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

Summary

The oviposition strategies were compared between two closely related gall midges, Asteralobia sasakii and A. soyogo (Diptera: Cecidomyiidae). They induce spherical, multilocular galls on the axillary buds of Ilex species (Aquifoliaceae). A. sasakii is known to have a narrower host range and induce larger galls that contain more numerous larvae than A. soyogo. The large A. sasakii galls have been considered to act as physical barriers to protect larvae from ectoparasitoid attack. We observed that A. sasakii induced 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. soyogo induced several small galls in a shoot, scattering their progeny. The number of A. sasakii galls per shoot was significantly smaller than that of A. soyogo. Our current observation, together with the previous data, demonstrates that A. sasakii lays eggs in clusters to lessen the threat of ectoparasitoid attack (aggregation), whereas A. soyogo spread the risk to progeny by scattering eggs in a single shoot using several host plant species (risk spreading). The oviposition strategy of A. soyogo also seems to be supported by its diversified life cycles with a polymodal emergence pattern.