Age-dependent male ejaculate investment in the West Indian sweet potato weevil, Euscepes postfasciatus

The sterile insect technique (SIT) is a method of biological control of pest insects. The efficiency of SIT depends on the ability of released sterile males to mate with and inseminate wild females. Males usually spike the numerous other seminal substances with their ejaculate and transfer them to females during mating. These substances can control female remating propensity or reproductive output. In this study, we examine the effects of male age and mating history on the number of transferred sperm and the degree of suppression of female remating propensity by the sequential mating experiment in the West Indian sweet potato weevil, Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae). This weevil is a major pest of sweet potato, Ipomoea batatas (L.) Lam. (Convolvulaceae), in the tropics and subtropics, including Japan, and an eradication program using the SIT for this weevil is in progress in Japan. Our results showed that female remating propensity was more strongly suppressed with male aging but not with male mating history. In contrast, the number of transferred sperms decreased significantly with male mating history but not with male aging: the number of transferred sperms was constant, regardless of the males’ age. These evidences suggest that male weevils allocate their investment more in seminal substances with aging than sperms. Since the release of sterile males at an adequate age would decrease the risk of sperm competition and enhance the utilization of the sterile sperm, the SIT should consider the maturation of the seminal substances in the target species.