Mike Rankin
Crops and Soils Agent - Fond du Lac County
University of Wisconsin - Extension
The advent of Bt technology has placed a powerful yield-enhancing tool into the hands of corn growers. The ability to genetically alter corn genetics such that a Cry protein is a component of plant tissue now has European corn borer (ECB) larvae dropping to their knees and pleading for Pepto-Bismol. Of course, it does no good.
How long this technology remains a viable tool to offset yield losses caused by ECB will depend largely on how farmers select and plant hybrids over the next several years. To understand how resistance to Bt might develop, it's helpful to first understand the basic principles of Bt technology.
"Make sure those critters die!"
Like people, not all ECB's inherit the same gene structure. A simplified model of gene resistance and susceptibility is presented in Table 1.
| Table 1. Effect of Bt Cry protein on different ECB gene types for resistance and susceptibility. | ||
Gene Type |
Reaction to Bt Cry Protein |
Effect on Corn Borer Larva |
ss |
Susceptible | Dies |
rs |
Susceptible or resistant, depends on dose | Low dose = survives High dose = dies |
rr |
Resistant | Survives |
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 for a refuge
With limited experience in refuge management, current guidelines 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. Blocks or strips within fields should probably be at least 8 rows in width. This will prevent the ECB larvae from ingesting pollen that has moved into the refuge strip from Bt plants. Current estimates are that 20 to 30% of corn acreage should be devoted to refuge areas of non-Bt corn hybrids.
One of the keys to refuge management is to pick a corn hybrid that is similar to the Bt hybrid being planted close-by. 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. EPA has issued conditional registrations that require seed companies selling Bt corn to develop and carry-out resistance management plans by 2001. It is the farmer's responsibility to comply with these plans.
For
more information contact Mike Rankin