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Refuges Help Evaluate Corn for Pest Damage

Refuges Help Evaluate Corn for Pest Damage
Check with your seed dealer to confirm refuge requirements.

Since corn hybrids containing biotechnology (Bt) traits were first introduced in 1996 for European corn borer, 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 (CRW) hybrids, the 20% refuge remained with the non-Bt CRW refuge required within or directly 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 insect 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 purchased Bt insect traits.

The refuge preserves insect pests that are susceptible to Bt. Should an insect pest evolve resistance to Bt corn, the resistance trait would likely not be passed on to the next generation because Bt susceptible individuals will be in much greater numbers than the Bt resistant individual. Because of this, mating would most likely 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 refuge strategy in corn has worked and corn pests have not evolved resistance to Bt. However, the refuge also serves as an important decision tool besides complying with the insect resistance management (IRM) requirement.

Refuges provide control

Refuges provide you the opportunity to evaluate your insect pest management plan. The question you should be asking yourself is "are the Bt traits working?" The refuge helps you 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 your refuge corn acres unprotected. You should consider treating refuge corn with a soil insecticide or seed treatment at planting to protect corn yield in your refuge acres, especially if you are planting corn on corn and CRW adult beetles were at or above established economic thresholds last fall. The refuge provides you the opportunity to evaluate your insect pest pressure levels during the growing season. Do not wait till harvest and compare yields between your Bt corn and your refuge corn because the information is only anecdotal and tells you nothing of insect pest pressure without in-season insect scouting observations. In insect pest management, there is more to evaluate than corn yield.

To determine the effect of Bt traits on insect pests, insect pests must be present. We can use direct or indirect methods to detect the presence of insect pests. For example, we could use traps to monitor pest populations or we could look for pest damage, root pruning on corn, ear or kernel feeding, etc. (Table 1). No matter the method, we are interested in answering this question: Are pests at economic thresholds that would result in yield loss? Pests are usually always 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 that the Bt traits are working. Next identify, what pests are present in the refuge to determine if the traits you purchased will control the pest. Bt traits are specific and target certain pests or pest complexes.

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

Table 1. Corn growth stages to monitor for damage caused by the larval stages of common corn insect pests.


Approximate range of corn stages to monitor


Black Cutworm

VE to V5

Pinholes in leaves

Plants cut off at soil

Corn Earworm

V8 to R6

Kernel feeding

European Corn Borer

V8 to R5

Pinhole feeding on leaves
Kernel feeding and ear drop
Stalks broken

Corn Rootworm

V8 to R6

"Goose-necked" stalks, root pruning, root tunneling


VE to VT

Leaf feeding beginning at the leaf margins inward to leaf midrib.

Western Bean Cutworm

VT to R6

Kernel feeding

for more information on individual corn pests see the following UW Extension website Corn Pests.
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