Effect of variation in host size on sex ratio, size, and survival of Syngaster lepidus, a parasitoid of Eucalyptus longhorned beetles (Phoracantha spp.): II

Abstract

Larvae of the cerambycid beetle Phoracantha recurva Newman colonize the inner bark of stressed or dying Eucalyptus trees. These cryptic larvae are parasitized by the solitary ectoparasitoid Syngaster lepidus Brullé. When denied a choice of host sizes in a previous study, parasitoid females produced only male progeny in 2 week old larvae, but when given access to only large larvae, the sex ratio of the progeny was significantly female biased. When female wasps were simultaneously presented with a range of host sizes in the study presented here, the sex ratio was significantly male biased for wasps produced from the smallest host class (2 weeks old), but was not different among the 3, 4, or 5 week old hosts. The sex ratio of progeny remained constant throughout the life of the ovipositing females. Fertility peaked during the second week of adult life, then declined to zero by the fourth week, with females producing about 22 progeny during their lifetimes. Longevity of females was significantly greater than that of males. Significantly more progeny were produced from 3 week old hosts than from any other age class. There was no difference in the number of progeny emerging from 2 or 4 week old hosts, but significantly fewer progeny were produced from 5 week old hosts. The size of emerging wasps increased with host size for both males and females. In Australia, a complex of parasitoids, uses Phoracantha larvae as hosts. S. lepidus prefers to use younger, smaller larvae, while other members of the parasitoid guild exploit larger larval hosts. The results from this study suggest tradeoffs between sex allocation and percent survival in different host size classes, and reflect the observations from previous field studies of this host–parasitoid complex in Australia.