Cotton growers in the desert West are breathing a collective sigh of relief. The Arizona Bt Cotton Working Group announced recently that pink bollworm is not developing resistance to Bt cotton as rapidly as had been predicted--thus extending the availability of a highly successful tool in an ongoing battle with the insect.

Researchers who are part of a team that monitors intensively for Bt-resistant pink bollworm are finding many fewer than anticipated, says University of Arizona entomologist Bruce Tabashnik, who is part of the monitoring program.

Five years after its commercial introduction, researchers are finding resistance in less than 1% of the insects rather than the much higher percentage that had been expected. "Based on the history of insect adaptation, there is a general sense that insects will become resistant," Tabashnik explains. "Some 500 insect species have adapted to one or more pesticides--it's an established pattern."

A surprise. While Tabashnik and other scientists are surprised by the lack of resistance that has developed, Arizona cotton growers are delighted. They have embraced Bt cotton with open arms, using it for more than 50% of their production. The biotech tool has helped them get a firm handle on controlling an insect that has been a devastating pest in Arizona and California for the past 35 years.

Growers in those states have applied more than 72 million acre-equivalents of pesticides at an estimated cost of $1.3 billion, says Larry Antilla of the Arizona Cotton Research and Protection Council, a major player in the Arizona Bt Cotton Working Group.

"In the past without Bt cotton, many growers had to spend more than $100 per acre to control pink bollworm. Now, more than ever, the economics are to the point that it's not how well can we survive without Bt cotton, but could we survive at all," he says.

So, tracking resistance to preserve the Bt tool is essential. It involves a multilevel approach that relies on close cooperation among the Arizona council, the University of Arizona, growers, industry and other stakeholders. The effort includes:

• Population monitoring, which provides insight into the suppressive effects of Bt cotton on pink bollworm.
• Paired field studies that sample pairs of adjacent Bt and non-Bt fields to measure the efficacy of Bt in areas of high pink bollworm pressure.
• Embedded refuge studies, which look at a single-row embedded non-Bt refuge strategy as an effective alternative to external refuges.
• A Rapid Response Team, which sends trained personnel to investigate reported cases of inadequate performance of Bt cotton.

The program builds on the work of Tim Dennehy, director of the Extension Arthropod Resistance Management Laboratory at the University of Arizona. He has been monitoring resistance development since the early '90s. "The strategies in place when Bt cotton was released commercially were largely based on computer modeling--we didn't have any field studies," Antilla says. "But when the commercial acreage of Bt cotton jumped dramatically in 1997, we knew we had to get a system for monitoring in the field and a database in place."

In addition to documenting the lack of resistance, the monitoring system is showing the value of the embedded refuges. There is agreement that a refuge strategy is vital to delaying development of resistance in insects, but the amount and placement is up for debate.

"There was some talk about increasing the mandated refuge size," Antilla says. "The theory is that if a teaspoon is good, a tablespoon would be better. Our monitoring system indicates that current requirements are more than adequate and that the embedded refuges are effective."

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