Pyrethroid resistant bollworms have caused pesticide failures in some Texas cotton fields, said a Texas A&M University entomologist.

Patricia Pietrantonio, with the insect toxicology lab in College Station, said resistance monitoring shows a significant number of bollworms trapped in several Texas counties survive pyrethroid applications.

Nueces County appears to be the most vulnerable, she said. But she's also confirmed resistance in Ellis, Gaines, Burleson, Uvalde, Williams and Hockley counties.

“Last year, with a few exceptions across the state, we saw high bollworm populations,” Pietrantonio told a group of cotton consultants at a recent Southwest cotton technology seminar, sponsored by Bayer CropScience, in San Antonio.

“In Nueces County last year, we found heavy bollworm populations and some pesticide failures,” she said. In some cases, as many as 50 percent of bollworm moths trapped and treated in a vial with cypermethrin survived rates that should have been lethal.

“At that rate, we would expect to see field failures,” she said. “Problems in 2004 began about a month earlier than in 2003.”

She said survival in Burleson County indicated “full-blown resistance. It was progressive from April through September. At rates at which all worms should have died, 50 percent did not.”

She said in Ellis County, the problem is “not as severe, but is still resistance. In Gaines County, farmers made a lot of applications for pink bollworm and field agents saw some bollworm resistance but no field failures.

In some cases, however, survival rates ranged from 30 percent to 50 percent.

“In Uvalde, most resistance showed up toward September. In Williams, we saw a lot of pressure and poor control at full rates.”

She said Tom Green County growers had not made pyrethroid applications before the bio-assay began. “We detected some resistance but at low frequency.”

Pietrantonio said entomologists have observed bollworm survival in Texas since 1987. Increased use of pyrethroids, she said, compounds the problem. In some areas, growers use pyrethroid insecticides on corn, grain sorghum and other crops, in addition to cotton.

“Overall, we've seen quite a bit of resistance and the risk seems to be greatest in Nueces County.” Ellis, Uvalde, Williams, Gaines, Burleson and Hockley follow in order of most to least risk, she added.

“In Gaines, Burleson and Hockley, we still have a pool of susceptible bollworms.”

She recommends that rowers practice product stewardship as a key to resistance management.

“Use the full rate of pyrethroid,” she said. “If you mix a pyrethroid with another kind of insecticide, use the full rate of the pyrethroid and a lower rate of the new chemistry.”

She also recommends balancing pyrethroid use in other crops and to consider the potential for enhancing pest resistance.

e-mail: rsmith@primediabusiness.com