February 7, 2008 - A University of Arizona researcher has declared that populations of cotton bollworms in Arkansas and Mississippi are now resistant to first-generation Bt cotton.

"What we're seeing is evolution in action," said Bruce Tabashnik, a University of Arizona entomologist and the lead researcher in a study about potential bollworm resistance worldwide. "This is the first documented case of field-evolved resistance to a Bt crop.”

But an Arkansas entomologists ­ whose data was the basis of the conclusion ­ said today that the data does not support that kind of sweeping conclusion.

The Arkansas entomologist, Randy Luttrell, said he “respectfully disagrees” with Tabashnik’s interpretation.

Questions about Bt resistance were raised late last year when Tabashnik and Luttrell appeared in a symposium on the subject at the Entomological Society of American national meeting in California. Today, a press release about the Arizona study was issued ahead of an article that will appear in the February issue of the journal Nature Biotechnology.

Tabashnik is one of the article’s authors and is listed as a lead researcher in the study.

In the study, Arizona entomologists analyzed published data from monitoring studies of six major caterpillar pests of Bt crops in Australia, China, Spain and the U.S., including Luttrell’s data.

Luttrell said today that the study compared bollworm populations from 1992 and 2002. The 1992 data were based on work Luttrell did while at Mississippi State University, while the 2002 data came from work he and Ibrahim Ali did at the University of Arkansas, where he now is a faculty member.

“The response (survival rate of bollworms) was greater in 2002, which at face value would make it appear that there might be resistance,” Luttrell said today. “But when you look closer, there are other important factors that have to be taken into account.”

Among them, he said, are:

• Bt cotton’s overall weakness on bollworms. “From the start, Bt cotton (Monsanto’s Bollgard varieties) really didn’t provide control of bollworms, and it was clear from the beginning that there were insects that survived,” he said. “We saw that effect several years before Bt cotton was commercially introduced.”

• Highly variable populations. Bollworm populations in some seasons and/or geographic areas seem to be more able to “slip through” Bt cotton, Luttrell said. Even in 1992, a population of bollworms identified in Texas had the ability to escape the effects of Bt cotton at a rate comparable to some “problem” populations found since 2000.

• The role of “avoidance” behavior or mechanisms. Soon after the commercialization of Bt cotton, entomologists and consultants found larger, surviving worms in blooms and under dried bloom tags where less Bt toxin tended to be expressed. Researchers have debated whether this is simply an indication of where some moths lay eggs or if some worms ­ after initial feeding ­ move from an area with concentrated toxin to a part of the plant with lower Bt levels.

“Roger Leonard (Louisiana State University entomologist) finds that some worms will take a bite in the upper part of the plant, don’t like what they’ve found, and then spin down with their silking gland into an open flower,” said Gus Lorenz, Arkansas Extension Entomologist. “We know there’s less expression down there. That’s well documented. So, how much survivability is due to increased tolerance and how much is due to some type of behavioral modification? That question has yet to be answered. I do know that we’ve had bollworms going through Bt cotton ever since it came on the market.”

Luttrell today referred to Tabashnik “as probably the world’s leading authority on Bt resistance, so when he writes a paper like this, it has impact.” And, he added, “An honest interpretation of the data cannot eliminate the possibility (of resistance). In other words, you can’t look at the data and say, ‘This hasn’t happened.’”

But, Luttrell continued, questions about population variability and behavior traits make it difficult for him to conclude that bollworms are resistant to the Bt toxin.

Luttrell said that, in fact, some inherited genetic trait may be at play. But it can't be determined with the data on hand whether this trait has to do with the insect’s tolerance to Bt, it’s avoidance behavior or other survival traits.

The subject gets cloudier when trying to compare lab-based analysis to real-world conditions, he added.

“Resistance in the laboratory may or may not translate to field resistance,” he said, referring to comparisons made of collected populations. “With bollworm and Bt, one should consider the effects of fitness costs and large populations from non-cotton hosts that may work to dilute field resistance, even if the insect has the genetic capacity for resistance.”

In the report, Tabashnik and his colleagues said that they believe Bt resistance is limited worldwide. The researchers noted that Bt cotton and Bt corn have been grown on more than 400 million acres worldwide since 1996, “generating one of the largest selections for insect resistance ever known."

Even so, the researchers found that most caterpillar pests of cotton and corn remained susceptible to Bt crops.

"The resistance occurred in one particular pest in one part of the U.S.," Tabashnik said. "The other major pests attacking Bt crops have not evolved resistance. And even most bollworm populations have not evolved resistance."

The field outcomes refute some experts' worst-case scenarios that predicted pests would become resistant to Bt crops in as few as three years, he said.

“The only other case of field-evolved resistance to Bt toxins involves resistance to Bt sprays," Tabashnik said. He added that such sprays have been used for decades, but now represent a small proportion of the Bt used against crop pests.