Inventory and assessment of foliar natural enemies of the soybean aphid (Hemiptera: Aphididae) in South Dakota

Authors: Hesler, Louis

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/14/2014
Publication Date: 6/1/2014
Citation: Hesler, L.S. 2014. Inventory and assessment of foliar natural enemies of the soybean aphid (Hemiptera: Aphididae) in South Dakota. Environmental Entomology. 43:577-588.

Interpretive Summary: Soybean aphid is a major pest of soybean in northern production regions of North America, and insecticides are the main method of controlling outbreaks of the aphid. While they have been crucial in controlling soybean aphid and preserving soybean yield, alternative control methods are needed to lessen the effects of insecticides on human health and the environment and reduce the risk of soybean aphid developing resistance to insecticide. Knowing more about soybean aphid’s natural enemies and their impact is critical for biological control to become an effective alternative management strategy. Although soybean is a major field crop in South Dakota, knowledge about its natural enemies and their impact on soybean aphid is lacking. This study was conducted in field plots in eastern South Dakota during July and August of 2004 and 2005 to identify the natural enemies of soybean aphid in open field plots. It also used methods to exclude natural enemies and ants, and thereby determined their impact on resulting soybean aphid population levels. In the open field plots, soybean aphid densities reached a plateau of several hundred aphids per plant in 2004, and peaked at roughly 400 aphids per plant in 2005. Despite such densities, a relatively high frequency of aphid-infested plants lacked any arthropod natural enemies. The abundances of arachnids, green lacewings, and immature lady beetles were correlated with soybean aphid densities. Caging tests showed that arthropod natural enemies reduced soybean aphid levels in only one of two years. Excluding ants from soybean plants made no difference in soybean aphid levels. Overall, the results suggest that there is room for more natural enemies to exploit soybean aphids and enhance biological control to make it an effective management strategy in South Dakota.

Technical Abstract: Soybean aphid (Aphis glycines Matsumura) (Hemiptera: Aphididae) is a major pest of soybean in northern production regions of North America, and insecticides have been the primary management approach while alternative methods are developed. Knowledge of arthropod natural enemies and their impact on soybean aphid is critical for developing biological control as a management tool. Soybean is a major field crop in South Dakota, but information about its natural enemies and their impact on soybean aphid is lacking. Thus, this study was conducted in field plots in eastern South Dakota during July and August of 2004 and 2005 to characterize foliar-dwelling, arthropod natural enemies of soybean aphid, and it used exclusion techniques to determine impact of natural enemies and ants (Hymenoptera: Formicidae) on soybean aphid densities. In open field plots, weekly soybean aphid densities reached a plateau of several hundred aphids per plant in 2004, and peaked at roughly 400 aphids per plant in 2005. Despite these densities, a relatively high frequency of aphid-infested plants lacked arthropod natural enemies. Lady beetles (Coleoptera: Coccinellidae) were most abundant, peaking at 90 and 52 percent of all natural enemies sampled in respective years, and Harmonia axyridis Pallas was the most abundant lady beetle. Green lacewings (Neuroptera: Chrysopidae) were abundant in 2005, due mainly to large numbers of their eggs. Abundances of arachnids and coccinellid larvae correlated with soybean aphid densities each year, and chrysopid egg abundance was correlated with aphid density in 2005. Three-week cage treatments of artificially infested soybean plants in 2004 showed that non-caged plants had fewer soybean aphids than caged plants, but abundance of soybean aphid did not differ among open cages and ones that provided partial or total exclusion of natural enemies. In 2005, plants within open cages had fewer soybean aphids than those within cages that excluded natural enemies, and aphid density on open-cage plants did not differ from that on non-caged plants and those accessible by small predators. In a separate 3-yr experiment, exclusion of ants from soybean plants did not lead to differences in soybean aphid density compared with ant-accessible plants. Overall, these results suggest that the soybean-aphid natural enemy guild is unsaturated and could be enhanced to improve biological control of soybean aphid in South Dakota.