If past history is any indication of soybean aphid pressure to be seen in 2007 – be prepared. Not only are soybean aphids more predominant during odd years, but counts of aphids overwintering this year were high, indicating this could be a banner year for soybean aphids.

“Illinois and Minnesota reported particularly high counts of overwintering soybean aphid,” says Travis Kriegshauser, Pioneer Hi-Bred International, Inc., senior research associate in Waterloo, IA. “At this point, it is important to prepare for the possibility of significant soybean aphid populations and anticipate the need to scout for this pest beginning in late June through August.”

To aid in monitoring this pest, a number of state university Web sites and USDA track the movement of aphids. Growers can monitor sites such as http://www.sbrusa.net (select soybean aphid in the upper right-hand corner drop-down menu), and begin scouting when aphids appear in their area.

Identifying soybean aphids

Soybean aphids are small and yellow with distinct black cornicles (“tailpipes”). At only one-sixteenth inch long (the size of a pinhead or smaller), they cannot be distinguished from other aphids with the naked eye. The soybean aphid is the only aphid in North America known to extensively colonize in soybean fields.

Soybean aphids overwinter as eggs on a woody shrub species known as buckthorn. The eggs hatch in the spring into wingless types, which establish on buckthorn for two generations. The third generation emerges, produces wings and migrates to soybean fields and other acceptable hosts.

“The soybean aphid can produce up to 15 generations during the summer on soybeans before migrating back to buckthorn in the fall as winged females,” says Kriegshauser. “Once on buckthorn, the winged females give birth to wingless females, which mate with males developed on soybeans to produce the overwintering eggs.”

Management options

Kriegshauser notes that soybean fields in areas where aphids were detected the previous year are at greatest risk. The northern states bordering the Great Lakes have the highest annual probability of aphid infestations.

“Management decisions regarding soybean aphids are difficult due to the explosive potential of aphid populations and the interaction of aphids with climatic conditions and natural predators,” says Kriegshauser.

Scout carefully to determine if treatment is needed, and time insecticide treatments to maximize their effectiveness. Economic threshold guidelines are currently established at 250 aphids per plant.

“Scout at least 80% of the field,” says Kriegshauser. “Select 20-30 plants in 10 random areas of the field. If you count 250 or more aphids per plant and populations are on the increase, it’s time to spray.”

Pioneer, in collaboration with Kansas State University, also has developed a technique for screening soybean lines for their ability to naturally reduce the rate of growth, survival and reproduction of soybean aphids that feed on soybean plants.

“This type of resistance is called antibiosis,” says Kriegshauser. “Pioneer has characterized current soybean varieties for their aphid antibiosis. The next step is to identify sources of exceptional antibiosis and incorporate this trait into other varieties.”

Today, aphid antibiosis ratings can be used as a management tool to help in determining field scouting priorities and insecticide application decisions.