Insemination success discrepancy between long-term and short-term copulations in the provisioning shield bug, Parastrachia japonensis (Hemiptera: Cydnidae)

Abstract

The duration of copulation in the gregarious shield bug, Parastrachia japonensis Scott (Hemiptera: Cydnidae), is of two types, the far more prevalent short-term copulation (average, 15 s) and the long-term coupulation (average, 23 min). Both types were thought to be equally effective in inseminating females. Recent evidence has suggested that there is, in fact, a discrepancy in insemination success between the two duration types of copulations. We carried out manipulated field studies to clarify the difference in insemination success between the two duration types and to determine whether there is some physical or physiological variability in females or males that might affect female receptivity to a long-term copulation. The findings indicated that, although a small percentage of short-term copulations resulted in some sperm transfer, long-term copulations were a far more effective way for males to inseminate females. Further, females experiencing long-term copulations were found to be at a slightly more advanced stage of ovarian development than those experiencing only short-term copulations, and may be deciding whether a long-term copulation occurs. Male size does not appear to affect copulation duration. It is concluded that the long-term type of copulation is the actual effective copulation duration in this species and the objective of all females. Possible factors that might contribute to the prevalence of these two copulation durations are discussed.