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Combating Resistance with a Refuge
Insects have developed resistance to a broad range of insecticides used in agriculture. The risk of insect resistance to insect-protected genetically modified (GM) crops is real but may be minimized with proper planning. In order to preserve the benefits of GM crops, Monsanto develops and implements insect resistance management (IRM) plans for each of its insect-protected products. Refuges are a key component of these efforts. In this backgrounder, we explore the refuge concept and its use.
Role of Refuges in Pest Management
In insect pest management, refuges are areas with little or no pesticide use that promote the survival of beneficial organisms and preserve susceptibility of insect pests. For beneficial organisms, refuges may provide needed food resources such as host plants, nectar, or pollen, as well as physical shelter needed for survival and reproduction. In some cases, refuges can be created by limiting insecticide use to only pest-infested areas within agricultural fields. Refuges can also be created by specific farming practices such as low or no-tillage or through the addition of organic matter, such as manure or straw.
A more recent use of refuges is to conserve susceptible pests for the purpose of combating resistance to insecticides. In this context, the refuge encourages multiplication of insects susceptible to insecticides. Any rare resistant individuals in insecticide-treated fields then mate with much more abundant susceptible individuals produced in refuges, thereby preserving pest susceptibility to the insecticide.
Refuges can take many forms, depending on the abundance of wild and cultivated vegetation in an area and insecticide use practices. Unstructured or Natural refuges commonly consist of alternate crops and uncultivated/wild host plants, including volunteer crop plants or weeds, on which insects can proliferate. Structured refuges are those that result from intentional planting of a crop on which insects will proliferate.
Refuges and Genetically Modified Crops
Commercialization of the first GM crops with insecticidal traits (so-called Bt crops) occurred in the mid 1990’s. These crops offer many benefits but pose concerns that insects susceptible to the Bt traits will develop resistance. To combat this risk, Monsanto has worked with scientists and government regulators to implement IRM programs that include refuges wherever Bt crops have been commercialized. For example, in the US, farmers are required to plant refuges for their Bt crop fields. Depending on the Bt crop and region, the size of structured non-Bt crop refuge required varies between 5 and 50%. In one case — with Bollgard II cotton in the southern states ranging from west Texas to the east coast — farmers can use natural refuge and need not plant a structured refuge.
Below are examples of structured non Bt-corn refuges that may be planted in association with Bt corn (corn containing a trait that protects it from damage by lepidopteran insects), depending on applicable requirements.
Farmers' Perspective on Structured Refuges
Structured refuges represent a significant cost to farmers in terms of time and money. It takes extra time to plant refuge blocks of conventional varieties and these frequently have substantially reduced yield, relative to Bt varieties. However, producers understand that these investments are necessary in order to keep Bt crops effective. In response to a survey performed by the Department of Agricultural Economics and the Department of Agricultural Statistics at Purdue University, most farmers agreed that refuges benefit all farmers and that the time and effort spent on refuges was justified by the benefits of maintaining resistance control (Alexander et al., 2006). Producers with experience planting Bt crops and implementing refuges have significantly more positive views towards refuges than those without experience.
Future Developments
Monsanto is actively pursuing ways to simplify and improve IRM programs for Bt crops. Stacking of multiple insecticidal traits in crop varieties is being undertaken to produce complementary pest control mechanisms in Bt crops, thus justifying smaller refuges. This is being coupled with region- and crop-specific analyses to identify landscapes where natural refuge would be suitable for combating resistance. Replacing structured refuges with natural refuges would enable farmers to reduce costs, while at the same time reducing the potential risk of pest resistance. Yet another option currently under scientific and field investigation is refuge in a bag, which comprises a mixture of Bt crop seed and conventional (refuge) seed in the same bag. As with natural refuge, refuge in a bag offers promise for simplifying IRM requirements for farmers.
References
Alexander, C. 2007. Insect Resistance Management Plans: The Farmers' Perspective. 10(1): 33-43.
Frisvold, G., Reeves, J. 2008. The Costs and Benefits of Refuge Requirements: The Case of Bt Cotton. Ecological Economics. 65(1): 87-97.
Gustafson, D., Head, G., Caprio, M. 2006. Modeling the Impact of Alternative Hosts on Helicoverpa zea Adaptation to Bollgard Cotton. Journal of Economic Entomology. 99(6): 2116-2124.
Landis, D., Wratten S., Gurr, G. 2000. Habitat Management to Conserve Natural Enemies of Arthropod Pests in Agriculture. Annual Review on Entomology. 45: 175-201.
Laxminarayan, R., Simpson, R. 2002. Refuge Strategies for Managing Pest Resistance in Transgenic Agriculture. Environmental and Resource Economics. 22: 521-536.
Piggott, N., Marra, M. 2007. The Net Gain to Cotton Farmers of a Natural Refuge Plan for Bollgard II Cotton. AgBioForum. 10(1): 1-10.
