Insect control using liquid and granule formulations of Bacillus thuringiensis (Bt) has been practiced since the 1970's. Bt is a naturally occurring soil bacterium that produces insecticidal crystal proteins that cause toxic reactions when ingested by some insects, including European corn borers (ECB). The Bt toxins are only active in alkaline-based stomachs and are harmless to humans, other animals and beneficial insects.

There are over 60 known insecticidal Bt crystal proteins. Current commercial Bt biotechnology has resulted from inserting the crystal proteins into the genetic makeup of several major agronomic crops. ECB control using Bt biotechnology is considerably more efficient than its external Bt or other insecticide application counterparts.

As corn producers, there are two components of Bt corn production that you should consider. First, be aware of corn market segregation issues concerning the mixing of Bt and non-Bt corn grain. This mixing can occur biologically through pollen drift or through planting, crop rotation, harvest, transportation or storage operations. Secondly, we need to manage against the development of Bt resistance in targeted insect pests.

"MFA's grain is directed toward the domestic market, where Bt has not been a major issue," said Roger Caffrey, MFA director of grain operations. Even though at this time co-mingling is not a major concern in our region, we should become familiar with this issue, as it may become important.

Resistance selection pressure is very high, given the large number of ECB per generation, multiple ECB generations per season, and that ECB larvae ingest Bt from Bt corn during their entire life cycle. In fact, Bt resistance by some moth species has already occurred in areas where Bt foliar sprays are used.

With genetic Bt technology, there are several reasons to believe that resistance can be suppressed. Crop breeders have a wide and diverse potential gene pool to select Bt sources from.

As crop producers and agronomists, we can manage against resistance. The most accepted approach to Bt resistance management is the high-dose refuge strategy, which relies on separate, but co-dependent components.

The high dose component is based upon the fact that Bt crops produce enough toxins (20 to 60 times the lethal dose) to kill virtually all non-resistant and partially resistant ECB in the larval stage. Truly resistant ECB represent a minute amount of the overall population.

The second component is to plant non-Bt corn refuge areas close enough to all Bt cornfields that the few resistant ECB will mate with susceptible ECB moths. The resulting offspring will be susceptible or only partially resistant to Bt.

The size, design, distance from Bt corn, and other components of non-Bt refuges are being debated, with no documented most-effective method. However, EPA has developed standard requirements.

Currently, only 80 percent of a producer's corn acreage can be planted to Bt hybrids (minimum 20 percent refuge area). If synthetic insecticides are planned in the refuge area, then producers are advised to only plant 60 percent of their corn acreage to Bt hybrids. This reduction is required because of the dilution of non-Bt resistant moths entering the genetic pool as a result of insecticide exposure. Currently, producers cannot use a neighbor's non-Bt cornfields or other ECB host crops (potatoes, oats, sorghum and weedy areas) as refuge areas. Because of Bt cotton technology, the corn planting refuge acreage requirement increases to 50 percent in certain cotton growing regions.

The non-Bt refuge must be planted within 0.5 miles from Bt cornfields. The recommended distance is 0.25 miles, especially if insecticides are to be used in the refuge area. These distances are needed to ensure the co-mingling of Bt resistant and susceptible moths (ECB moths commonly fly less than 1 mile to find mates).

There are several different refuge designs. Non-Bt refuges can be planted in separate fields, large blocks within Bt fields, outer or end rows within Bt fields or in alternating strips within Bt fields. Because corn borer larvae can move between plants, do not mix Bt with non-Bt seed within the same row. For this reason, the alternating strip method (not recommended) requires that the non-Bt strips be at least 6 rows wide. Another reason to avoid within field stripped refuges is the difficulty in insecticide spraying or harvest management differences that may be required between the Bt and non-Bt strips. The University of Nebraska has developed a web-based interactive refuge builder design tool that can aid your Bt refuge planting strategy. It is available at: http://agbiosafety.unl.edu/education/refugebuilder/refugebuilder.htm. Refuge areas need to be planted at the same time as Bt areas, using corn hybrids of similar agronomics. This will ensure that moth emergence and mating between Bt resistant and non Bt resistant ECB will occur.

All corn growers using Bt technology share the responsibility of protecting against resistance.