University of Illinois

No. 2/April 3, 1998

Stewardship of Bt Corn

As corn planters begin to roll, we need to re-emphasize the importance of stewardship of Bt-corn. Many more acres of Bt-corn will be planted this year for management of European corn borers. The "efficacy story" about Bt-corn has been told countless times at meetings, on radio programs and spot ads, and in print media articles and ads for months. Although we Extension entomologists have discussed the potential virtues of Bt-corn,we also remind growers about their obligation to practice good stewardship of this exciting new technology for insect management. Specifically, we strongly encourage corn growers to plant non-Bt-corn refuges in or adjacent to fields of Bt-corn. The objective of planting non-Bt-corn refuges is to slow down the development of resistance to the Bt endotoxin within our European corn borer population. Non-Bt-corn refuges encourage the survival of corn borers that have not been exposed to Bt, and these susceptible moths should find and mate with any (hopefully rare) individuals that are homozygous for resistance to Bt. The resulting heterozygous progeny should still be susceptible to the Bt endotoxin.

You've heard many of us say that the arrangement and amount of non-Bt-corn refuges are based upon theoretical models; and some people have asked, "Why should I plant non-Bt-corn just because these theoretical models indicate it's a good idea?" We understand the question, but we cannot support an attitude that non-Bt-corn refuges are not necessary. They are. If European corn borers become resistant to the Bt endotoxin, we will have repeated the same mistakes we made by overusing insecticides. Entomologists at three different universities have developed laboratory colonies of European corn borers that became moderately resistant to Bt after seven or more generations, proving that the gene(s) for resistance is (are) present in the native population. If we place too much selection pressure on European corn borers by planting too much Bt-corn, resistance to Bt will develop. And we will lose the benefits of the technology and hurt our chances for ever being able to use other transgenic crops.

Although the exact amount and placement of the optimal non-Bt-corn refuge is not known, and maybe never will be, university entomologists throughout the north-central region have agreed upon the following suggestions. In areas where mostly corn and soybeans are produced, growers should plant at least 20 to 30% non-Bt-corn. If a grower wants to protect the non-Bt-corn refuge from an economic infestation of corn borers, the refuge size should be increased to 40%. The non-Bt-corn refuge should be planted at a similar time and in close proximity to Bt-corn. These recommendations and other information about Bt corn can be found in North Central Regional Extension Publication NCR 602, Bt-Corn and European Corn Borer: Long-Term Success Through Resistance Management. This publication may be obtained from Information Technology and Communication Services at the University of Illinois. Inquire about this publication by calling (217)333-2007 or (800)345-6087.

Although the optimal arrangement of Bt-corn and non-Bt-corn is not known, ideally the non-Bt-corn refuge should be planted as a block within the same field as Bt-corn. This arrangement mostly likely would foster better interaction among corn borer moths that emerge from both Bt- and non-Bt-corn. However, regardless of how a grower decides to arrange a non-Bt-corn refuge (within reason), we will be content as long as a refuge is planted. Each grower who plants Bt-corn must accept the responsibility, and, in our opinion, the obligation to protect the technology. Development of corn borers resistant to Bt will be detrimental for everyone involved with agriculture.
Kevin Steffey ( and Mike Gray (,Extension Entomology, (217)333-6652