University of Illinois

No. 17/July 16, 1998

Treating Soybeans to Prevent Western Corn Rootworm Egg Laying: Confusion

On July 13, we began our on-farm "root digs" across east-central Illinois. Thus far, we have evaluated root injury in treated and untreated strips in 12 growers' fields from the following counties: Iroquois, Grundy, Kankakee, Livingston, and Vermilion. Although we have yet to wash and rate the roots for injury, we observed lodging in many of the fields. So far, we have not observed impressive densities of western corn rootworm beetles in first-year cornfields as were commonplace last year. Perhaps the wet spring reduced survivorship of the larvae in many saturated fields. Hopefully, by next week we can share some of the actual root ratings from our on-farm trials.

We have received a disturbing number of inquiries regarding the treatment of soybean fields for western corn rootworm beetles to prevent egg laying. Our thoughts on this topic were made very clear in many of our articles in last year's Bulletin. Not much has changed since then. The following questions and answers are designed to clarify the confusion on this controversial subject.

What's the threshold for treating western corn rootworm beetles in soybeans?There isn't a threshold for treating western corn rootworm beetles in soybeans.

Can't I just use the threshold designed with sticky traps to trigger an insecticide application to my soybean field to suppress egg laying? No. The threshold discussed in issue no. 15 of this Bulletin was designed to assist growers in making decisions regarding the use of a soil insecticide the following spring. Much research remains before we can determine when peak egg laying occurs in soybean fields. Treating soybean fields with insecticides too early or too late may result in unsatisfactory root protection next year.Although a good deal of research has been conducted on the timing of egg laying in cornfields within a continuous corn-production system, the same cannot be said of soybeans.

Because of the widespread soil insecticide failures last year, and the scattered reports this season, I've heard that it makes some sense to spray beetles in soybeans and also use a soil insecticide at planting next spring. Is this a good pest management practice? No. First of all, this fails to make any economic sense. Secondly, with the considerable concern being focused on the organophosphate class of insecticides (Food Quality Protection Act), from an overall agricultural and public-policy vantage point, this approach appearsvery risky indeed. From a resistance-management perspective, the use of insecticide applications aimed at adults (this year in soybeans) and larvae (next spring at planting) is a recipe for disaster. We've been down this road before with corn rootworms. In the 1950s and 1960s, corn rootworms were treated in some fields twice each season with broadcast chlorinated-hydrocarbon insecticides. Resistance soon ensued and spread across the Corn Belt in short order. Currently, resistance to methyl parathion (Penncap-M) andcarbaryl (Sevin) has been confirmed in several locations in Nebraska. Entomologists are now concerned over the possible development of cross resistance to the soil insecticides. Bottom line--two insecticide treatments aimed at preventing larval injury is a bad approach.

Does the idea of treating soybeans fields to suppress egg laying have any potential merit? Yes. Last year, many soybean fields and first-year cornfields were treated with SLAM (insecticidal bait containing cucurbitacins and carbaryl) and eventually Sevin XLR Plus in an areawide research project just south of Sheldon, Illinois. The research effort is a United States Department of Agriculture funded project that involves cooperation among USDA, Midwest Consulting Service, Purdue University, and University of Illinois personnel. Overall, 45 producers responsible for the production of about 12,000 acres of corn and soybeans are involved in this large experiment. Last season, 40, 21, and 6 soybean fields were treated once, twice, and three times, respectively, with SLAM to prevent egg laying by western corn rootworm adults. Due to resurging densities of beetles, many soybean fields also were eventually treated with Sevin XLR Plus. This summer we intend to dig roots from those fields treated last year to judge the merits of this approach to rootworm management. Bottom line--the treatment of soybean fields to prevent egg laying has potential merit; however, more research is clearly warranted on this topic.

Can rootworm beetles be managed in cornfields to prevent egg laying? Yes. This technique has been used popularly in many areas of the Corn Belt, particularly in some western states. Long-term use of this management strategy does increase the selection pressure for development of insecticide resistance. Please refer to details in the following article regarding this rootworm management approach.

Mike Gray ( and Kevin Steffey (, Extension Entomology, (217)333-6652