Different Oviposition Strategies in Two Closely Related Gall Midges (Diptera: Cecidomyiidae): Aggregation versus Risk Spreading

Abstract

The oviposition strategies were compared between two closely related gall midges,Asteralobia sasakiiandA. soyogo(Diptera: Cecidomyiidae). They induce spherical, multilocular galls on the axillary buds ofIlexspecies (Aquifoliaceae).A. sasakiiis known to have a narrower host range and induce larger galls that contain more numerous larvae thanA. soyogo. The largeA. sasakiigalls have been considered to act as physical barriers to protect larvae from ectoparasitoid attack. We observed thatA. sasakiiinduced galls more frequently on the axillary bud at the tip of the shoot, indicating that the females concentrated their eggs at a single oviposition site. In contrast,A. soyogoinduced several small galls in a shoot, scattering their progeny. The number ofA. sasakiigalls per shoot was significantly smaller than that ofA. soyogo. Our current observation, together with the previous data, demonstrates thatA. sasakiilays eggs in clusters to lessen the threat of ectoparasitoid attack (aggregation), whereasA. soyogospread the risk to progeny by scattering eggs in a single shoot using several host plant species (risk spreading). The oviposition strategy ofA. soyogoalso seems to be supported by its diversified life cycles with a polymodal emergence pattern