by Ric Bessin
Kentucky Pest News

Surveys of corn growers in several Midwestern states have shown that for the vast majority of users of Bt corn,
they are using refuges to combat the development of resistance to Bt corn. This is good, but it could be better. All users of Bt corn need to be using refuges in order to delay resistance by these pests.

What has made this issue a bit more complicated is that there are now two different guidelines for refuges with
Bt corn. The reason is that the refuge for Bt corn that controls rootworms is not identical to those used for Bt corn that controls corn borers. While there are similarities between the two schemes, there are some differences to account for differences in the biology of corn borers and corn rootworms. But one fact remains true, growers using Bt corn are required to use a refuge for resistance management.

Required Refuge For Yieldgard Corn Borer And Herculex Bt Corn

The refuge for YieldGard Corn Borer and Herculex corn is any corn not possessing those traits. The minimum
size of this refuge is 20% of the total acreage. For example, if 100 acres of Bt corn (one of the two types
listed above) is grown, then the grower must plant at least 25 acres of corn that does not control corn borers.
The refuge needs to be located close to the Bt corn to be effective. If the grower plans to spray for corn borers in the non-Bt corn, then the refuge needs to be within 1/4 mile of the Bt corn. If the grower will not spray for corn borers, then the refuge can be up to 1/2 mile from the Bt corn. In general, the closer the refuge, the more effective it is . If the grower does spray for corn borers, the grower should not use a Bt containing sprays, as this would reduce the effectiveness of the refuge.

Required Refuge for YieldGard Rootworm Corn

The refuge for YieldGard Rootworm corn is similar is size to that for Bt corn that controls borers. The refuge
for Bt rootworm corn is 20% of the total corn acreage. However, the refuge for Bt rootworm corn must be
immediately adjacent the Bt corn. A possible field arrangement would be to have the Bt and non-Bt blocks
of corn next to each other or directly across the road from each other. Another arrangement is to split a field
with the refuge to one side or encircling the Bt rootworm corn.

The reason for having the Bt rootworm corn so close to the refuge is that the female rootworm beetles on travel a short distance from their emergence site before mating. Corn borers on the other hand often leave the field to move to weedy action sites to before mating. The goal of an effective refuge for Bt corn is to ensure adequate mixing of the adults to have random mating, and to have at least 500 susceptible adults for each potentially resistant individual that may emerge from the Bt corn.