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Issue No. 1, Article 9/March 18, 2004

Development of Economic Thresholds: Show Us the Science

Soon after the variant western corn rootworm began to wreak havoc on first-year cornfields across east-central Illinois in 1995, we began plans to develop a sampling protocol and an economic threshold for producers to use in predicting the likelihood of economic larval infestations in rotated corn. We were fortunate to have the support of C-FAR funding and the generous cooperation of many producers. After several years of on-farm re-search that involved a large team of entomologists, graduate students, producers and Extension educators, we developed an economic threshold that was based on the deployment of yellow sticky traps (Pherocon AM traps) in soybean fields. We submitted the results of our research to the scrutiny of other scientists through the peer review process and ultimately published our findings in the Journal of Economic Entomology (February 2001). While the use of Pherocon AM traps in soybeans and the economic threshold (5 beetles per trap per day) do not explain more than 27% of the variation in corn rootworm larval injury in first-year cornfields, they do represent the only science-based approach that we have seen published to date on this vexing challenge. To date, producers have largely relied on these traps in areas of the state in which the variant western corn rootworm is new and expanding its range. Use of Pherocon AM traps in regions of Illinois where the variant western corn rootworm is well established is noticeably lacking. Instead, the common management approach is simply to use a soil insecticide each spring and assume that all first-year cornfields support an economic density of corn rootworm larvae. We know that this is not the case. In the early 1990s, we confirmed that only about 50% of continuous cornfields across northern Illinois had economic infestations of corn rootworm larvae. Despite this information, over 90% of these corn acres were treated each spring with a soil insecticide. Recent on-farm surveys of corn rootworm larval damage in first-year cornfields also indicate that not every field warrants an insecticide treatment, even in the "heart" of the problem area.

Pherocon AM trap in soybean field.

Close-up of Pherocon AM trap.

Some final thoughts--there continues to be considerable interest about the use of broadcast insecticidal treatments in soybean fields to suppress egg laying by variant western corn rootworm adults. We have argued and continue to argue against this approach. Research conducted at the University of Illinois over a 3-year period (1999 to 2001) indicates that egg laying in soybean fields occurs over a protracted period beginning in early July throughout August. Oviposition was 10% complete in soybean fields (Iroquois County) in 1999 through 2001 on July 19, 20, and 25, respectively. By July 31, 30, and August 10, egg laying was 50% complete for each of these years. Egg laying was 90% complete by August 15, 12, and 26 in soybean fields in 1999, 2000, and 2001, respectively. These data clearly substantiate the variation in the timing of egg laying in soybean fields (varies from year to year) and the large length of time in which oviposition occurs. Spraying soybean fields to suppress egg laying by the variant western corn rootworm in late July may prove futile as egg laying persists even into late August and early September. Multiple broadcast treatments of insecticides to soybean fields does not appear to be an attractive option, particularly when one considers the potential harmful effects on nontarget populations. Reductions in beneficial insect densities may trigger an outbreak of certain pests, such as the soybean aphid. So, be very leery of economic thresholds suggesting that if western corn rootworm densities reach a given level per square foot in soybean fields, treatments may be warranted. Show us the science to support this recommendation.--Mike Gray and Kevin Steffey

Mike Gray

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