Insect Resistance Management for Bt Plant-Incorporated Protectants, Regulation of Biotechnology under TSCA and FIFRA, US EPA
- 1 US EPA
- 2 Overview of Insect Resistance Management (IRM)
- 3 How Does EPA Manage Resistance to Bt Crops?
- 4 How Does EPA Monitor Bt Crops for Pest Resistance?
- 5 What Happens if Resistance in Bt Crops Is Detected?
- 6 How Does EPA Manage Refuge Compliance?
- 7 Information Sources (How Do I Get Additional Information on IRM?)
- 8 Rootworm — basic control measures
- 9 Abstract
- 10 What Is a Control Measure in Healthy and Safety?
Overview of Insect Resistance Management (IRM)
Description of Bt PIPs
Bt plant-incorporated protectants (PIPs) are plants that have been genetically altered to produce proteins that are harmful to certain insect pests. This has been accomplished by transferring specific genetic material from a bacterium, Bacillus thuringiensis (Bt), to the plant’s genome. The genetic material encodes these proteins that have specific toxicity to certain insect pests, but do not affect mammals or non-target organisms.
Beginning in 1995, EPA has registered numerous cotton and corn PIPs that have since been widely adopted by growers in the United States and other countries. For example:
- Bt cotton PIPs are designed to control lepidopteran (moth) larvae (budworms and bollworms).
- Bt corn PIPs have been developed for the control of lepidopteran larvae (corn borers) and/or coleopterans (corn rootworm beetles).
Benefits of Bt PIPs
Bt crops have provided substantial human health, environmental and economic benefits. Growers planting Bt crops may need to use less conventional (chemical) insecticides for pest control, which has both human health and environmental benefits. At the same time, growers may realize increased crop yields through better pest control and lower overall input costs.
Furthermore, Bt is well known as a low risk pesticide with little or no toxicity to mammals or non-target organisms. Pesticide products containing microbial Bt have been used in organic agriculture for many years. These benefits may be eroded, however, if insects develop resistance to the Bt PIPs.
Risk of Resistance for Bt PIPs
Like most pesticides, insects are capable of developing resistance to Bt proteins. In Bt PIPs, this risk may be heightened by the fact that:
- Bt proteins are expressed at high levels in most or all plant tissues;
- the proteins are produced by the plant continually during the growing season (i.e., throughout the lifespan of the plant); and
- some of the major target pests, such as European corn borer, corn rootworm, and pink bollworm, feed almost exclusively on corn or cotton.
These factors can increase insect exposure to the controlling toxins (Bt protein) and hence, increase selection pressure for resistance. That means that if the toxin kills susceptible insects, those that survive and reproduce are more likely to be resistant to the toxin.
Why IRM for Bt PIPs?
EPA places a high value on the efficacy of Bt PIPs and on preserving their significant agricultural and environmental benefits. Therefore, the Agency is committed to maintaining effective oversight of these products to mitigate the development of resistance in the target pests. To counter the threat of resistance, EPA has mandated the implementation of an Insect Resistance Management (IRM) plan for each commercially registered Bt PIP. The goal of IRM is to delay the onset of resistance for as long as possible, though it is important to note that it may not be possible to entirely prevent resistance from evolving.
How were the current IRM strategies developed?
EPA has used the best available science to develop the current IRM strategies for Bt PIPs:
- IRM plans are developed using a risk assessment process in which the Agency evaluates information on the biology of the target pests and the nature of the PIP (i.e., how much of the Bt toxin is expressed by the plant and how susceptible the target pests are to it).
- EPA also considers the potential for cross resistance, in which resistance to one Bt PIP confers resistance to other Bt PIPs.
- Finally, EPA evaluates simulation models that evaluate how resistance can evolve to PIPs among target pests and which IRM strategies work best.
Detailed IRM assessments for specific PIPs, including summaries of the data and information reviewed to develop the strategy, can be found in EPA’s “Biopesticide Registration Action Documents” (BRADs) for each registered PIP. View information about available BRADs.
To aid in developing IRM plans for Bt PIPs, EPA has frequently sought advice from the FIFRA Scientific Advisory Panels (SAP). These panels consist of scientists who are experts in resistance management and have provided feedback and recommendations to EPA on how to best manage resistance concerns. View a list of the relevant SAP meetings for IRM.
How Does EPA Manage Resistance to Bt Crops?
The Risk of Resistance: Pyramids vs Single Toxin Products
Many of the registered Bt PIPs are deployed as “pyramids” that include two or more Bt proteins targeted at the same target pest(s). Pyramids are inherently more durable and less at risk of resistance than varieties that contain only one Bt toxin. For this reason, pyramided Bt products generally have lower resistance management requirements than single trait products (see the “Refuge” section below).
