Inoculation of refrigerated non-viable eggs of Riptortus clavatus (Heteroptera: Alydidae) to enhance parasitism by egg parasitoids in soybean field

Abstract

Riptortus clavatus Thunberg (Heteroptera: Alydidae) is an important soybean pest in Korea and Japan. Chemical application is a common control method against R. clavatus although it is not always effective at reducing crop damage. As a biological control, the release of refrigerated stink bug eggs has been suggested because the eggs become non-viable but can still be parasitized by parasitoids. We evaluated the inoculation of refrigerated eggs of R. clavatus as a method to enhance natural parasitism. Overall, 9,000–10,600 refrigerated eggs of R. clavatus were inoculated in soybean fields. A higher seasonal parasitism rate on both natural and released host eggs was found at the treated sites. Gregarious parasitoids, including Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae) and solitary Gryon japonicum Ashmead (Hymenoptera: Scelionidae), emerged from natural eggs. On the released eggs, the parasitism rate by gregarious parasitoids was greatly reduced for an unknown reason. The proportion of bean pods with stylet sheaths as an index of crop damage was also significantly reduced; however, neither the seasonal number of unparasitized eggs nor nymphs and adults of R. clavatus was affected by host egg inoculation. This is the first trial to inoculate non-viable host eggs to enhance natural parasitism.