Density-dependent responses of soybean aphid (Aphis glycines Matsumura) populations to generalist predators in mid to late season soybean fields

Since the arrival of the soybean aphid, Aphis glycines, to the U.S. in 2000, year to year abundances have varied substantially. Although, field studies have suggested that several resident generalist predators have the ability to suppress A. glycines populations, factors responsible for its population dynamics remain unclear. For natural enemies to regulate herbivore populations, predation rates must increase as prey population densities increase (i.e., density dependence). Experiments were conducted in 2004–2006 to assess the ability of the existing natural enemy community to exert density-dependent effects on A. glycines populations. Aphid densities were manipulated on individual plants (transplanted to the field) or in small plots to contain a range of aphid densities from low (10 per plant) to high (>1000 per plant). Populations were then monitored weekly. Caged controls were included to measure population growth in the absence of predators. In 2006, an additional treatment was included to quantify the proportion of aphids emigrating from plants. In all experiments, a strong density-dependent decline in population abundance was observed as aphid populations on plants with low initial densities (<50 aphids per plant) tended to increase while plants with high initial densities (>100 aphids per plant) decreased by as much as two orders of magnitude over a 1–2 week period. We estimate that fewer than 5% of aphids emigrated from plants when aphid densities were below 4000 per plant. This suggests that the extreme declines in aphid populations over these brief experiments were not due to aphids leaving plants, but rather, due to an aggregative response of predators to high density prey populations. Given the rapidity and degree of density-dependent population decline in three consecutive years of study, we suggest the resident community of natural enemies in Midwestern soybean may have great potential to regulate soybean aphid populations. 2007 Elsevier Inc. All rights reserved.