What's in the toolbox? Aphids & other soybean insects

University of Minnesota Extension
Farm Forum

Robert Koch, University of Minnesota associate professor of entomology and Extension entomologist, gives updates on soybean aphid and other insects to keep an eye out for as we head into August.

Soybean aphid continues to be a perennial pest in Minnesota with outbreak severity varying each year. Problem fields can also be scattered across the state, so someone the next county over or even a neighbor treating for aphids is not a guarantee you have an issue in your own field. So far this year, most areas of the state have not seen treatable levels of soybean aphids, but growers should continue to monitor for aphids as soybeans mature.

Koch stressed the importance of scouting to determine if you have enough aphids to consider any insecticide application while discussing the continued validation of soybean aphid economic thresholds. Soybean aphid does not cause economic damage to justify the cost of treatment until approximately 670 aphids per plant on average (i.e., the economic injury level), so the economic or action threshold of 250 average aphids per plant is already a conservative trigger point to prepare an insecticide application in order to prevent populations from reaching 670 aphids. Soybeans can tolerate smaller aphid populations without affecting yield, so treating at levels near 50 to 100 aphids is unlikely to produce any yield benefit.

Koch also discussed tools available to manage soybean aphid. Pyrethroid resistance issues are a concern across Minnesota, and remaining insecticide groups need to be used wisely to prevent the loss of another insecticide group. Research is ongoing in Koch’s lab with other collaborators to examine pyrethroid-resistant populations and deliver other management tools, such as aphid-resistant soybean, to growers.

Other insects such as thistle caterpillar, Japa

nese beetle, and other defoliators have not been a widespread issue this year, but growers should continue to watch for them through August. As with aphids, soybean plants can tolerate what at first glance may appear to be heavy defoliation.

Soybean aphid on susceptible, left, and resistant soybean. Photo by Anthony Hanson