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Controlling Insect Resistance to Bt Crops

Crop and Soil Environmental News, September 1996

Charles Hagedorn,
Extension Biotechnology Specialist

The last issue of "CSE News" introduced the plans approved by EPA and USDA to delay the evolution of resistance to Bt occurring as a result of the use of Bt crops. In this issue, details of the potato, corn, and cotton management plans are described, and the monitoring and mitigation (eradication) procedures they contain. The plans currently available to the public outline broad scientific approaches that might work to monitor and control the evolution of Bt resistance if sufficient resources were committed on a long-term basis to their implementation. In light of the practical uncertainties associated with prevention strategies, the current plans for monitoring intensify concern that neither the government nor the companies can assure that Bt will not be lost as a result of the use of Bt crops.

Evaluation of Plans

To be effective, a monitoring plan must contain a method of detecting the emergence of resistant insects early enough for eradication to be possible. When Bt crops are used over large areas, identifying small pockets of resistant organisms is a considerable challenge. In general, there are two approaches to monitoring: 1) collecting insects more or less at random in the area of Bt crop use and testing them for tolerance to Bt in a laboratory or 2) soliciting reports of insect damage in Bt crops from farmers and testing insects from those fields. Once resistant insects are discovered, mitigation measures must eradicate the resistant organisms. Different pests require different mitigation measures. If the pest is sedentary, for example, eradication measures may be limited to the farm on which the resistant organisms were identified. If the pests are mobile, eradication measures may have to be applied to a much wider area.

The Potato Plan In terms of mitigation and monitoring, the potato plan is the skimpiest of the three. This is because the plan is not a condition of Monsanto's Bt-potato permit, but only a voluntary submission. The plan promises to sample populations of the Colorado potato beetle through time looking for changes in resistance. Scientists on USDA's scientific advisory panel on Bt resistance agreed that the random collection approach would not be likely to detect resistant organisms at an early enough stage to allow eradication. Therefore, this sampling (although useful for other purposes) does not constitute a valid method for mitigation of resistance. The plans contain no discussion of approaches to enlist farmers' cooperation in the detection of the kind of damage on Bt crops that might indicate resistance. Neither is there any discussion of how the company plans to eradicate resistant pests, once discovered, or over what area it would apply such measures.

The Corn Plan

The corn plan, required of Ciba-Geigy and Mycogen as a condition of Bt-corn permits, is much more detailed than the potato plan. It requires that the companies monitor populations of insects in the area in which the corn is grown and in addition instruct their customers to report incidents of "unexpected" levels of corn borer damage to the company. The plan further requires Ciba and Mycogen to report confirmed incidents of resistance to EPA and to develop a long-term resistance management action plan for the affected area, including implementation of structured refuges. If the first measures are not successful in mitigating resistance, it requires the companies to voluntarily cease sales in the affected area.

This corn plan has given rise to several areas of concern. The structured refuges should be in place from the start to avoid the development of resistance, not adopted after resistance has emerged. And what does the plan mean by "unexpected" damage? Scientists expect some damage from corn borers in Bt corn because the level of toxin in the corn is not high enough to kill all the European corn borers that carry resistance traits. The lack of a truly high dose, in addition to undermining the success of the "high-dose plus-refuge" prevention), means that damaged corn could result from either corn borers that would have survived the toxin anyhow or newly emerging resistant organisms. This confusion could generate numerous false alarms. The corn plan needs to address this issue and indicate how the companies plan to instruct farmers to define "unexpected" damage. The corn plan should also lay out how and over what area the company would seek to eradicate any resistant borers it discovered. Corn borers have mobile life stages that may require treatment of fields far beyond those where the resistant population was discovered.

The Cotton Plan

Unlike the Bt corn and potato, Bt cotton is intended to control three pests: tobacco budworm, cotton bollworm and pink bollworm. Monitoring and mitigation plans must be tailored to the biology of each of these pests. The cotton plan submitted by Monsanto, also required as a condition of its permit, contains many of the same kinds of general provisions that apply to corn. On mitigation and monitoring, the plan contains references to two approaches discussed above: routine sampling of pests for changes in susceptibility to Bt and screening in response to performance inquires. Monsanto agrees with EPA scientists that the routine sampling-which is to be done in eight states for tobacco budworm and cotton bollworm (one sample site each in Louisiana, Mississippi, Arkansas, Alabama, Georgia, South Carolina, and North Carolina and two in Texas) and in one state (Arizona) for pink bollworm-will not be of practical use in early detection of Bt resistance. There are simply too few sites over too large an area to pick up resistance in time to eradicate the resistant organisms. Therefore, the monitoring in Bt cotton rests on the growers and crop consultants reporting "unusual pest survival" on Bt crops. As with corn, defining "unusual" survival is likely to be difficult and compounded by the fact that the three target pests are differentially susceptible to the Bt dose found in the Bt cotton. Scientists believe that Bt cotton delivers a genuinely "high" dose only for the tobacco budworm. For the cotton and pink bollworms, the dose is only midrange and may allow susceptible insects to survive.

As with the corn borer, lack of a high dose means that damage to Bt cotton observed by farmers need not indicate the evolution of resistance. It could be damage caused by either newly emerging resistant forms of budworm or susceptible forms of cotton bollworms that have not been killed by the Bt dose or other insects. Adding to the confusion, much of the damage in cotton is done by insect larvae which look very similar even among different species. In many cases, a farmer may not know whether he is observing tobacco budworm or cotton bollworm or some other insect. The plans need to give more specific instructions to growers and consultants on the lookout for damage. The mitigation measures discussed in the cotton plan also fail to make clear that insecticides may be needed beyond the field where the resistant pest was discovered. This is a significant problem because the cotton pests are mobile and may have moved beyond the field where they were discovered by the time the insecticide is applied. Companies should be prepared to monitor within a wide radius of a confirmed report of resistance to see how far insects have gotten and to recommend a plan, perhaps including cultural methods or foregoing use of the Bt cotton for a season or two, that covers all the farmers in the affected area.


In summary, the only monitoring program with a chance of working is farmer surveillance of crops. Implementation of such programs is challenging because farmer observations of crop damage could have causes other than emerging resistance, especially in the corn and cotton systems. This means that there could be numerous false alarms reported by growers and crop consultants. Following up on these false alarms will require a considerable resource investment by the companies. Since the plans do not seem to appreciate the possibility of false alarms, there is some doubt that they will be willing to make the necessary investment year after year to follow up on them. In addition, the mitigation measures outlined in the plans fail to take into account the potential mobility of resistant pests. How well these plans will work is being closely watched this first Bt-crop field season by regulatory agencies and the scientific community.

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