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Controlling Western Corn Rootworms in Soybeans: Confusion?

August 14, 2003

I recently was told by someone that the University of Illinois's recommendation regarding controlling western corn rootworm adults in soybeans to prevent egg laying was confusing. Please allow me to clear up the confusion: We do not recommend this practice; in fact, we strongly discourage it. I hope this clarifies our position.

It has come to my attention that some "consultants" (I use quotation marks because some of the consultants are pesticide manufacturing company salesmen) are recommending that producers should control western corn rootworm adults in soybeans to prevent them from laying eggs, thereby protecting the corn crop that will be planted next year. However, they also are recommending that the producers use a soil insecticide (granule, liquid, or seed treatment) next year, just in case. That's quite an insurance policy. By using a soil insecticide on corn the next year, there is no way to know whether the insecticide spray to prevent egg laying did any good or not unless the producer leaves an untreated check.

Most of you realize that controlling western corn rootworm adults in corn to prevent egg laying can protect the corn crop the following year. In fact, we have included this management alternative for corn rootworms in our recommendations for several years. (Read "Managing Corn Rootworms," pages 2 to 5 [particularly page 3] in the 2003 Illinois Agricultural Pest Management Handbook for more information.) This practice works because a threshold has been established, timing is understood, and a scouting protocol exists. And if everything is done correctly, no soil insecticide is necessary to protect the corn roots the following year. Prevention of egg laying by rootworm adults is intended to be an alternative management practice, not a supplement to larval control. The information needed to develop a procedure that prevents significant egg laying by rootworm adults was developed over a relatively long period by a large group of university and industry entomologists. The same type of research needs to be conducted in soybeans, with corn planted the following year, before we could recommend controlling corn rootworm adults in soybeans to prevent egg laying. Thus far, this type of research has not been conducted.

One threshold being used (by "consultants") for recommending insecticides to prevent western corn rootworm adults in soybeans to prevent egg laying is 0.2 to 0.3 adult per sweep of a 15-inch diameter sweep net. Interesting. I have yet to learn where that threshold came from. In addition, I am curious about timing of such insecticide applications. According to Joe Spencer, research entomologist at the Illinois Natural History Survey, western corn rootworm adults fly back and forth from cornfields and soybean fields for a relatively long period. Consequently, timing an insecticide application to prevent peak egg laying is guesswork at best.

I have been told that there are data to support insecticide applications to control western corn rootworm adults in soybeans to prevent egg laying. However, I have not seen these data. Therefore, I invite anyone who has such data to share them with me. I am willing to examine the data objectively and would be very interested in learning whether this alternative for rootworm management has any validity.

One final word of caution. The practice of controlling western corn rootworm adults in soybeans one year and then controlling the larvae in corn the following year is loaded with risk. This practice places a lot of selection pressure on the rootworm population, and the end result (the possibility for development of insecticide resistance) would leave many producers with few viable alternatives. So think about this before you make the decision to control adults in soybeans one year and then control larvae in corn the following year. Corn rootworms have adapted to every type of selection pressure we have placed on them (including crop rotation), so there's plenty of reason to believe they will continue to adapt.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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