SOYBEAN STEM FLY, MELANAGROMYZA-SOJAE (DIPTERA, AGROMYZIDAE), ON SUMATRA - SEASONAL INCIDENCE AND THE ROLE OF PARASITISM

The density of M. sojae and percentage parasitism was studied in unspr ayed soybean at 18 farmers' field sites in Aceh, North Sumatra and Wes t Sumatra provinces, Indonesia, during 1992 and 1993. M. sojae general ly infested soybean throughout the season: infestation was initially l ow, reached its peak from the fifth till the eighth week after plantin g and declined towards the end of the season. A complex of seven hymen opterous parasitoids had a high impact on the pest. Parasitism levels bulk up alongside with host density and remained high until just befor e harvest. The eucoilid Gronotoma sp. was the most prevalent species d uring the early and mid-season; a complex of pteromalids became domina nt towards the end of the season. In soybean planted successively at 2 -week intervals, the parasitism level in the first-planted crop built up slowly, whereas the parasitism level in the third-planted crop was high from the early crop stage onwards, which suggests that the third- planted crop benefited from parasitism build-up in the earlier-planted crops. The role of parasitoids in controlling beanflies in unsprayed soybean is discussed.