Combining Bt Cotton, Sterile Insects Prevents Destruction of Cotton Plants

Bruce Tabashnik, study leader and department head of entomology in the University of Arizona's College of Agriculture and Life Sciences, has used pest-resistant cotton and the release of sterile pink bollworm moths as a type of birth control that will help prevent destruction against cotton plants. Tabashnik and his team targeted caterpillars of the pink bollworm as the enemy. These insects, which were first discovered in 1917, are one of the top destroyers of cotton plants worldwide. To remedy this issue, researchers used computer simulations to analyze over a decade of field data before conducting tests. Once testing began, Tabashnik used Bt cotton, which is a genetically engineered crop that contains a gene taken from the bacterium Bacillus thuringensis and gives the plant a protein capable of killing certain insects. But Bt cotton cannot kill off cotton-destroying insects alone because it only kills certain insects, not all. Fortunately, pink bollworm caterpillars fall into the category of insects that are killed after eating Bt cotton, but the problem is that these caterpillars can become resistant to the toxins after a period of time, rendering the Bt cotton useless to a certain extent. "The most widely used strategy to delay resistance is to set aside refuges...patches with regular, non Bt cotton where the pest can feed without ingesting the Bt cotton," said Tabashnik. "If you plant Bt cotton with no refuges, the vast majority of the caterpillars will die, but a tiny fraction will be resistant. These rare resistant survivors will emerge as adults and mate with each other." Refuges make it so a majority of the insect population is non-resistant to the toxins. Since the amount of non-resistant caterpillars/moths outnumber those who are resistant, the chances of two resistant insects mating are slim. But the problem is that the pests are still alive and reproducing, whether they're resistant or not, thus destroying cotton plants. Refuges do not completely resolve the issue. To keep unwanted visitors away from the cotton plants, researchers obtained large numbers of pink bollworms and sterilized them. Then, the sterile bollworms were released into the crop fields to block reproduction. "When a sterile moth mates with a fertile, wild moth, the progeny won't be fertile," said Tabashnik. "The sterile insects soak up the reproductive potential of the wild population. If you have a high enough ratio of sterile to wild moths, you can drive the reproduction of the wild population to zero." Tabashnik and his team began testing this strategy in 2006. It is the first method of pest control that combines Bt cotton and sterile moths. As it turns out, this strategy has proved to be very effective. From 1990 to 1995, cotton growers lost $18 million per year to the management of pink bollworm. From the time Tabashnik and his team began testing until 2009, the pink bollworm population decreased by 99.9 percent. In 2009, only two pink bollworm larvae were found in 16,600 bolls of non-Bt cotton in Arizona. Along with this decreased population came a large reduction in insecticide spray purchases. Between 2006 and 2009, cotton growers' cost to manage pink bollworm fell from $18 million to $172,000. "We are running the pesticide treadmill in reverse," said Tabashnik. "Our new approach has resulted in huge environmental gains. We are using cutting-edge technology to create sustainable cotton farming practices." This study was published in Nature Biotechnology on November 7.