In a year full of surprises what is one more? Soybean aphids are now showing up in unexpected places. I can only remember once previously when soybean aphids were found in any noticeable numbers as far south as Champaign. They have predominantly been a pest of the northern Illinois counties and the states to the north. However this year aphids have shown up in huge numbers along the I-70 corridor as well as all across north-central Illinois.

The soybean aphid was first noticed in Illinois in 2000. Prior to 2000 this pest had never been seen in the Corn Belt. Its arrival caused quite a stir in the agricultural community, especially in Northern Illinois and Wisconsin. By the end of the 2000 growing season small populations of aphids could be found throughout the state of Illinois.

This aphid over-winters as eggs on a woody shrub called buckthorn. This plant is fairly common in fencerows and scrub areas. Some buckthorn species have even been used in landscaping. In the spring the aphids hatch out as a winged aphid and fly to soybean fields. They mate and produce a wingless population of aphids. This wingless population can build rapidly. Under certain conditions the population can double every 2-3 days. These aphids damage the plants by sucking the plant sap and stealing needed moisture and nutrients from the plant. This added stress causes pods to abort and seed size to be reduced. They can cause secondary problems, the plants get coated with a sticky residue that is a good growth medium for a black fungus called sooty mold. This layer of "goo" can actually reduce the amount of sunlight reaching the leaves. Early infestations cause yield losses averaging over 40% in one study in Minnesota.

When the population reaches a certain threshold the wingless aphids will produce a winged generation that will fly off and look for new fields to infest.

Because of the speed with which this pest can develop, good scouting is critical. The economic threshold for soybean aphids is 250 aphids per plant (field average) with 80% of the plants infested. This conservative economic threshold allows for time to make the decision and arrange for an insecticide application. Spraying after the R6 growth stage generally does not provide any economic return.

Now is the really critical time for making a good decision on spraying. If I was scouting your field the first thing I would determine is the growth stage. R6 is also called "full seed" this is determined when a green seed entirely fills the pod cavity at one of the top 4 nodes of the plant. If the field is R6 or later then forget about aphids. Prior to R6 I would scout the field selecting plants at random and counting for aphids. The previous stage R5, called "beginning seed," and is determined when a seed reaches 1/8 inch diameter in the top four nodes. If the threshold of greater than 250 aphids per plant is reached I would consider spraying with a pyrethroid family insecticide. The Pyrethroids (Warrior, Mustang Max, Asana, Baythroid) have a long residual, and work very well at temperatures below 90°F.