Refuge-in-the-Bag Strategy for Managing Insect Resistance to BT Maize
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2015, Research Information Ltd. All rights reserved. Bacillus thuringiensis (Bt) is a common soil-dwelling bacterium, which can produce insecticidal proteins. Most of these Bt proteins are specifically toxic to some insect pests. Sprayable Bt insecticide formulations have been used for control of agricultural and health pests for many years, especially in organic farming. Bt insecticides are considered safe to the environment, people, soil decomposers, pollinators, parasitoids, and wildlife. Thus the United States government views the insecticidal properties of Bt as a public good. Alternatively, Bt insecticidal proteins could be effectively utilized by transferring these external Bt genes into plants. Transgenic Bt plants expressing Bt proteins within the plant tissue can directly kill target insects when the plant tissues are consumed. Genetically modified maize (GM maize) expressing Bt proteins for managing insect pests was first commercialized in 1996 in the United States and several other countries. Since then, Bt maize was widely accepted as a common tool to control a variety of maize pests. By 2014, >19.5 million hectares of Bt maize were planted globally (James 2014). In the same year in the United States, nearly 12.0 million hectares or 80% of the total field maize in the country was planted to Bt maize. Effectiveness of Bt maize against the target insect pests, in general, has been well documented in numerous studies. However, evolution of resistance in target pest populations is a great threat to the long-term efficacy of Bt maize products. Because the widespread use of Bt technology places consistently strong selection pressure on target pest populations, the development of resistance to Bt proteins is a potential problem. Intensive use of Bt maize and cotton has resulted in the development of documented field resistance to Bt proteins and subsequent reduced efficacy or control failure in at least six cases in the world (Tabashnik et al. 2013; Huang et al. 2015). Therefore, it is important to implement effective insecticide resistance management (IRM) programs to ensure the sustainable use of Bt insecticidal proteins either as a biocontrol agent or expressed in transgenic crops. Since 1996, the United Sates Environmental Protection Agency (U.S. EPA) has deployed a high dose/refuge IRM strategy for planting Bt maize. This strategy requires the Bt plants to produce a high dose of Bt proteins that can kill resistant heterozygote (RS) insects that carry one copy of the resistant genes. The high dose/refuge strategy requires planting a portion of non-Bt refuge crops along with the Bt crops. The purpose of planting non-Bt refuges is to sustain enough survival of susceptible insect populations such that these susceptible individuals develop without selection for resistance. Ideally, rare resistant insects (RR) originating from Bt plants will mate with these susceptible insects (SS) from the non-Bt refuge plants, and their resulting offspring are RS. Since RS will be killed by the high dose Bt proteins in the Bt plants (Figure 1), resistance frequencies in the pest populations should be maintained at low levels for a long period of time. Before 2010, Bt maize crops expressed only a single Bt protein for a target pest and the non-Bt refuge was required to be arranged in a structured form that was planted as blocks or strips along with the Bt plants in the fields (Figure 1). After many years of commercial planting of Bt crops, the high dose/structured refuge strategy appeared to be successful (Huang et al. 2011). However, the success of the structured refuge depends largely on growers complying with the non-Bt refuge maize planting. Therefore, there is a high risk for resistance evolution to Bt crops when farmers ignore the non-Bt refuge requirements. During the early years, a relatively high rate of compliance (e.g. 86-92%) was reported for U.S. Bt maize growers, but compliance rates have declined in recent years (Smith et al. 2012). Growers compliance with the structured refuge requirement has been an issue.
