Quantifying predation on soybean aphid through direct field observations

The soybean aphid, Aphis glycines (Hemiptera: Aphididae), is the most important insect pest attacking soybeans in North America since 2000. Several studies have documented strong impacts of generalist natural enemies on A. glycines populations using predator exclusion cages and correlating predator and aphid abundances. However, to date no studies directly observed and quantified the natural enemy groups that attack A. glycines in North America under field conditions. In 2005, we conducted 72 h of direct observations of predation on natural populations of A. glycines in Michigan. The observations were conducted during three consecutive weeks during morning, afternoon and evening, in replicated 1 m2 areas. A total of 643 predators within 12 groups and 211 predation events on A. glycines were observed. Transient predators such as Harmonia axyridis and Coccinella septempunctata (Coleoptera: Coccinellidae) accounted for most of the observed mortality and were very effective on a per capita basis, despite relatively short residence time in the observed patches. Transient predators responded positively to increased A. glycines field densities. Resident predators, particularly Orius insidiosus (Hemiptera: Anthocoridae), were abundant and accounted for many predation events, but they were not as effective on a per capita basis and did not respond to changes in aphid density. Lower predation was obtained on A. glycines located at the bottom portion of the plants and on alate aphids. We concluded that coccinellids exerted most of the mortality observed and have the potential to rapidly respond to changes in aphid density with high per capita rates of predation.