By Mari N. Jensen

A cotton boll with a pink boll caterpillar inside. Photo credit: Timothy Dennehy.

Biotech cotton has beaten back pink bollworm eight years running, reports a team of scientists from The University of Arizona (UA) in Tucson.

The surprising finding is good news for the environment. Arizona farmers who plant the biotech cotton known as Bt cotton use substantially less chemical insecticides than in the past.

Insect pests sometimes evolve resistance to such chemicals in just a few years, a fate that was predicted for biotech crops genetically altered to produce Bt toxin, a naturally occurring insecticide.

"This is the most complete study to date for monitoring resistance to Bt crops," said team leader Bruce E. Tabashnik, the head of UA's department of entomology, a member of UA's BIO5 Institute and an expert in insect resistance to insecticides.

“We found no net increase in insect resistance to Bt. If anything, resistance decreased. This is the opposite of what experts predicted when these crops were first commercialized.” He added, “I’m definitely surprised.”

Tabashnik, Timothy J. Dennehy, a UA Distinguished University Outreach Professor of Entomology and extension specialist and a member of BIO5, and Yves Carriere, UA associate professor of entomology, will publish their research in an upcoming issue of the Proceedings of the National Academy of Sciences.

Bt cotton has been planted in Arizona since 1996. Now more than half of the state’s 256,000 acres of cotton fields are planted with the biotech plants. Without the protection provided by Bt cotton, some fields can have 100 percent of plants infested with pink bollworm caterpillars, which live inside the cotton boll, destroying the crop.

Dennehy said, “In an extreme infestation, you can have every single boll in the field infected." The caterpillars eat the seeds and damage the developing cotton fibers.

In contrast, when the caterpillars eat Bt cotton, they die.

Cotton fields in Parker Valley, Ariz. Bt cotton is the greener field in the foreground. The whiter swath of cotton in the background is a refuge field of non-Bt cotton. Photo credit: Timothy Dennehy.

Before the use of Bt cotton became widespread, pink bollworm was one of the top three insect pests of cotton in the Southwest. In 1995, losses from pink bollworm in Arizona cotton were estimated to be $8.48 per acre, totaling $3.4 million statewide. Cotton is grown in eight Arizona counties: Cochise, Graham, La Paz, Maricopa, Mohave, Pima, Pinal and Yuma.

"Moreover, the harsh insecticides used to control pink bollworm resulted in a host of other insect pests becoming more serious problems," Dennehy said.

Everything changed in 1996, he said, when Bt cotton and two other "soft" insect control tactics replaced a large amount of the harsh pesticides used on cotton crops. Spraying less chemical insecticides means more beneficial insects survive, further reducing the need for spraying.

By 2004, pink bollworm losses had fallen to nearly half of earlier levels, $4.34 per acre.

Tabashnik said, “Some of the other pests are not so much of a problem because we’re not killing their natural enemies with insecticides."

Dennehy added, “These soft toxins plus the good bugs acting together have driven pesticide use to historic low levels ... this is a wonderful success of integrated pest management.”

Since widespread adoption of Bt cotton in 1997, insecticide use on Arizona’s cotton crops is down 60 percent, said Tabashnik. The reduction in chemical pesticide use saves growers about $80 per acre. According to the Arizona Agricultural Statistics Bulletin, the value of Arizona's cotton crops for 2004 was estimated at $207 million.

The key to Bt cotton’s continued efficacy is the use of refuges – patches of traditional cotton intermingled with the fields of Bt cotton.

The refuges ensure that the few pink bollworm moths that are resistant to Bt are most likely to mate with Bt-susceptible pink bollworm moths that grew up in the refuges. The offspring from such matings die when they eat Bt cotton.

In contrast, if all of Arizona’s cotton was Bt cotton, only pink bollworm caterpillars that were resistant to the Bt toxin would survive. If resistant pink bollworm moths mated with each other, their offspring would be resistant and could feed on Bt cotton. Bt cotton would then become useless against pink bollworm.

The UA team used a combination of field surveys, laboratory testing and mathematical modeling to determine if pink bollworm had become resistant to Bt cotton.

The team did find Bt-resistant pink bollworm caterpillars in the field, but they were rare.

Tabashnik said that doesn’t mean the insects won’t bite back in the future. "It's not that pink bollworm can't beat Bt toxin, but that it hasn't beaten Bt toxin so far."

There's a new variety Bt cotton now available that has two different Bt toxins, he said. The team's next step will be to determine how to best use that combination of toxins to stay one step ahead of the pink bollworms.

The U.S. Department of Agriculture, the Arizona Cotton Research and Protection Council, the Monsanto Company, and Cotton, Inc. funded the research.