Planting Bt?.... don't forget the refuge

Mike Rankin
Crops and Soils Agent
University of Wisconsin - Extension

        Last year, the Environmental Protection Agency approved the use of Bt corn for another seven years.  However, they made a clear statement that corn growers needed to comply with recommended refuge requirements.  By doing so, this would insure the future use of Bt and other transgenic technologies.  As a grower, it is your responsibility to insure that these rules be followed.

Why a refuge?

        Like people, not all insects inherit the same gene structure.  This also applies to the European corn borer (ECB).  A simplified model of gene resistance and susceptibility is presented in Table 1.

        As can be seen, some ECB's are susceptible to the slightest hint of Bt seasoning (ss), others are either susceptible or resistant depending on the dose (rs), while a few are resistant (rr).  The first step to avoiding the build-up of resistant individuals is to make sure Bt Cry proteins are present in high doses of corn plants.  This insures that the rs individuals don't survive, get intimate with each other, and produce fully resistant (rr) offspring. Obviously, the ss individuals will also head for that big corn field in the sky.  The question now becomes, "How do we deal with the resistant (rr) guys?"  This is where the concept of planting a refuge enters the picture.           

What is a refuge?

        A refuge is simply a block or strip of non-Bt corn that provides a source of susceptible ECB moths (ss) that will mate with potential resistant moths (rr) emerging from nearby Bt corn fields.  The theory is to produce an overwhelming number of susceptible moths for every resistant moth.  This lowers the chances that the resistant moths will mate with each other.  Clearly, if this source of susceptible moths is not available to cross with the resistant types, a proliferation of rr individuals will occur rapidly.

Planning a refuge

        Much effort and thought has gone into the development of refuge guidelines.  They are based on predictive models and scientists' "best guesses".  ECB moths do not typically fly great distances.  The maximum distance currently being talked about is 1/2 mile or within the same half-section (320 acres).  For Fond du Lac County, this is probably too much of a distance to be effective.  Rather, growers will be better served by planting refuge blocks within the same field or in adjacent fields to insure moth movement between fields. The figure below illustrates some possible refuge plans.  The new rules state that blocks or strips within fields be at least four rows in width.  Quite frankly, eight rows will be much more effective.  This will help prevent the ECB larvae from ingesting pollen that has moved into the refuge strip from Bt plants.  By regulation, at least 20 percent of your corn acres need to be planted to a non-Bt refuge.

         One of the keys to refuge management is to pick a corn hybrid that is similar to the Bt hybrid being planted close-by or within the same field.  ECB infestation is largely governed by crop development.  First-generation ECB are attracted to those fields that are most advanced in growth.  Conversely, second generation ECB are attracted to later planted fields.  If the two fields are not "in sync" in terms of moth attraction, the purpose of planting the refuge is defeated.

            In summary, the potential threat of resistance by ECB to Bt corn necessitates that a resistance management plan be implemented.  The EPA has made it clear that refuge plans will be implemented or the technology will be lost.  It is the farmer's responsibility to comply with these plans.