The light‐brown apple moth, Epiphyas postvittana (Walker) 2. Uneven sex ratios and a condition contributing to them in the field

Whereas populations of Epiphyas postvittana (Walker), the light‐brown apple moth (LBAM), normally produced a slight preponderance of male adults in laboratory cultures, males tended consistently to be less abundant than females in samples drawn from Australian field populations. The sex ratio of the species was investigated with a view to explaining this fact. A condition, causing bisexually‐reproducing females of LBAM to form viable progenies comprising mainly female individuals, was found to affect 7% of females in population samples from the south‐eastern mainland of Australia, and 4% from Tasmania. A statistical model describing the distribution of the condition in field populations is given. The origin of the condition is unknown; the causative agent is believed to act by inducing the death of male embryos. The condition was only observed to be transmitted from mothers to daughters, of which some might produce apparently normal progenies, while others produced either all‐female or predominantly‐female progenies. The demographic performance of affected females differed from that of comparable ‘normal’ individuals only in fertility, which was significantly reduced. In a limited number of experiments, laboratory populations formed from field‐collected samples containing affected females, which produced a proportion of males, could only be maintained through subsidies of males from non‐affected stock. The present case was considered in relation to literature reports of similar conditions affecting a dipteron, a coleopteron, and a macrolepidopteron. In E. postvittana, it is surmised that the condition might contribute to optimizing the sex ratio of populations comprising affected individuals, thus increasing their adaptive fitness. A set of verifiable hypotheses are formulated concerning future trends in the condition's natural distribution and frequency.