The Effect of Host Size on Sex Ratio in the Ichneumonid Wasp, Trychosis cyperia, a Spider Parasitoid

Abstract

-Since parasitoid wasps can determine the sex of their offspring, they may use this ability to adjust sex in response to differences in the size of their hosts. To test this hypothesis, I measured the size, sex and development times of emerging ichneumonid wasps (Trychosis cyperia Townes), whose larvae are parasitoids of the eggs of crab spiders Misumena vatia (Araneae: Thomisidae), with a single larva per spider clutch. Over 70% of the emerging wasps were females. Host size was positively correlated with size of female, but not male, cocoons and adults. Spider clutches producing male wasps and clutches producing female wasps did not differ in size, but the resulting female wasps were larger than the males. In this sense females used host tissue more efficiently than did males. Probable parental laying dates, emergence dates, and development times of males and females collected in the field but reared in the laboratory did not differ significantly.