Feature Article

Bt Corn and European Corn Borer:
Long-term Success Through Resistance Management

The European corn borer (ECB), a billion dollar pest of field corn and sweet corn and the most damaging insect pest of corn throughout the United States and Canada, is the primary target of a new technology to manage insect pests, called Bt corn. Bt corn hybrids were created by inserting genes from the bacterium Bacillus thuringiensis, (or Bt). These genes produce a protein toxic to some caterpillars, such as the ECB. Also contained in the genetic package is a promoter to "switch on" production of the proteins by corn and a marker to verify the package is functional. Because these hybrids contain an exotic gene, they are commonly called transgenic plants. Several seed companies now offer Bt corn hybrids to farmers and the number of hybrids is increasing rapidly.

These hybrids provide protection against the ECB equal to, and usually far greater than, optimally-timed insecticides; most larvae die after taking only a few bites. Although conventional Bt insecticides may perform as well as synthetic insecticides, their performance is not always consistent because of toxin sensitivity to UV radiation, heat and desiccation, incomplete coverage of feeding sites, or reduced toxicity against older larvae. Modifying a corn plant to produce its own Bt protein overcomes these liabilities. The protein is protected from rapid environmental degradation. Plants produce the protein in tissues where larvae feed, so coverage is not an issue. Finally, the protein is present whenever newly-hatched larvae try to feed, so timing of Bt application is not a problem. The result is an efficient and consistent built-in system to deliver Bt proteins to the target pest. However, because Bt corn provides unprecedented control of ECB through a simple seed choice, widespread use could set the stage for resistance.

ECB may have the potential to develop resistance to Bt. Insects are known for their ability to develop resistance to certain insecticides rapidly. Resistance occurs particularly when insecticides are used repeatedly and at high concentrations. More than 500 species of insects and mites have developed resistance to insecticides and miticides. A recent Midwestern example in corn includes adult western corn rootworm resistance to Penncap-M in Nebraska. In addition, laboratory colonies of more than 15 different insect pests have developed resistance to Bt proteins, including Indian meal moth, tobacco budworm, beet armyworm, pink bollworm and Colorado potato beetle. Moreover, the diamondback moth, a worldwide pest of cole crops, has developed high levels of resistance to Bt insecticide in field populations in Hawaii and Florida.

Many factors contribute to the development of resistance. Some of these for the ECB include predictions for widespread use of Bt corn, high season-long mortality, and two or more generations per year. Recent laboratory studies in Minnesota, Kansas, Delaware and Iowa confirm that ECB (collected from Minnesota, Iowa, and Kansas) can develop moderate levels of resistance to Bt insecticides or Bt proteins. Resistant ECB strains in these studies require 30-60 times more toxin to kill 50% of a test population of larvae compared with nonresistant ECB strains. This modest level of Bt resistance developed in relatively small lab populations after seven to nine generations of exposure. These labortory results, however, do not prove that ECB will develop resistance to Bt corn under more complicated field conditions. Bt corn and ECB in the field pose a dramatically different situation than larvae feeding on Bt insecticides in laboratory diet.

Scenarios of resistance development by ECB are suggested by studies of insecticide resistance in many insects and by resistance to Bt insecticides by tobacco budworm and diamondback moth. In any population of ECB, a few of the borers will have two copies of genes for resistance (rr), some will have one copy of the gene (rs) and most will have none (ss). Resistance genes are likely to be rare. On Bt corn, ECB with one or more copies of resistance genes will survive better and produce more offspring. This improved survival or reproductive success results in a "selective advantage." As the Bt corn acreage increases, and with it the proportion of the ECB population exposed to Bt corn, more larvae carrying resistance genes could survive to adulthood. The overall population of Bt-resistant individuals increases with each generation. At some point, control failure could occur with resistant larvae reaching infestation levels in Bt corn fields similar to levels found in non-Bt corn fields.

