Soybean Aphids

Urbana-Champaign, Illinois (August 21, 2008)--The number of soybean aphids has increased dramatically during the past week in certain northern Illinois soybean fields. If not done previously, now is the time to sample or monitor fields to determine aphid population. Don't let potential rapid increases in soybean aphid numbers go unnoticed.

The continued moderate temperatures and a relative lack of natural enemies currently in soybean fields will benefit population growth of soybean aphids, notes Kevin Steffey, Extension entomologist.

The insect, first discovered in Wisconsin in 2000, is yellow and about one-sixteenth inch long. Colonies of soybean aphid can be found on the stem and young leaves of growing plants and on the undersides of older plants. They have a very complicated life cycle and buckthorn serves as the overwintering host.

Insecticides applied too early may kill early-season predators, allowing soybean aphid populations to increase later. Entomologists feel that weather and natural enemies play a big role in regulating soybean aphid populations. High temperatures and many predators will suppress soybean aphids. Cooler temperatures (low 70's to mid 80's) and low numbers of predation establish the potential for soybean aphid problems. Soybean aphid populations can double within two to four days, if temperatures are between 70 and 86 degrees F.

Extension entomologists in the north central region indicate an action threshold of 250 or more soybean aphids per plant at the R1 (beginning flowering) through R5 (beginning seed) stage of development, 80 percent of the plants are infested, and natural predators are not suppressing aphid population. According to Steffey, this threshold incorporates an approximate seven day lead-time between scouting and treatment to make insecticide application arrangements and handle weather delays.

Soybean aphid numbers from weekly samples in 26 commercial fields from Woodford County to Stephenson County can be found at this University of Illinois web site, www.ipm.uiuc.edu.

Bottom line - watch soybean fields closely to determine if soybean aphid numbers, and natural enemies, are increasing, remaining stable, or decreasing.

Further information is available at University of Illinois Extension offices.

Contact:
Jim Morrison
Extension Educator, Crop Systems
morrison@uiuc.edu