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Posted: June 2014

Increasing Resistance of Pests to Genetically Modified, Insect-resistant Bt Crops

Bt crops have been genetically modified to create toxins that are lethal to certain insects. Corn (maize) and cotton are the primary Bt crops that have been grown commercially since 1996. Between 1996 and 2012 a cumulative total of over one billion acres of cropland have been planted with Bt crops. In 2013 three-quarters of the corn and cotton in the United States consisted of Bt varieties (link formerly reported here is no longer available:

Since the advent of Bt crops, ecologists have known that target insects would evolve resistance to Bt toxins. It was a question of how long this would take. In order to delay insect resistance, a so-called “refuge strategy” was mandated in the U.S., Australia, and other countries. Farmers have been required to grow non-GM, non-resistant plants alongside the GM plants. The idea is that “most of the rare resistant pests surviving on Bt crops will mate with the relatively abundant susceptible pests from nearby refuges of host plants without Bt toxins. If inheritance of resistance is recessive, the progeny from such mating will die on Bt crops, substantially delaying the evolution of resistance” (Tabashnik et al. 2013). It seems that this strategy has slowed the emergence of resistant insect populations, which, however, have begun to appear around the world in recent years.

Tabashnik et al. (2013) analyzed the results of 77 different studies from five continents that had monitored resistance to Bt crops in agricultural fields. Thirteen major pest species—ones that would be killed by consuming Bt crop tissue—were examined. Up until 2005, only some populations of one pest species had evolved resistance to Bt to the extent that the crops were being noticeably damaged (“reduced efficacy”). By 2012, in some populations of five of the thirteen species, “reduced efficacy” was reported. In such cases more the fifty percent of the individuals in the population were resistant to the Bt crops and the resistance had evolved in less than ten years. They found an additional pest species in which more than fifty percent of individuals in some populations had evolved resistance, but with crop damage not yet being reported. Four additional pest species had populations with low numbers (one to six percent) of resistant individuals.

In the worst case, resistance evolved in three years. This occurred in Puerto Rico, where the fall armyworm (Spodoptera frugiperda) became resistant to Bt corn and did extensive damage to the corn. It was withdrawn from the market in Puerto Rico in 2007. And although no Bt corn has been commercially planted on the island since then, the majority of armyworms remain resistant to Bt toxins (Storer et al. 2012). Storer et al. write, “we do not anticipate a rapid reversion to susceptibility in Puerto Rico.”

For a description of growing insect resistance to Bt corn in Iowa, see this report.

In the best cases, according to Tabashnik et al., “efficacy can be sustained for 15 years or more.” But resistance will evolve at some point, which has led biotechnology companies to market GM crops that produce more than one type of Bt toxin in an effort to slow the rate at which pests become resistant. But that such insecticidal plants with multiple modes of action represent a viable “long-term resistance management” (Storer et al. 2012) is illusory. We know that weed species have evolved resistance to more than one herbicide over a relatively short period of time (see our report on glyphosate resistance).

Tabashnik, B. E., T. Brévault, and Y. Carrière (2013). “Insect Resistance to Bt Crops: Lessons from the First Billion Acres,” Nature Biotechnology vol. 31, pp. 510-21. doi:10.1038/nbt.2597. Also available here:
Note: Tabashnik reports that he is the “coauthor of a patent on engineering modified Bt toxins to counter pest resistance” and that “Dow AgroSciences, Monsanto and Bayer CropScience did not provide funding to support this work, but may be affected financially by publication of this paper and have funded other work by B.E.T.”
Storer, N. P., M. E. Kubiszak, J. E. King et al. (2012). “Status of Resistance to Bt Maize in Spodoptera frugiperda: Lessons from Puerto Rico,” Journal of Invertebrate Pathology vol. 110, pp. 294-300. 10.1016/j.jip.2012.04.007.
Note: Storer works for Dow Agrosciences, which marketed Bt corn in Puerto Rico.

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