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Refuges evaluate efficacy of traits

Since corn hybrids containing biotechnology traits for European corn borer were introduced in 1996, a refuge of non-Bt corn consisting of 20% of the acreage has been required within a half-mile of the Bt corn. In 2003, with Bt corn rootworm hybrids, the 20% refuge remained, with the non-Bt refuge required within or adjacent to the field.

This season, with the addition of more Bt traits with multiple modes of action that target corn rootworm and a range of caterpillar pests, refuge acres have been reduced to 5% for some multi-trait Bt corn hybrids. Check with your seed dealer to confirm refuge requirements (20% or 5%) for your 2010 seed selection if you bought Bt insect traits.

The refuge preserves insect pests that are susceptible to Bt. Should an insect pest develop resistance to Bt corn, the resistance trait likely would not be passed on to the next generation because Bt-susceptible individuals will be in much greater numbers than Bt-resistant individuals. Thus, mating most likely would occur between a Bt-resistant individual and a Bt-susceptible individual. The resulting offspring would be Bt-susceptible since the Bt resistance trait is not dominant in insect pests.

To date, the corn refuge strategy has worked, and corn pests have not evolved Bt resistance.

Refuges provide control

Refuges also allow you to evaluate your insect pest management plan. The question you should ask yourself is: “Are the Bt traits working?” The refuge helps answer this question by serving as a control or untreated check in which you can compare the efficacy of the Bt corn.

This is not to suggest that you leave refuge corn acres unprotected. Consider treating refuge corn with a soil insecticide or seed treatment at planting to protect yield, especially if you are planting corn on corn and rootworm adult beetles were at or above established economic thresholds last fall. The refuge provides the opportunity to evaluate your insect pest pressure levels during the growing season. Do not wait until harvest and compare yields between Bt corn and refuge corn, because the information is only anecdotal and tells nothing of insect pest pressure without in-season insect scouting.

To determine the effect of Bt traits on insect pests, pests must be present. There are direct and indirect methods for detecting the presence of insect pests. For example, traps can monitor pest populations, or growers can look for pest damage: root pruning on corn, ear or kernel feeding, etc. (see chart). No matter the method, you should answer this question: Are pests at economic thresholds that would result in yield loss? Pests are usually present, but not always at levels in which yield losses will result. If pests are at economic thresholds in the refuge and not in the Bt corn, this suggests the Bt traits are working.

Next, identify what pests are present in the refuge to determine if the traits you purchased will control them. Bt traits target certain pests. Information on type and numbers of corn insect pests and crop damage estimates will allow you to make informed pest management corn hybrid selections next season.

The corn refuge is an important tool to use when monitoring corn insect pests. You bought the traits. Now make sure they were worth the investment by evaluating your corn for pest damage.

Volenberg is the Door County Extension ag educator.

Monitor for damage caused by common corn insect pest larvae at these growth stages


For more information on individual corn pests, see the following University of Wisconsin Extension website: www.entomology.

This article published in the June, 2010 edition of WISCONSIN AGRICULTURIST.

All rights reserved. Copyright Farm Progress Cos. 2010.