Growers and seed companies will face the primary impacts of ECB resistance to Bt corn. Initially, while seed companies and entomologists develop strategies for countering ECB resistance, producers in problem areas might lose the option to use Bt corn. Organic growers who rely on Bt insecticides also could lose a valuable management option in these areas. Resistance effects could be minor, though, if hybrids that express alternative proteins are effective and if they are introduced rapidly into problem areas. ECB, however, could develop cross resistance to two or more of the proteins. If entire groups of proteins are neutralized by resistance development, growers could permanently lose Bt corn and Bt insecticides as valuable management tools. This would be unfortunate for organic growers and other producers who rely on Bt insecticides. In addition, the failure of a voluntary, proactive resistance management plan could create more regulatory pressure for future transgenic crop technologies. This could limit the use of a transgenic Bt approach for other high-value crops, such as sweet corn.

The potential threat of resistance by ECB to Bt corn necessitates a management plan to delay or avoid the risk of resistance. Resistance management is a key element of good IPM practices. Consequently, the EPA has issued conditional registrations that require companies selling Bt corn to develop and carry out resistance management plans by the year 2001. Proactive use of "refuges" of non-Bt corn is encouraged to delay development of resistance by ECB and to preserve the longevity of Bt corn technology.

Resistance management in Bt corn is currently based on two complementary principles: high dose and refuge. Plant geneticists designed Bt corn to produce very high levels of Bt proteins, much higher than levels found in Bt insecticides. The intent is to kill all ECB larvae with no genes for resistance, plus those with one copy of a resistance gene. The assumption inherent in this resistance management approach is that Bt hybrids have achieved this high-dose objective. If a high-dose objective is not achieved, then corn borer larvae with one copy of a resistance gene may survive to adulthood and mate with other resistant moths. Most of the offspring from these matings would be resistant to Bt corn. The second principle of the resistance management plan is the use of refuges. The purpose of a refuge is to provide a source of ECB, not exposed to Bt corn or Bt insecticides, to mate with potential resistant moths emerging from nearby Bt corn. The goal is to produce an overwhelming number of susceptible moths to every resistant moth. A refuge is any non-Bt host of ECB, including non-Bt corn, potatoes, sweet corn, cotton or native weeds that occur near Bt corn (within the same 1/2 section, 320 acres). The question is, how large a refuge is needed to provide enough susceptible moths? In any given year, approximately 20-30% of ECB larvae should not be exposed to Bt proteins. This is based on current knowledge of ECB biology, pesticide resistance studies and computer simulation models. To be effective, ECB moths must emerge from the refuge at the same time as resistant moths and be close enough to mate with resistant moths. Although some ECB moths can fly substantial distances, many moths fly less than a mile from their emergence site. Consequently, each farm should have one or more refuge areas next to Bt corn.

The actual amount of refuge required will vary among regions, farms, and corn production systems. Always the goal is to prevent Bt protein exposure to 20-30% of the larval population. In continuous corn and corn-soybean rotations, the primary available refuge is non-Bt corn, so 20-30% of the corn acreage should be non-Bt corn. In continuous corn areas where ECB is typically sprayed with insecticides, the refuge should be increased to 40% to compensate for larval mortality. Where the total corn acreage is small and much of the local ECB population is associated with alternative hosts that do not contain Bt proteins, a smaller refuge may be suitable. This assumes that corn borers from alternative hosts emerge at similar times as corn borers from corn. When the proportion of the local ECB population that flows through non-Bt hosts is unknown, a 20-30% non-Bt-corn refuge may be the simplest and best insurance to delay resistance.

Monitoring for the development of resistance to transgenic plants will provide information that is essential to managing ECB resistance. Monitoring is necessary to learn whether a field control failure resulted from resistance or other factors that might inhibit expression of the Bt protein. The extent and distribution of resistant populations can be mapped so that alternative control strategies can be adopted in areas where resistance has become prevalent. Finally, detecting resistance may be possible before control failures occur, if monitoring techniques are sensitive enough to provide complete discrimination between resistant and susceptible individuals.

- Adapted from Ostlie, K. R., W. D. Hutchison and R. L. Hellmich. 1997. Bt Corn and European Corn Borer. NCR Publication 602. University of Minnesota, St. Paul, Minn. 20 p.

For more information on Bt corn and implementing a resistance management plan for European corn borers order the full 20-page publication through your extension office. It will be available for $3.50 in Minnesota but prices will vary between states.

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