Predators suppress Aphis glycines matsumura population growth in soybean

The soybean aphid, Aphis glycines Matsumura, is an invasive pest of soybean, first discovered in North America in 2000. We studied the ability of the existing predator community in soybean to suppress A. glycines population growth during June-August 2002, in field studies using predator exclusion and sham cages or no-cage controls. Cages were infested with uniform initial densities of A. glycines adults, and subsequent populations of aphids and predators were monitored. After 2 wk, exclusion and sham cages were switched, with aphid and predator density followed for additional 2 wk. The experiment was repeated a second time, allowing observation of predator community response to both low and high density aphid populations over several time periods and stages of soybean development. Cages had minimal effects on temperature, relative humidity, or soybean growth. In contrast, predator communities and aphid populations were strongly affected by cage treatments. In the first trial, the activity of foliar-foraging predators effectively prevented A. glycines population growth maintaining populations below 10 aphids per plant (adult + nymphs) ill sham cages, while populations exceeded 200 aphids per plant in exclusion cages. After cage switch, these high A. glycines populations in the former exclusion cages were rapidly colonized and reduced by nearly an order of magnitude within 2 wk by a combination of generalist and specialist predators. The second trial produced qualitatively similar results, but at much lower aphid densities. The most abundant predators in both trials included: Harmonia axyridis Pallas, Orius insidiosus (Say), and Leucopis spp. These studies demonstrate that existing predator communities comprised of a mixture of indigenous and naturalized species can suppress A. glycines population density in soybean. The impact of existing predator communities should be further investigated as a component of A. glycines management in United States soybean production systems.