Do sterile males and females synergistically suppress pests targeted by the sterile insect technique?

The sterile insect technique (SIT) is a pest control method that releases a large number of sterile pests into the field to inhibit mating between wild pests, ultimately resulting in the eradication of wild pest populations. It has often been discussed whether releasing one sex (mainly males) of sterile insects enhances the pest control efficiency of SIT compared to releasing both sexes (bisexual release). The control efficiency of sterile females may be negatively affected by their release, because sterile males lose the opportunity to mate with wild females. In this study, we investigated whether sterile females have a positive effect on the control efficiency of SIT and whether there is an additive or synergistic effect of releasing both sexes using two weevil species, Cylas formicarius and Euscepes postfasciatus, which are serious pests of the sweet potato Ipomoea batatas and targets of SIT in Okinawa. The results showed that in both species, the reproduction rate of wild pests was significantly suppressed in the case of the sterile female-only release. Interestingly, the most effective method varied between male-only and bisexual release depending on the density of sterile individuals and weevil species. Synergistic effects of both sexes on bisexual release were observed but varied with the density of sterile individuals and weevil species. High-density bisexual release was most effective for the SIT of at least two weevil species.