Key NC-205 Committee of U.S. academic experts updates its European Corn Borer resistance managment advice to EPA in a May 24, 1999 memo
This memo is in response to an invitation from USDA and EPA for NC205 to provide further comment on resistance management strategies to preserve the insect suppression benefits provided by transgenic Bt corn. NC205 members had the opportunity to review the Industry Stewardship Proposal (ISP) for Insect Resistance Management (IRM) of Bt corn provided to us on April 19, 1999. Summarized below is an update of NC205 views.
The NC205 resistance management strategy is based on a high-dose/refuge concept. Events that do not provide high-dose performance are not considered in this high-dose refuge resistance management strategy. At this time we do not have scientific information as to what additional issues and parameters should be used to guide resistance management when growing nonhigh-dose hybrids.
Based on our current scientific understanding of the situation and a review of published and unpublished data available to us, planting schemes which place all Bt corn within one half mile of the non-Bt corn refuge are preferred and refuge plantings within one quarter mile would be even better. We understand that practical considerations, for example, unusual field configurations, can create problems in grower implementation of IRM options.
The purpose of the non-Bt corn refuge is to provide unselected, susceptible mates for any resistant individuals surviving within nearby plantings of Bt corn. Whether insecticides can be applied to refuges without significantly harming the resistance management strategy is a very complex and controversial issue. Several members of NC205 are investigating this matter. More definitive scientific results may be available in a few months.
NC205 recognizes that our present 20 percent unsprayed/40 percent sprayed refuge guideline presents practical difficulties for growers. Growers cannot predict before planting whether they will need to spray and therefore, growers will not know whether a 20 percent or 40 percent refuge strategy is appropriate.
Newly acquired and preliminary scientific evidence indicates that a 20 percent sprayed refuge may be adequate in most corn growing areas where economic thresholds for European corn borer are not regularly exceeded. This situation presently describes most of the Corn Belt east of the High Plains. It also may apply in High Plains areas where up to 25 percent of the acres have been treated for European corn borer over a large area. If these rates of insecticide application to non-Bt corn refuge were to continue, it is possible that a sprayed 20 percent refuge is adequate in most years.
All members of NC205 recognize that a greater risk of resistance occurs in areas where insecticide application has been historically high. This situation frequently occurs in the High Plains area where growers commonly spray for Southwestern corn borer (SWCB). Heavy use of Bt corn coupled with potential use of highly effective insecticides in treating refuges for SWCB will expose the resident European corn borer population to substantial selection pressure. A refuge management strategy that is more conservative than the one applied across the greater Corn Belt, yet less restrictive than the one proposed for areas growing both corn and cotton, may be most appropriate in the heavily treated areas jointly infested with SWCB and ECB.
In summary, the scientific evidence is still unclear regarding the efficacy of a sprayed refuge for IRM. Given the uncertainties surrounding the efficacy of sprayed refuges, we minimally recommend: 1) insecticide treatment of refuges should be based on scouting and accepted economic thresholds, 2) treatment should be with a product that does not contain Bt or Cry toxin, 3) records should be kept of treated refuges and shared with the EPA, 4) the potential impact of sprayed refuges should be monitored closely and evaluated annually, and 5) monitoring for resistance should be most intense in higher risk areas, for example where refuges are treated with insecticides.
All stakeholders must be vigilant and engaged in assessing product performance and susceptibility levels as part of an ongoing resistance management program. Such assessments will be important in insuring that resistant populations do not go undetected. Remedial action may need to be taken when necessary to circumvent potential resistance outbreaks and loss of this transgenic Bt technology as a pest management option. Field monitoring and communication among all stakeholders in assessing the continued performance of the technology is essential to enhancing the life span of this technology.
Implementing IRM will require ongoing dialogue among stakeholders. NC205 will continue to report the best available science based techniques for delaying the development of corn borer resistance. We recognize that growers are the ultimate stewards of this technology and they need IRM options that are both effective and practical. It is recognized that some growers are likely to implement a simple IRM plan that has the potential to be less effective in delaying resistance than the scientifically optimal plan.
The potential tradeoffs between effectiveness and practicality in IRM plans is a point that must have a prominent place on the agenda of all stakeholders. It is important that plans can be modified annually or as new information becomes available. An ongoing dialogue is essential to assure that the best IRM plans (science based and practical) are identified and implemented.
NC205 is ready to continue facilitating interaction among all stakeholders. We will continue to develop additional educational materials to communicate IRM options and other important information with growers.
This memo is submitted on behalf of the members of NC205. It is based on extensive deliberations via e-mail and conference calls. We appreciate the opportunity to provide this document. Please do not hesitate to be in contact regarding this document and other matters.
Eldon Ortman Administrative Advisor NC205
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Last Updated on 5/25/99
By Karen Lutz