University of Illinois

No. 21/August 15, 1997

Controlling Rootworm Adults in Soybeans to Prevent Egg Laying

We have become aware that some growers in east-central Illinois are having their soybean fields treated with insecticides to prevent western corn rootworm adults from laying eggs in the soybeans to "protect" next year's corn crop. Furthermore, some dealers and/or consultants are recommending the practice. I don't think I can state our response any more clearly than this: If a grower is having his or her soybeans treated for control of corn rootworms, or if a dealer or consultant is recommending this practice, he or she is making a mistake. As we have stated several times already, a threshold for this practice has not been developed; the best we'll be able to offer is a preliminary, tentative threshold based upon the work done in 1996 and 1997. In addition, timing of insecticide applications to prevent western corn rootworm adults from laying eggs in soybeans is not known. Treating for western corn rootworms in soybeans is a knee-jerk reaction to a challenging problem.

Eventually, we may have some guidelines for controlling western corn rootworms in soybeans to prevent them from laying eggs. Several university and private researchers are working on this aspect of rootworm management. However, right now it's just research; the data have not been analyzed, and the results are not conclusive. If a dealer or consultant recommends the practice of controlling rootworms to prevent them from laying eggs in soybeans, he or she takes responsibility for the program's effectiveness next year when rootworm larvae begin to feed. What if the practice doesn't work?

Another scenario that's even more frightening is that some fields of soybeans will be treated to control western corn rootworms this year, and the grower has every intention of applying a soil insecticide next when he or she plants corn. Again, I can't be more blunt: We will not condone this practice. Treating two different life stages of an insect with an insecticide is one of the surest ways to accelerate the development of insecticide resistance. In east-central Illinois where crop rotation does not seem to work for management of rootworms, what option will growers have if rootworms become resistant to insecticides? We would be out of options, folks; and the consequences could be significant. You are already aware that western corn rootworms have developed resistance to methyl parathion and carbaryl in Nebraska; it can happen here, too.

Before you try everything available to control rootworms, think about it first. Don't simply jump onto the first treadmill that rolls by. Be smart when you plan a rootworm management program. Don't make the problem worse than it already is.
Kevin Steffey and Mike Gray, Extension Entomology, (217)333-6652