How Science Wiped Out the Invasive Pink Bollworm in the U.S.
By Melissa Mayer
The very hungry caterpillars emerged from their eggs and bored into nearby cotton bolls, sinking their mouthparts into cottonseeds and blocking production of valuable lint as they burrowed through the bolls in their quest to tank up before their final molt. That was bad news for the cotton industry—starting in 1917 when the invasive pink bollworm (Pectinophora gossypiella) first showed up in the United States.
Arizona cotton growers lost $32 million due to crops damaged by the pink bollworm in 1990. They spent another $16 million on insecticides to control the pest, but those can’t really reach the caterpillars protected inside the bolls. In 1996, Bt cotton hit the market. This genetically engineered crop produces insecticidal proteins from a bacterium (Bacillus thuringiensis, or Bt), so when susceptible insects—like pink bollworm larvae—chomp down on that cotton, they don’t survive the meal.
That made a huge difference in controlling the pest, but researchers knew the clock was ticking. Pink bollworm larvae rapidly evolved resistance to Bt cotton in the lab and in cotton fields in India.
What happened next—a coordinated eradication program that wiped out the pink bollworm in the cotton-growing regions of the U.S.—is featured in the January 5 issue of Proceedings of the National Academy of Sciences. “I wanted to make sure that this great success of collaboration between farmers and entomologists… took its place in the history books and the textbooks,” says Bruce Tabashnik, Ph.D., head of the Department of Entomology at the University of Arizona and lead author of the article.
A Not-So-Modest Proposal
When Tabashnik first heard about the eradication proposal nearly two decades ago, he wasn’t a fan of the idea. “I thought it wouldn’t work,” he says, chuckling. “The adaptability of insects is one of the reasons they’re so difficult to control, manage, and—particularly—eradicate. So, when I knew that the growers in Arizona were thinking about this, it made me very anxious, because at that point things were being managed well in Arizona.”
Tabashnik agreed to run the computer models—fully expecting the math to quash the idea—but the results immediately won him over. “Anything we did that was realistic worked,” he explains. “So, I changed sides and became an advocate.”
The next step was convincing the Environmental Protection Agency. At that time, the EPA required farmers to plant areas of regular cotton called refuges so that any Bt-resistant moths would be more likely to mate with susceptible moths. However, the new plan would work best if farmers could plant all Bt cotton.
They paired that with another strategy called sterile insect release. In a facility in Phoenix, scientists bred pink bollworms and zapped the adults with radiation to render them sterile. Then, pilots flew over cotton fields, releasing scads of these sterile moths so they outnumbered wild type moths, making it tough to find a fertile mate.
Over the course of the eradication project, from 2006 to 2014, they dropped 11 billion sterile moths over Arizona cotton fields. For the first two years, those steriles outnumbered the wild moths by 2-to-1, which was enough to set off a cascade that ballooned to 2,000-to-1 by 2010. “When you get 2,000 steriles to one wild insect, it’s really hard for the wild population to sustain itself,” says Tabashnik. “2012 was the last year a wild pink bollworm moth was caught in Arizona. One was caught, and the ratio was 600,000 steriles to that one.”
Parallel programs ran in the rest of the region: California, New Mexico, Texas, and portions of Mexico. The Pink Bollworm Technical Advisory Committee, which included Tabashnik, wanted to see four consecutive years with zero pink bollworm catches before considering the pest eradicated. The U.S. Department of Agriculture made that declaration official in 2018—a century after the pink bollworm problem first showed up.
Pink bollworm eradication saved U.S. farmers $192 million between 2014 and 2019. It’s a feat that required careful coordination between scientists working in the cotton industry, government, and academia. It helped that pink bollworm caterpillars depend almost entirely on cotton as a host plant and that their Bt resistance is inherited recessively—but it’s possible the strategy could work for other pests too.
Because the pink bollworm is still thriving in dozens of other countries and global markets are so interconnected, Tabashnik thinks reintroduction of the pest is likely. They’ll monitor continuously and keep an arsenal of interventions at the ready. And there are other Bt toxins that kill resistant pink bollworm that could be engineered into cotton in the future.
Tabashnik hopes the story serves as a guidepost for other complex problems facing society. “It’s a great example of putting science to work to achieve something that benefits everybody,” he says.
Proceedings of the National Academy of Sciences
Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: firstname.lastname@example.org.
Tabashnik photo by Andrea Mathias.