Other Bt PIP products include “stacked” toxins that target multiple target pests (e.g., lepidopteran stalk borers and corn rootworm). These stacked PIPs allow grower to manage different pest complexes with the same product.
The Role of Refuges in Resistance Management
The primary resistance mitigation measure for Bt crops has been the use of refuges. Simply put, a refuge is intended to provide a source of large numbers of Bt-susceptible insects to counter any resistant insects. Overall, the IRM refuge strategy has largely been successful in delaying insect resistance.
It is called refuge because the insects living on these non-Bt plants are sheltered from the pesticidal effects of Bt and are consequently not pressured into developing resistance to Bt proteins. If one assumes that any insects emerging from the Bt field will be resistant to Bt (and few in number), susceptible insects from the refuge should be able to overwhelm the resistant insects and dilute the frequency of resistance genes in the pest population. Typically, a refuge is a portion of a farmer’s operation that is planted to a non-Bt variety of the crop.
Refuges have a size component – usually a percentage of the total Bt crop planted – and must be planted close enough to the Bt field(s) to ensure that the susceptible insects are able to mate with any resistant ones. The size of the refuge is determined by resistance risk. For example, refuge requirements for Bt corn are larger in southeastern cotton-growing regions due to a target pest (corn earworm) in this area that feeds on both corn and cotton. Successive generations of this insect can be exposed to PIP proteins in both Bt corn and Bt cotton during the same growing season, which increases the resistance risk.
For Bt PIPs, there have been three different refuge approaches:
- Structured refuges are a dedicated portion of the farming operation devoted to a non-Bt variety. These refuges are planted as discrete fields (blocks), border rows surrounding a Bt field(s), or rows within the Bt field(s). The key components for structured refuge are its size (as a percentage of the corresponding Bt crop) and proximity to the Bt field(s). Refuges must be able to generate a sufficient number of susceptible insects and be close enough to the Bt field so that susceptible insects (from the refuge) and resistant insects (from the Bt fields) can interact and mate.
- Seed blends (“refuge-in-the-bag”) incorporate non-Bt seed (refuge) with Bt seed in the same seed bag. The advantage of seed blends is that growers don’t need to coordinate the planting of a separate refuge – refuge compliance is therefore assured.
To date, seed blends have been approved for some Bt corn PIP products. EPA has held several FIFRA Scientific Advisory Panel (SAP) meetings on seed blends – for more information on these meetings refer to the links listed below under “Information Sources.”
“Natural refuge” refers to wild hosts, weeds, or other cultivated crops that can serve as a source of susceptible insects. Such a refuge can be effective if the target pest(s) feeds on multiple plant hosts and doesn’t specialize solely on the Bt crop. Natural refuge has been approved only as an IRM strategy for Bt cotton in the southeastern United States. EPA held an SAP meeting on natural refuge in 2006 – for more information refer to SAP links listed below under “Information Sources.”
Registered Bt corn and cotton products for commercial use are required to use one of the above refuge strategies. Structured refuges and seed blends have been employed for Bt corn products, while natural refuge has been used for Bt cotton in the southeastern United States.
The following tables list the current refuge requirements for Bt crops.
|Bt Corn Type||Target Pests||Structured Refuge||Proximity to Bt Fields||Seed Blend|
|Single toxin||Lepidoptera||Corn Belt: 20%
Cotton regions: 50% 1
|1 or separate 20% refuges Cotton regions: separate 2 20% (CRW) and 50% (Lep) refuges||Combined refuge 2 : adjacent or within Separate refuges 3 : adjacent (CRW) and 1||1||Areas with Leps: 1 “Cotton regions” refers to cotton production areas in the following states: Alabama, Arkansas, Georgia, Florida, Louisiana, North Carolina, Mississippi, South Carolina, Oklahoma (only the counties of Beckham, Caddo, Comanche, Custer, Greer, Harmon, Jackson, Kay, Kiowa, Tillman, Washita), Tennessee (only the counties of Carroll, Chester, Crockett, Dyer, Fayette, Franklin, Gibson, Hardeman, Hardin, Haywood, Lake, Lauderdale, Lincoln, Madison, Obion, Rutherford, Shelby, and Tipton), Texas (except the counties of Carson, Dallam, Hansford, Hartley, Hutchinson, Lipscomb, Moore, Ochiltree, Roberts, and Sherman), Virginia (only the counties of Dinwiddie, Franklin City, Greensville, Isle of Wight, Northampton, Southampton, Suffolk City, Surrey, Sussex) and Missouri (only the counties of Dunklin, New Madrid, Pemiscot, Scott, Stoddard).
2 “Combined refuge” refers to a single refuge meant to address both lepidopteran and coleopteran target pests.