Biotech Cotton Beats Pink Bollworm for Eight Years Running

University of Arizon-Tucson finishes most complete study on monitoring resistance to Bt crops. Compiled by staff

Published on: Oct 20, 2005

Biotech cotton has been beaten pink bollworm solidly over the last eight years, and still going strong. A team of scientists from the University of Arizona in Tucson, recently finished the most complete study to date for monitoring resistance to Bt crops.

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.

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, says, "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 adds, "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.

Dennehy says, "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.

Before the use of Bt cotton became widespread, pink bollworm was one of the top three insect pests of cotton in the Southwest. "Moreover, the harsh insecticides used to control pink bollworm resulted in a host of other insect pests becoming more serious problems," Dennehy states.

Everything changed in 1996, he says, 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.

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

Dennehy adds, "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."

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.

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 says 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 says. 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.