected females and prefer the latter. Although there have been many investigations of male-killer infection rates in H.axyridis, little is known about the influence of host mating on male-killer infection dynamics. We investigated copulation rates and changes in infection frequency in a wild population of H.axyridis in western Japan. Almost all infected females collected each year laid fertilized eggs and had therefore mated. Mean infection rates of females collected each year were 13% in 2003, 15% in 2012 and 23% in 2013. Statistical analysis showed that neither the copulation rate nor the infection rate differed significantly among years. These results suggest that the infection rate of H.axyridis with male-killing Spiroplasma is kept approximately constant and that there is no difference in the chance of mating with infected and uninfected females.