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The Environmental Protection Agency (EPA) recently released a newly drafted proposal for regulations related to Bacillus thuringiensis (Bt) resistance management. The goal of these regulations is to extend the lifespan of current Bt traits in corn and cotton, as well as any future products that may contribute to less conventional pesticide use. The proposal will change some of the current insect resistance management (IRM) strategies and also incorporate additional strategies to reduce the risk of resistance. The proposal is specifically focused on Lepidopteran pests such as corn earworms (cotton bollworm), fall armyworms, and western bean cutworms. Of these, only corn earworms and fall armyworms occur in Alabama.

A Need for Change

The current IRM plan was created when Bt corn and cotton were first introduced more than 20 years ago. The original plan addressed resistance monitoring strategies, grower compliance, and mitigation plans in the case of Bt-resistant populations. This newly drafted plan is needed because of the documented resistance of caterpillar pests and the threat of additional resistance in the future.

Corn earworms are currently resistant at some level to all the Bt toxins on the market except for one. The Vip3A gene is the only remaining Bt toxin with no resistance for corn earworm across the cotton belt. Bt resistance has also been documented for fall armyworms and western bean cutworm.  Many of these cases were documented by academic scientists prior to widespread issues. However, they did not meet the EPA’s regulatory definition of pest resistance, which relies on laboratory bioassays paired with economic damage in the field. This method has failed to detect early reports of resistance in a timely fashion. Without early detection, mitigation plans are often put in place too late.

The New Proposal

The new proposal addresses several risk factors that have led to increased resistance development. These risk factors include planting single-trait Bt corn, a limited number of highly effective Bt traits, a lack of refuge compliance, and others. Many changes are put forth in the proposal. Some of these changes, that are characterized below, are major changes that will substantially affect Bt corn and cotton production. The entire document of changes can be read online.

Definition of Resistance

Previously, field failures went through a series of regulatory tests to ensure the insect population was in fact resistant before reporting it to the EPA or public. This process made it almost impossible to inform growers or the EPA of Bt resistance in a field or area before it became widespread. The new proposal addresses this issue with a new definition of resistance. Field failures are now considered cases of practical resistance. The unexpected injury (UXI) levels in the table below are the thresholds to trigger this definition. Seed companies can still make collections and conduct laboratory tests if they wish to refute the determination of practical resistance. Fields with UXI that meet thresholds will automatically begin a resistance mitigation program while the insect population is laboratory tested to confirm resistance. UXI events will be communicated to growers, local Extension personnel, crop consultants, and others as part of an enhanced communications strategy.


Unexpected Injury (UXI) Levels

CropBt ToxinSample SizeUXI Threshold
CottonVip3A100 bolls/fruits/squares6% injury
CottonCry2100 bolls/fruits/squares12% injury
CornVip3A30 earsTen percent of the ears have second instar larvae or exit holes present and greater than 60 damaged kernels.
There is greater than 75cm² injury with second instar larvae or exit holes present.
CornCry230 earsFifty percent of the ears have second instar larvae or exit hole present and 600 damaged kernels.
There is greater than 240cm² injury with second instar larvae or exit holes present.


Resistance Monitoring and Communication

To monitor for UXI, sentinel plots will be used throughout the cotton belt and in the Great Lakes Region/Nebraska. The sentinel plots will be used in addition to grower reported UXI. These sentinel plots will be monitored regularly for UXI. If the UXI in these plots is above the determined thresholds, they will be deemed practically resistant. UXI events above threshold are treated the same if they are a grower field or one of the sentinel plots. For any resistant population, companies are encouraged to implement best management practices in the region. There will also be increased communication following an UXI event. The affected registrant (i.e. seed company) is responsible for a regional communication plan that alerts growers, consultants, Extension personnel, seed distributors, and state/federal authorities.

Public Comment Needed

The EPA is seeking feedback on its plan from growers and others in the agricultural industry before it is finalized. The public, 60-day comment period is open through November 9. Comments on the plan can be submitted online.

The EPA is especially concerned about comments made in reference to three options put forth in the proposal. The EPA will not take a position on these options until it has read all comments from stakeholders. These three option are as follows:

  1. Phase Down of Single Traits and Non-functional Pyramids. A short term phase down of single trait corn products controlling Lepidopteran pests and a long-term phase down of compromised pyramid corn and cotton products.
  2. Increasing Percent Refuge in Seed Blend Products. An increase from 5 percent refuge-in-a-bad in Bt corn pyramid products to 10 percent refuge nationwide.
  3. Refuge Compliance Monitoring. Mandatory registrant on-farm refuge compliance visits, measures for non-compliant growers, increased seed dealer record keeping. Registrants would be required to implement these measures with growers.
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