Extension Ag Update
July/August 2008
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Soybean Aphid Scouting

Ellen Phillips, Extension Educator – Crop Systems, University of Illinois Extension, 708-352-0109, ephillps@uiuc.edu

Keep an eye on population trends of soybean aphids and their natural enemies. Are they increasing or decreasing? Cooler weather (upper 70's and low 80's) can be beneficial to aphid populations which can increase quickly, doubling in 2-4 days.  If temperatures move above 90 degrees then populations may decline.  

Another weather factor that appears to influence aphid population is heavy rainfalls (> 1”), especially early season. A research study utilizing a rainfall simulator showed that after a 2” rainfall on V3-V4 soybeans there was a 45% reduction in aphid populations after 5 days.  The impact of rain decreases as soybean plant increase in size.
Aphid populations are also regulated by natural enemies so monitor these as well when scouting soybean fields. The Multicolored Asian lady beetle, lacewing larvae, syrphid larvae and Minute Pirate Bug are just a few of the beneficial insects in our soybean fields.  To see pictures of these helpers check out the following web pages.

The Good Guys

Soybean Aphid Biological Control

Only repeated scouting will help determine what’s happening to the soybean aphid populations. "Speed scouting" (developed by Univ. of MN) can decrease scouting time for aphids. More information about this method is on website www.soybeans.umn.edu/crop/insects/aphid/aphid_sampling.htm

The threshold for treatment is 250 aphids per plant at the R1 (beginning flowering) through R5 (beginning seed) stage of development when 80 percent of the plants are infested, and natural predators are not controlling aphid population.  The threshold is designed to give about seven days of lead time between scouting and applying insecticides.  Insecticide recommendations can be found in the 2008 Illinois Pest Management Handbook at www.pubsplus.uiuc.edu/IAPM-08.html. For weekly updates on pests and their control look in The Bulletin www.ipm.uiuc.edu/bulletin

Continue scouting after treatment.  If insecticides are applied too early, early-season predators may be killed, allowing soybean aphid populations to rebound later.

For further information about Soybean Aphids, checkout the University of Illinois IPM webpage at http://www.ipm.uiuc.edu/fieldcrops/soybeans/insects.html