The soybean crop is looking almost as good as the corn crop, but there are a couple of very dark clouds still on the horizon that farmers are nervously watching. Soybean rust and soybean aphids are on the move.

After a slightly slow start, soybean rust has been has now become very active in Texas and Louisiana. With new detections is Arkansas and Oklahoma last week pushing the disease even farther north. These infestations are more worrisome than the Florida, Georgia and Alabama detections have become almost common over the last two years. It is more likely that prevailing weather patterns will bring viable spores from these new areas.

Closer to home, soybean aphids are approaching economic levels in northern-western and northern Illinois counties. A quick survey of soybean fields in Mclean, DeWitt, Macon, Piatt and Champaign counties last week indicated aphids are present but at still very low levels.

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, the economic injury level (generally when the cost of control equals the cost of the yield loss) is slightly greater than 600 aphids per plant. This conservative economic threshold allows for time to make the decision and arrange for an insecticide application.