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Commentary on Corn Rootworm Management in Soybeans

July 26, 2002
On July 23, a research team from the University of Illinois evaluated rootworm larval injury in experimental plots located near Monmouth and DeKalb, Illinois. The level of root pruning was severe, particularly in the DeKalb study, with plants stunted and under severe moisture stress. On July 25, similar treatments (soil insecticides and seed treatments) will be evaluated at our Urbana site. We hope to share the results of these trials in August.

While in DeKalb, we learned that producers in nearby counties were becoming increasingly concerned about the level of first-year corn rootworm injury they were experiencing in recent years. Some have inquired whether or not it makes economic sense to treat western corn rootworm adults in soybean fields in an attempt to prevent egg laying. By treating soybean fields, they hope to eliminate the need for a soil insecticide the following spring in first-year cornfields. As readers of the Bulletin should know by now, we do not support this management tactic for the western corn rootworm variant.

Our research has shown that egg laying in soybeans occurs for a very long period (late July through August). Repeated broadcast insecticide treatments to prevent egg laying would not be cost-effective. Because of the mobility of western corn rootworm adults, many treated soybean fields would be successfully invaded again and again following the degradation of the insecticide treatment on soybean foliage. Insecticides applied to soybean fields are not selective, and many beneficial insects also would be killed by these treatments. We've been fortunate this summer (so far) that soybean aphid densities have not exploded in Illinois. The hot and dry weather may be largely responsible for this development; however, natural enemies have played a role in reducing densities of soybean aphids. Let's enhance our natural enemy populations, not reduce them by applying nonselective insecticides in a futile attempt to reduce western corn rootworm egg laying in soybean fields.

A final note on this topic is worth mentioning. We have discussed (many times) how to sample soybean fields for the western corn rootworm variant and also how to utilize an economic threshold in making decisions regarding whether or not to use a soil insecticide during planting. Our approach involves the use of 12 Pherocon AM traps (evenly deployed) across a soybean field. The traps should be placed in soybean fields beginning the last week of July. All 12 traps should be changed on a weekly basis, and sampling should continue through the first 3 weeks of August. A trap capture average should be calculated at the conclusion of the monitoring program.

If five adults per trap per day are caught, it suggests that sufficient egg laying may have occurred to cause an average root injury rating of 3.0 (some limited root pruning) to corn (left untreated) the following season. If 10 adults per trap per day are caught, severe root pruning (one node of roots destroyed) may occur to corn left untreated (no soil insecticide used) next spring.

To date, I am unaware of any thresholds that have been developed and critically reviewed for sweep net counts in soybean fields. So, please be very wary of any management advice that relies on sweep net thresholds. Keep in mind, sweep net captures vary considerably among individual people and also are dependent on the time of day in which sweeping occurs. Thus, we have chosen to rely on Pherocon AM traps as a more reliable sampling approach for the western corn rootworm variant in soybean fields.

One last piece of advice: don't wait until September or October to begin asking whether or not you should expect first-year corn rootworm injury in 2003. Take the time now to monitor your soybean fields.--Mike Gray

Pherocon AM trap in soybean field.

Author: Mike Gray

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

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