By Leslie Mertz, Ph.D.
Three countries used the same tactic to fight the same pest on the same crop, but they had wildly different results. Why? A new article published in June in the Journal of Economic Entomology explores this intriguing tale of three countries and the broader lesson it holds for pest management.
Starring in this trilogy is the pink bollworm (Pectinophora gossypiella), a caterpillar pest that can devastate cotton. To fight this invasive insect, millions of growers worldwide have reduced their need for chemical sprays by planting transgenic cotton, called Bt cotton, which was genetically engineered to produce caterpillar-killing proteins from the bacterium Bacillus thuringiensis (Bt), says the article’s lead author Bruce Tabashnik, Ph.D., Regents’ professor and head of the University of Arizona (UA) Department of Entomology. According to the Tabashnik and coauthor Yves Carrière, Ph.D., professor of entomology at UA, Bt toxins kill some voracious pests including pink bollworm but are not harmful to people or to most beneficial insects. (Learn more in this fact sheet about Bt from the Entomological Society of America.)
At first, Bt cotton worked well against pink bollworm in all three countries, but this adaptable pest harbors mutations that confer resistance to Bt toxins. These mutations were rare before Bt cotton was commercialized. However, when two resistant caterpillars develop into moths and mate, their offspring are also resistant. If unchecked, the proportion of resistant insects increases every generation. Conversely, if a resistant moth mates with a normal, toxin-susceptible moth, the progeny remain susceptible. So, one strategy is to plant some non-Bt cotton—a refuge—allowing normal caterpillars to survive, become moths, and mate with the resistant moths, Tabashnik says. This makes it less likely that two rare, resistant moths will find one another in the crowd. Today, most countries require refuges and use Bt cotton that contains two toxins, so caterpillars need two mutations to survive.
In the remarkable story of pink bollworm versus Bt cotton in the world’s top three cotton- producing countries, here’s what happened:
United States: Through a carefully designed program that brought together farmers, government agencies, and University of Arizona research and extension scientists, growers in Arizona began planting Bt cotton when it was introduced in 1996. Exceeding the refuge requirement mandated by the U.S. Environmental Protection Agency, they planted at least 25 percent of their cotton acreage as non-Bt cotton statewide. This was enough to keep resistance at bay, Tabashnik says. Then in 2006, the strategy switched: Billions of sterile moths were dropped from airplanes into cotton fields, so the rare resistant moths were most likely to mate with sterile moths, which yielded no fertile offspring. The double whammy of Bt cotton and sterile moths, along with other tactics, eventually led to the October 2018 declaration by the U.S. Department of Agriculture that—after plaguing farmers for a century—the pink bollworm was eradicated from the nation’s commercial cotton-growing areas.
China: Growers in China are not required to plant refuges of non-Bt cotton, primarily because the Old World bollworm (Helicoverpa armigera) has been the main pest there, Tabashnik says. Unlike the pink bollworm, which feeds almost exclusively on cotton, the Old World bollworm eats many other crops that serve as refuges. With non-Bt cotton refuges scarce, pink bollworm resistance to Bt cotton began to increase. Coincidentally, to boost yield and reduce their production costs, cotton seed companies started selling second-generation hybrid seeds that generated fields with 25 percent non-Bt cotton plants randomly mixed with Bt cotton. The growers were not aware of this change, but apparently preferred the hybrid seed because of its short-term economic and agronomic advantages, Tabashnik says. With the increase in refuge, he says, the pendulum swung the opposite way. Resistance declined and pink bollworm was effectively suppressed by Bt cotton.
India: In India, Bt cotton seed comes with a small packet of non-Bt seeds, enough for growers to plant 20 percent of their cotton as refuges. Unfortunately, growers haven’t planted the refuge seed, in part because they haven’t been given sufficient education about the value of refuges, Tabashnik says. With meager refuges, widespread resistance to Bt cotton producing one toxin evolved in less than a decade. Before most growers switched to Bt cotton producing two toxins, pink bollworm was already resistant to the first toxin, and resistance to the second toxin evolved rapidly. In India today, he says, Bt cotton provides little or no control, pink bollworm is rampant, and growers must switch back to the tactics they used before, such as shortening the cotton season.
“So, what have we learned? If you want to slow the evolution of resistance, make sure there are sufficient refuges, and don’t rely on a single control tactic. That’s a general principle that can be applied to any transgenic crop,” Tabashnik says. “The theory was already absolutely clear on this point, and small-scale experiments provided some evidence. Now we also have this tale of three countries that is one of the most compelling comparisons supporting the idea: Refuges can delay resistance.”
Journal of Economic Entomology
Leslie Mertz, Ph.D., teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.