Sex ratios in field populations of two parasitoids (Hymenoptera: Chalcidoidea) of Coccus hesperidum L. (Homoptera: Coccidae)

摘要

Several assumptions and predictions of host-quality-dependent sex allocation theory were studied with data obtained for the parasitoid Metaphycus stanleyi on its host, brown soft scale (Coccus hesperidum) in a California citrus grove and in the laboratory. Scales ceased growing after parasitization by M. stanleyi. Thus, parasitoid fitness gains with host size and adult size were similar in males compared to females. Females consistently emerged from larger hosts than males. Mean host sizes of females versus males, and of solitary versus gregarious parasitoids, varied with the available host size distribution. Only females emerged from hosts in the upper size range, and a variable ratio of males and females emerged from hosts in the lower size range. It was concluded that the sex ratio of field populations of M. stanleyi is driven largely by the available size distribution of C. hesperidum. In addition, predictions resulting from theoretical analyses of sex allocation in autoparasitoids were tested with data obtained on Coccophagus semicircularis parasitizing brown soft scale in the field. The sex ratio of C. semicircularis was consistently and strongly female biased (ca. 90% females). Based on available theoretical analyses, it is suggested thatthis sex ratio pattern may have resulted from a very low encounter rate of secondary hosts coupled with a strong time limitation in C. semicircularis females. This explanation was the most plausible, given constraints stemming from the detection of secondary hosts, their variable location within primary hosts, and their handling times. Finally, the size of hosts which yielded single versus multiple parasitoids, and the sizes of these parasitoids, were compared. These comparisons suggested that: (1) M.stanleyi females gauge host sizes precisely, and in terms of female offspring; thus a fitness penalty is not incurred by females which share a host, while males benefit from sharing a host, and; (2) instances where multiple C. semicircularis emerged froma single host were probably the result of parasitism by different females, or during different encounters by a single female.