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As tolerance builds

The growing insect tolerance to pyrethroids, the “most spectacularly effective insecticide used on cotton,” means farmers must focus on proper usage, use other materials or combinations of materials to try to sustain its use, says a North Carolina State University entomologist.

In their heyday, pyrethroids gave a blanket of protection against bollworms. Those days may be coming to a close. Researchers are documenting the development of insect tolerance or resistance to pyrethroids in North Carolina and southern Virginia, says John van Duyn, North Carolina State University entomologist. “I would say that in the next five or 10 years, we may be out of business with these materials — depending on how we use pyrethroids.

“Growers won't replace pyrethroids, and their effectiveness probably won't be duplicated with new chemistry, but it's to our advantage to try to keep them around as long as we can,” van Duyn told a large group of farmers at the recent Southeast Cotton Conference. “Pyrethroids still work pretty well for us, so it's to our advantage to manage the resistance.”

He offers these tips to manage insect resistance or tolerance to pyrethroids.

  • Restrict use. For example, when cutworms are in the field, and there are no bollworms. Otherwise, use materials just for major moth flights. And use a non-pyrethroid. Use alternative insecticides, such as Bt cotton, to some extent. Other examples include Tracer and Stewart, new chemistries that are quite good, but costly, van Duyn says.

  • When using a pyrethroid — Karate, Ammo, Scout X-TRA, Baythroid or Decis — make sure that everything is done right the first and second applications, in order to kill as many tolerant insects as possible. Resistant or tolerant caterpillars are easier to kill in the egg stage, or soon thereafter, than when they get a little size on them.

  • Don't use pyrethroids on soybeans and peanuts. In peanuts, the alternatives to pyrethroids aren't quite as good, but in soybeans there are good alternatives for bollworms, also known as corn earworms and pod worms.

  • Report pyrethroid failures to your county agent so we can keep up with this.

The North Carolina State entomologist found a trend toward insect resistance or tolerance to pyrethroids in the field. Scientists captured healthy, male moths in the fields and treated them with zero, five micrograms and 10 micrograms of cypermethrin (Ammo) in laboratory vials. Van Duyn and colleagues kept the moths in the vials for 24 hours. “If they can fly away, they're alive and have complete resistance to the pyrethroid,” he says. “If they fly a little bit or if they're dead, then the insects are susceptible to pyrethroids.”

In North Carolina tests, in 1998, none of the bollworms were killed early in the season with the 10 microgram dose. At the five microgram dose, the bollworms had some survival. “We see an increase in survival or resistance to pyrethroids as we go through the season,” van Duyn says, who collected samples in the northeastern part of North Carolina.

In 1999, the trend was also toward an increased resistance to pyrethroids as the season progressed, van Duyn says. In 2000, he saw a little bit of survival in the five microgram application early in the season, picking up as the season went along, and increasing in the 10 microgram treatment.

In the central part of North Carolina, N.C. State entomologist Jack Bacheler and Dan Mott took samples from “regular” fields with no bollworm problems in Hyde, Beaufort and Washington counties and samples from “problem” fields in Pitt, Hyde, Hoke and Duplin counties. Insect resistance to pyrethroids in the five microgram application was in the 18 percent range, while the 10 microgram application hit 14 percent. “They had much higher survival rates in the problem fields,” van Duyn says. “This is another powerful piece of evidence that we are losing the efficacy of these pyrethroids.”

Across the line in Virginia, Ames Herbert, Virginia Tech entomologist, found survival rates in Suffolk and Southampton counties starting in June with the five microgram rate and increasing into June, July and August. He says 10 percent of the insects were resistant at the 10 microgram level.

Regarding the bollworms in North Carolina, van Duyn says the data suggest that the genetics for resistance be fixed with the population. “Normally, you can't pick this thing up until the process is over,” he says. This “thing” he's talking about is the progressive movement of the insect toward resistance to a material.

“When insects are completely tolerant, most of the process is invisible, and not even detectable with what we're doing in the lab. It's not until you get toward the end of the process that you begin to pick it up.”

“When insects are completely tolerant, most of the process is invisible, and not even detectable with what we're doing in the lab,” van Duyn says. “It's not until you get toward the end of the process that you begin to pick it up.”

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