EPA Panel Reports on Bt Crops

March 16, 2001

A panel of independent scientists convened by the Environmental Protection Agency has completed work on its report which provides input on EPA's preliminary comprehensive reassessment of all Bacillus thuringiensis (Bt) corn, cotton, and potato plant incorporated protectants. EPA solicited advice from the scientific advisory panel (SAP) during a public meeting held Oct. 18-20 to ensure that the agency's reassessment of currently-registered biotechnology products utilizes the most current scientific understanding, and meets the most stringent public health and environmental safety standards.

The panel generally agreed with the 1998 EPA-SAP which concluded that a refuge/high dose strategy must be used within the current understanding of the technology. Some members of the panel believed that the limit for high dose should be increased, but the majority concluded that there was insufficient need to warrant a change. One recommendation was that EPA should clarify that the 25X definition of high dose is a provisional one, and that their determination of high-dose status for any Bt crop may be changed if it is determined that the inheritance of resistance in insects from a Bt crop in the field is such that the 25X definition is inappropriate.

A high dose plant, the panel said, would be one that expressed the toxin at 25 times the dose required to kill 99% (LC99) of a susceptible insect pest population. Studies have concluded that for the refuge/high dose approach to be effective, Bt cultivars must produce a toxin concentration that is high enough to kill most insects which are heterozyogous for resistance. The 1998 SAP assumed that a toxin concentration that was 25 times the concentration that would kill susceptible insects would be sufficient to kill heterozygotes.

A number of public commenters challenged the need for raising the high dose limits. Some of the concerns expressed included: 1) the heterozygotes tested to date do not survive on current high dose cultivars, 2) in some of the cases used by Caprio, inheritance of resistance was dominant and, 3)use of the LC50 values is not appropriate and; 4) increasing the dose beyond 25- fold might exacerbate problems associated with non-target exposure.

The SAP indicated that no single number could be rigorously defended as the cut off for a high dose for any key pest species at the present time, because the resistance properties of every resistance allele in the field cannot be predicted. Any definition of high dose must therefore be imprecise.

For cotton, the panel concluded that the impact of differential expression of Bt toxin among cultivars on resistance management is dependent upon the level of toxin expression and the species of insect. As long as the expression level of all cultivars exceeds the definition of "high dose" then variation among cultivars should not be a cause of concern. If the expression level of any cultivar is below that of the defined "high dose," then the impact on resistance development could be significant.

The panel discussed the EPA's current requirement of a 20% corn refuge in the North and a 50% refuge in the South which is based on the cotton boll weevil being exposed to both Bt corn and Bt cotton in the South. EPA uses strict political boundaries in distinguishing North from South. This approach makes the assumption of no return migration in boll weevil.

Also the panel reviewed data relevant to the question of whether the boll weevil migrates both to the North in early summer and to the South in late summer. The northward migration is more clearly documented than the southward migration. The general consensus was that although southward migration was not proven, there was considerable circumstantial evidence of southward migration, and it should therefore be considered in resistance management. There is more evidence supporting southward movement than evidence against southward movement. The need for further scientific investigation of migration was emphasized.

The panel discussed how resistance management for Bt corn and Bt cotton could be affected by north-south migration. Such migration would mean that selection pressure from Bt corn in the corn belt, as well as the refuge provided by non-Bt corn in the corn belt, should be considered in resistance management.

Since Bt cotton and Bt corn do not produce a high dose for the boll weevil, plus the probable long-distance, cross continental movement of insect from the northern corn producing areas to the southeastern corn and cotton producing areas of the United States, the exact placement of refuges is not as critical as it is with insects for which there is an appropriate high dose.

This means that the percentage of Bt corn planted in some subregion of the northern corn belt should be more important to resistance management than the percent refuge on each farm. As long as the percent corn planted in such a region is below 50%, then the 20% refuge requirement for each farm in the corn belt may not be a concern. If the overall percentage of Bt corn in a region increases above 50%, then the issue of on farm refuge size may need to be reconsidered. If further studies demonstrated that migrating moths from specific corn belt states with high percentages of non-Bt corn contributed significantly to the following season's boll weevil population, then non-Bt contributions of CBW from corn could be considered in determining the cotton refuge size.

Existing EPA registrations for corn and cotton Bt products (also called "plant incorporated protectants") will expire in September. These products are regulated by EPA as pesticides because they have been engineered to contain a naturally-occurring substance called Bacillus thuringiensis (Bt) which acts to combat insect pests. EPA included this time-conditional provision for plant incorporated protectants to ensure that these products be reevaluated regularly for safety to human health and to the environment.

EPA's next step is to proceed with its comprehensive evaluation of the existing Bt registrations, taking into account all data available to the Agency, the SAP's recommendations, any newly submitted information and public comments. EPA will then publish the revised reassessment for public comment, which will include proposed registration decisions on the Bt products.

The entire SAP report is available on the Internet at http://www.epa.gov/scipoly/sap/2000/october/octoberfinal.pdf.