Winged soybean aphids are beginning to build and show up in the Midwest suction traps network.

See the map on line here. Click on the individual traps to see the counts.

Dave Ragsdale, the entomologist at University of Minnesota, believes that southern Minnesota is on the verge of a major outbreak with many fields now over threshold and temperatures near the optimum for soybean aphid reproduction. Suction traps in Wisconsin are collecting soybean aphids. as well as a trap in Kansas. Dave Voegtlin, the aphid expert from University of Illinois, believes these are migrants, most likely from fields in Southern Minnesota or northern Iowa.

In Michigan, no winged SBA have been caught yet in traps (as of July 18 – new suction trap samples were sent in Friday July 25). However, many fields are infested at a high rate – for example, 80 to 90% of the plants in my fields at the MSU Ent farm on campus are infested, with per plant populations around 20-25 SBA/ plant. When I warn about a ‘trigger’ that could set these populations off in late July or early August, I mean:

• high percentage of plants infested
• lack of predators
• dry conditions, temps at least in the mid to high 80s
• potassium deficient soils

Under optimal conditions, aphids can increase rapidly and be over threshold in a matter of a week. At least walk a few fields to make sure aphids remain low. On the other hand, don’t spray fields as insurance or throw an insecticide in with fungicide or roundup applications just because you are already driving over the field. An insecticide kills predators and can actually be the trigger that turns a small aphid population into an outbreak.

Dr. Christina DiFonzo
Field Crops Entomology Program
Michigan State University

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