Effects of host size and laboratory rearing on offspring development and sex ratio in the solitary parasitoid Agrothereutes lanceolatus (Hymenoptera: Ichneumonidae)

Agrothereutes lanceolatus is a solitary ectoparasitoid attacking mature larvae, prepupae and pupae of tortricid and pyralid moths, including Homona magnanima, which is an important pest of tea. Adult A. lanceolatus are sexually dimorphic and vary considerably in size in the wild, suggesting they parasitize a variety of hosts of different sizes. Here, the effects of host size on offspring survival, size and sex ratio in A. lanceolatus were investigated. Wild-caught and laboratory-reared parasitoids were used to identify the effects of laboratory rearing on the above three biological parameters. The percentage of hosts producing parasitoid offspring was not dependent on host size regardless of parasitoid generation. Offspring survival was lower in laboratory-reared generations than the F1 generation. Large parasitoids emerged from large hosts. The offspring sex ratio (proportion of females emerging) was positively correlated with host size. The sex ratio at the egg stage (= primary sex ratio) inferred from maternal fertilization act also changed in response to host size, although there was imperfect control of female egg deposition. Thus, the observed sex-ratio resulted from maternal control of offspring sex ratio. A striking difference was detected in the offspring sex ratio in the different generations. Offspring sex ratio was more male-biased in laboratory reared generations. These results suggest that inbreeding depression caused a reduction in offspring survival and the male-biased sex ratio in the laboratory. The sex-ratio difference could result either from the difference in host size in the field and laboratory or from the production of diploid males in the laboratory.