Soybean aphid (Aphis glycines Matsumura) is an invasive insect pest from Asia, which was first detected in North America in 2000. In the US, natural enemy communities of A. glycines are primarily dominated by predators, with relatively low abundance and diversity of parasitoids. During 2004 and 2005, we conducted a field experiment to evaluate the joint impact of predators and parasitoids on A. glycines populations and to examine the role of predators in limiting parasitoid impacts via intraguild predation (IGP). The experiments were performed using natural parasitoid populations in the Biodiversity Plots of the KBS-LTER site, Michigan. Using tomato cage frames covered in mesh we differentially excluded all natural enemies (< 1 mm mesh) or only large predators (2 mm mesh), thus allowing natural parasitism to occur. The effect of the complete assemblage of natural enemies was tested using un-caged plants and plants enclosed with sham cages, with plants caged without aphids and totally un-manipulated plants serving as controls. Soybean aphid populations were regulated throughout the season by the complete assemblage of natural enemies, resulting in a trophic cascade that increased plant height, biomass and yield. Small size predators and parasitoids delayed peak aphid populations for approximately two weeks but did not prevent yield loss (42%). Parasitism was higher within cages providing refuge from IGP by large predators, with up to 30% IGP in un-protected treatments. However, because parasitoid impacts were low even in the absence of large predators, IGP did not prove important in limiting A. glycines parasitism.