The Refuge Builder

Insect Resistance Management of Bt Corn Using Refuge. Access the Refuge Builder Bt corn hybrids are engineered to produce an insecticidal protein from, Bacillus thuringiensis, a naturally occurring soil bacterium. Bt is toxic to European corn borers and a few other insects. Bt has been used for many years in its topical form by organic farmers and home gardeners, only recently has it been engineered into plants. European corn borers exposed to Bt producing plants generally consume Bt at higher levels and over a longer time compared to foliar applications of Bt. This high level of toxicity and increased exposure time creates a higher level of selection pressure, increasing the threat of resistance by European corn borers to the Bt toxins. Because of this increased threat of resistance and the importance of Bt to both organic and non-organic farmers, it was decided necessary to create a set of strategies to reduce the occurrence of resistance. An important concept of insect resistance management is refuge plantings. Refuge is any host plant (non-Bt corn, potatoes, oats, sorghum, and some weeds) not producing or treated with conventional Bt formulations. In the current resistant management plan for Bt corn, the refuge must be non-Bt corn. Refuge supplies a source of susceptible European corn borer that could mate with corn borer that are resistant to the Bt toxins. Mating between resistant and non-resistant adults, produce offspring susceptible to the Bt toxin.

The EPA and Canadian Food Inspection Agency have created similar guidelines for resistance management requirements. On each farm, growers may plant up to 80 percent of their corn acres with Bt corn. At least 20 percent of their corn acres must be planted with non-Bt corn and treated only as needed with insecticides. Decisions to treat the refuge should be based on economic thresholds. Conventional Bt products (liquids or granules) must not be used on the non-Bt refuge. Plant non-Bt corn refuge within, adjacent to, or near the Bt cornfields. If the grower intends to treat the refuge it is encouraged that it be placed within 1/4 mile of the Bt field. In any case, the refuge must be placed within 1/2 mile of the Bt field. If refuge is established as strips within a field, the strips should not be narrower than 4 rows, preferably 6 or more (for the Refuge Builder the minimum of 6 rows is used). If possible, locate refuge plantings to protect potentially vulnerable non-host insects (e.g. Monarch butterfly). Refuge plantings can serve as buffer zones between the Bt cornfield and the habitat of non-target insects. Non-Bt corn hybrids for use as refuges should be selected for growth, maturity, and yield traits similar to the Bt hybrid used in the remainder of the field. Refuge Design Considerations Linear blocks, brackets, or border refuge plantings are relatively easy to plant, treat, monitor, and harvest. They have the added advantage of acting as buffer areas between the Bt corn and non-target habitat or non-GMO cornfields. Strips have the advantage of providing susceptible European corn borer to all parts of the Bt field, but they also have several drawbacks. Strips cannot be treated separately from the Bt corn. Harvest may be difficult if non-Bt strips dry down differently than the Bt corn. Also, it is difficult to keep track of where the strip rows begin or end, so monitoring is more difficult. Do not plant strips narrower than six rows or mix seed. This increases the risk of resistance occurring because European corn borer larvae often move from plant to plant. Corn borer larvae that can survive eating small amounts of Bt (low level resistance or tolerance) can end up on a non-Bt plant and survive. The design for planting strips will depend on your planter. For example, dedicating three end row units of a 12-row planter will effectively give you a 25 percent refuge and maintain the 6-row strip size. If you have a 6-row planter you can achieve the 25 percent, 6-row minimum refuge by splitting the planter into three units of Bt and three units of non-Bt and only strip one-half of the cornfield. Four-row or single-hopper planters are not suitable for this refuge option. The Bt-susceptible European corn borer from the refuge must be present at the same time as possible Bt-resistant European corn borer from the Bt cornfield. To achieve this the corn hybrid in the refuge should be agronomically similar (e.g. similar days to maturity) to the Bt hybrid, planted at the same time as the Bt field, and managed in the same manner as the Bt field. In this way the European corn borer moths will be equally attracted to the refuge and Bt cornfield. Larvae also will develop at the same rates and emerge as adults at the same time. Using a neighbor's cornfield as a refuge is not allowed because the hybrid selection, planting time, pest control, and other production activities are not under the control of the grower planting the Bt corn. Planting only non-irrigated pivot corners as refuge is not recommended because the corn plants in these areas are significantly different and less attractive to European corn borer moths than the corn under irrigation. Remember, the idea is to produce Bt-susceptible European corn borer moths. The closer the refuge is to the Bt field the better. This brings Bt-susceptible European corn borer in close proximity to any Bt-resistant European corn borer that may survive in the Bt cornfield. Female European corn borer generally mate close to where they emerge as adults, so having nearby refuge increases the chances that a susceptible European corn borer will mate with a resistant one. You can combine refuge configurations to meet the required 20 percent refuge. The Refuge Builder Disclaimer The Refuge Builder is an interactive exercise that teaches the basics of refuge management for Bt corn. It is not intended to include every aspect of refuge management (i.e. recommendations for individual Bt hybrids by industry registrants, geographic location, or growing region). The Refuge Builder only refers to generalized Bt corn type and does not deal with the intricacies of each Bt corn type or other plants that produce Bt. The refuge builder should not be used as an all-inclusive guideline for planting Bt crops, however, all of the concepts introduced are a part of the EPA and CFIA regulations for refuge management. Access the Refuge Builder. References Canadian Plant Inspection Agency, Plant Health and Production Division, Plant Biosafety Office. Insect Resistance Management of Bt Corn in Canada. February, 8, 1999. EPA, Office of Pesticide Programs. EPA on Insect Resistance Management in BT Crops. May 27, 1999. Nebraska Cooperative Extension. Resistance Management for European Corn Borer and Bt Transgenic Corn: Refuge Design and Placement. NF00-425 Revised November 2000.