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Tuesday November 21, 2000

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Toxins save cotton fields from pests, UA study says

By Blake Smith

Arizona Daily Wildcat

Genetically-modified cotton allows farmers to harvest

Many of the pests destroying Arizona's cotton crops are not resistant to a natural protein toxin, as originally believed, a UA entomology department study released today found.

During a three-year study, UA researchers examined the effects of genetically modifying many of Arizona's cotton fields to include the Bt (Bacillus thuringiensis) bacterium, which kills crop-destroying insects before they ruin valuable fields.

Scientists collected cotton samples from area fields, looking for pink bollworms - one of the most destructive pests to cotton crops.

They let the pink bollworms mate, put their offspring on a wheatgerm diet laced with Bt bacterium and waited three weeks to see if the newborns had died from the effects of the toxin, said Bruce Tabashnik, head of the University of Arizona entomology department.

"We found that one in 10 developed a resistance in 1997," Tabashnik said. "It is higher than expected, but the Bt toxin is still effective."

Before the introduction of the Bt toxin, Arizona farmers were losing about $170 per acre each year. Since the pink bollworm began destroying cotton crops in Arizona 30 years ago, the farming industry has lost nearly $1.3 billion in potential revenue.

Tabashnik and other researchers also found that a resistance to the Bt toxin can only be passed on to offspring if both parents possess the gene. Pink bollworms have only become resistant to the toxin in a laboratory setting, not in cotton fields.

The diamondback moth, another cotton crop pest, has been the only insect to become resistant to Bt toxin while living in the fields, Tabashnik added.

Larry Antila, staff director for the Arizona Cotton Research and Protection Council, said the diamondback moth has been a frequent target of the Bt toxin, which could increase the likelihood that a resistance is passed on over time.

Antila added that other pests may become resistant more slowly because they may not reproduce as successfully as the diamondback moth.

"I think that eventually there will be resistance (among all of the pests)," Antila said. "To think that complete resistance will never happen is foolish."

But, the Environmental Protection Agency is attempting to slow down the rate of resistance in cotton crop pests.

They require farmers who grow Bt cotton to also plant refuges of cotton without Bt toxin to allow for the survival of pests that are not resistant.

This allows for the possibility of diamondback moths that live in non-Bt fields to mate with moths that live in Bt toxin fields. This will keep resistance from spreading quickly, because the likelihood that the insect residing in the non-Bt field carries the resistant gene is low.

Today, about 40 percent of Arizona's cotton fields have not been treated with Bt because of the EPA regulation.

Antila said that while farmers know those crops are probably going to be destroyed by pests, they realize it helps in the long run with keeping the resistance spread to a minimum.

Once the cotton field pests become completely resistant to the Bt toxin, a different killer could be brought in to fight off the insects.

Plans are in the works to introduce a new toxin in Arizona called Cry2.

Until that time, farmers will continue to use Bt toxin to keep more of their crops from being destroyed by pests.

Antila said he thinks of the Bt toxin as a tax. Farmers do not gain any yield from using the toxin, it just prevents massive losses in crops.

Blake Smith can be reached at