China, the top cotton-producing country in the world at 16 billion pounds per year, first introduced Bt cotton in 1997. For the first decade, Bt cotton was highly effective against both cotton bollworm and pink bollworm. Recently, however, early signs of resistance to Bt cotton have been detected in caterpillars collected from the field, and growers may have to increase the use of insecticides if resistance evolves further, perpetuating a dangerous cycle, according to the University of Arizona (UA).
“We are seeing warning signs,” said Xianchun Li, a Chinese native and associate professor of entomology at UA’s College of Agriculture and Life Sciences. “We want to transfer the successful technology that we have here to China to help them prevent further development of resistance.”
Bruce Tabashnik, head of the department of entomology at UA, agreed, saying, “What I would love to see is China able to sustain control of these major pests with Bt cotton so it can reduce its use of insecticides.”
In order to achieve this goal, Li, Tabashnik, and Kongming Wu, a top researcher with the Institute of Plant Protection at the Chinese Academy of Agricultural Sciences in Beijing, have joined forces and obtained a $475,000 grant from China’s National Science Foundation to study resistance in pink bollworm in China and to develop strategies against it.
The team is collaborating with growers, biotechnology companies, the U.S. Department of Agriculture and the U.S. Environmental Protection Agency to manage resistance and eliminate insecticide sprays against pink bollworm in Arizona. And, thanks to the effort, Tabashnik reports that Arizona is now a poster child for resistance management in Bt crops. “Pink bollworm is rare now in Arizona. We have very strong evidence that there is no resistance in the field,” he said.
In order to mirror these results in China, samples of pink bollworm larvae in Chinese fields are being collected and studied at the DNA level.
“We will track changes in the frequency of resistance genes in the field and use computer simulations to help determine the best strategies for delaying resistance,” Tabashnik said.
It is likely strategies that have worked well in Arizona will benefit Chinese growers.
Three of these strategies are:
1) Refuges: This method, which consists of planting standard cotton plants that do not make a Bt toxin, enables survival of insects that are susceptible to the toxin. According to the team, refuges are planted near Bt crops with the goal of producing enough susceptible insects to make it unlikely two resistant insects will mate with each other. This method, used from 1996 to 2005 in Arizona, is considered to have made a sizable difference in pest pressure and resistance levels.
2)Sterilization: A highly successful multi-pronged attack on pink bollworm was launched in 2006 in Arizona. Several times each week, airplanes released billions of sterilized pink bollworm moths over all cotton fields in Arizona. Matings between these sterile moths and wild pink bollworm moths do not yield fertile offspring. Because Bt cotton kills nearly all pink bollworm caterpillars, the rare resistant moths are most likely to mate with the abundant sterile moths – an evolutionary dead end. Although the sterile insect release technique is not new, this is the first time it’s been used jointly with a Bt crop. According to UA, this multi-faceted strategy continues today.
3) Dual Toxin Bt: Bt cotton engineered to make two distinct toxins is successfully used in Arizona. This way, pests arising with resistance to one toxin are killed by the other toxin. According to the researchers, switching to two-toxin Bt cotton could help to stall resistance in China.
The goal of the team’s research, according to Li, is to prevent the return of the resistance crisis in China that occurred in the 1990s. “So much pesticide was used that the pests became resistant. You could actually throw a caterpillar into the pesticide and they would swim around. It wouldn’t kill them. Farmers couldn’t grow cotton and it really hurt the economy,” Li said. “We hope to catch this in the early stages.”