Relationships amongst sex ratio of progeny in Anastatus disparis (Hymenoptera: Eupelmidae), sperm depletion and decreased fecundity

Abstract

Sex allocation in haplodiploid arthropods has implications for fitness and has received much attention with regard to parasitoid wasps. In addition to environmental variables, maternal characteristics (such as age) are important factors that shape sex allocation. Despite this, maternal characteristics have received less research attention. Older females may experience an increase, a decrease, or no change in the sex ratio [male/(female\u2009+\u2009male)] of their offspring compared with younger females. Using mated Anastatus disparis (Ruschka) (Hymenoptera: Eupelmidae), a quasi-gregarious parasitoid, we determined experimentally the sex of offspring in relation to the age of their mother. As most studies predicted, the result showed that, as maternal A. disparis grew older, the sex ratio of offspring increased. However, most females continued to produce offspring of both sexes until their death. Sperm depletion was a likely reason for the increase in numbers of male offspring in response to maternal age. However, both fecundity and the number of male offspring declined overall in the whole colony, suggesting that the increase in sex ratio amongst offspring was mainly due to decreased fecundity in older maternal A. disparis. Overall, both sperm depletion and decreased fecundity played a role in determining the sex ratio of progeny from older mothers.