Insect resistance to Bt Crops

Authors: Coates, Brad

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 11/1/2018
Publication Date: 11/28/2018
Citation: Coates, B.S. 2018. Insect resistance to Bt Crops. In: Proceedings of the 30th Annual Integrated Crop Management Conference, November 28, 2018, Ames, Iowa. AEP302-2018:37-41.

Interpretive Summary: The evolution of insecticide resistance within pest insect populations is a threat to sustainable crop production practices. A summary of the current insect species that have developed resistance to corn hybrids that express Bacillus thuringiensis (Bt) toxins was generated in an extension and outreach effort by a USDA scientist at the Corn Insects and Crop Genetics Research Unit in Ames, Iowa. The proceedings paper and the accompanying presentation will educate crop production professionals, including crop consultants from around the Midwest United States, on the geographic scope and severity of Bt toxin resistance. ARS and university research devoted to uncovering the causal genetic mechanism(s) of Bt resistance in western corn rootworm, corn earworm and western bean cutworm are highlighted. Furthermore, science-based strategies for insect resistance management and integrated pest management will be provided for distribution to growers. The information presented will benefit those interested in the practical impact of Bt toxin resistance on grower practices.

Technical Abstract: Prior to the development of insecticides, agricultural crop production suffered from drastic yield reductions as a result of insect feeding damage, which reduced both producer profitability and product quality. The invention of foliar and in-furrow chemical insecticides in the post-World War II era offered a means of control, but were not ideal due to difficulties in the timing of application, low persistence, environmental contamination, and non-target insect effects. Transgenic corn expressing the insecticidal Bacillus thuringiensis (Bt) protein toxin Cry1Ab targeting the European corn borer (ECB), Ostrinia nubilalis, was first commercialized in the mid-1990s. Production of an insecticidal protein by crop plants eliminated many of the drawbacks of chemical insecticides, and offered producers season-long control of insect feeding damage while increasing environmental stewardship. Transgenic Cry1Ab corn hybrids were so effective at controlling ECB feeding damage such that this pest has virtually been eliminated as a concern to growers and crop consultants. While this success encouraged early adoption by 2000 only 19% of corn acres were planted with these hybrids. However, a combination of favorable commodity prices from 2011 to 2014, and an increase in the availability and variety of insect pests controlled by Bt corn, contributed to the subsequent rise in acreage planted, reaching 82% in 2018 (USDA, Economic Research Service, 2018). Prior experiences with chemical insecticides have led to the belief that the increased exposure of insect populations to an insecticidal toxin can eventually lead to the selection of resistant individuals. In attempts to circumvent the onset of target pest insect resistance to Bt crops, the U.S. Environmental Protection Agency and seed corn companies implemented a high-dose refuge (HD/R) strategy for insect resistance management (IRM). The HD/R operates under the premise that insect resistance to Bt toxins is rare and functionally recessive, meaning heterozygote individuals carrying one copy of a resistance allele (Sr genotype) are functionally susceptible, and the Bt dose in a transgenic crop is sufficient to kill all heterozygotes as well as susceptible individuals. In addition, under the HD/R strategy producers are mandated the plant a refuge crop consisting of non-Bt corn on 20% acreage (block refuge), or use a 5 or 10% blended refuge in a bag (RIB) seed product, on which large number of susceptible individuals will grow and survive. Subsequent random mating of an overwhelming excess of susceptible individuals with rare individuals that are homozygous resistant (rr genotype) that develop on a Bt crop, will result in functionally recessive heterozygotes the next generation. Despite these efforts to preserve the utility of Bt toxins, several insect species have developed resistance and are currently causing damage to Bt corn. The following will provide a summary of the extent of resistance in insect populations, the genetic mechanisms by which resistance develops, and how resistance can be managed.