Occasional interspecific competition and within-plant microhabitat preference in egg parasitoids of the bean bug, Riptortus clavatus (Hemiptera: Alydidae) in soybean

We examined the occurrence of interspecific competition among egg parasitoids of Riptortus clavatus (Thunberg) (Hemiptera: Alydidae) in within-plant microhabitats of soybean fields in Fukuoka during the planting seasons from 1985 to 1987. R. clavatus eggs were found mainly on leaves and pods, and rarely on stems of soybean plants. They were parasitized by Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae), Gryon japonicum (Ashmead) and G. nigricorne (Dodd) (Hymenoptera: Scelionidae). During three planting seasons, O. nezarae was more abundant than Gryon spp. O. nezarae parasitized more than 70% of hosts on leaves but less than 50% of those on stems and pods. Gryon spp., especially, G. nigricorne, parasitized more hosts on pods than on leaves or stems. Multiparasitism by O. nezarae and Gryon spp. rarely occurred in hosts from all three parts of the plant. Exploitation competition was more likely to occur in the early part of the season during 1985 and 1986 when overexploitation by O. nezarae was likely to reduce reproduction of Gryon spp. in soybean. Since interspecific competition was infrequent, the different parasitism patterns by O. nezarae and Gryon spp. on various plant parts were probably due to their differences in microhabitat preference.