Cotton insect control in South Carolina — like in other Southeastern states — has evolved in the years since the Boll Weevil Eradication Program. And, while insect pest management has become somewhat simpler, the threat of resistant bollworms and budworms still remains, says Mitchell Roof, Clemson University Extension entomologist.
"The Boll Weevil Eradication Program has gotten us where we are today," says Roof. "After we eradicated the boll weevil in 1985, beneficial insects became much more important to us, and budworm thresholds were increased."
Prior to the completion of boll weevil eradication, cotton producers in South Carolina made an average of 12 to 14 insecticide treatments per year, he says. About half of those sprays were made with pyrethroids and the other half were organophosphates aimed at controlling boll weevils, adds Roof.
"After eradication, we dropped our number of insecticide treatments down to four to six per year. This remained our system for a number of years. Our cost of insect control after the eradication of the boll weevil was reduced by $75 to $80 per acre," he says.
Consequently, beneficial insects became more important to South Carolina cotton growers, according to Roof. Prior to boll weevil eradication, growers would make pin-head square treatments in mid-June, taking out beneficial insects. Also during this time, tobacco budworm problems would develop in June, he notes.
"After boll weevils were eradicated, our growers became much more conservative with beneficial insects — they even learned some of them by name. These beneficials often controlled early worms. In fact, they pretty much took care of early season tobacco budworms in South Carolina."
Beneficial insects that once were considered problems, such as fire ants, became helpful predators, says Roof. Growers probably depend on fire ants and big-eyed bugs more than any other beneficial insects, he adds.
The threshold for treating tobacco budworms eventually was raised, he says, to 15 small worms or 20 percent square damage in June before bloom. Growers previously were treating at six worms or 10 percent square damage.
In 2000, less than 10 percent of South Carolina’s conventional cotton acreage was treated for tobacco budworms, says Roof. In other years, growers probably didn’t spray as much as five percent, he adds.
Bt cotton, he says, was readily accepted by South Carolina cotton producers. "In the past two years, about 80 percent of our acreage has been planted in Bt varieties. About 95 percent of our cotton acreage is planted in Roundup Ready varieties. This leaves only about two percent of our acreage being planted in conventional cotton. We’ve probably averaged one to two bollworm treatments per year in the past two years."
Researchers first found resistant bollworms in Hamptom County, located in the southern part of South Carolina, in 1996, says Roof. "Growers in that area of the state were spraying more than in any other area. They were making between six and eight sprays per year. They weren’t getting the job done with Karate in 1996.
"It wasn’t unusual to find 20 to 25 percent square damage in August in some of the fields in that area. So, we wondered what would happen in 1997. We found two fields in Orangeburg and Calhoun counties in 1997 where worms couldn’t be controlled. In 1998, we found five farms in the area ‘below the lakes’ with control failures."
There were no reports of control failures in 1999 and 2000, says the entomologist.
Vial tests to determine bollworm survival rates were conducted from 1996 to 2000 using Karate, says Roof. "Normally, a susceptible population of bollworms would be wiped out by 2.5 micrograms of Karate. But in Hampton County in 1996, we saw 30-percent survival at the five microgram level.
In 1997, pheromone traps were set to catch moths and test their resistance. Resistance was much less in moth captures from the general population than in those taken from field situations, he says. "We still saw considerable survival at 2.5 micrograms and some even at the 10 microgram level."
Combined bollworm data from 2000 in South Carolina showed much higher survival rates in June, says Roof, including about 30 percent survival at the five microgram level and 20 percent survival at 10 micrograms. For the entire year, survival rates were less than five percent at five micrograms and less than one percent at 10 micrograms.
"High survival rates in June tend to show us that resistance seems to be increasing. Resistance appears to be increasing in bollworms in South Carolina."
Turning to budworm resistance, Roof says a resistant population of the insects was found in Sumter County in 1998. "With Bt cotton, it probably won’t be much of a problem. Yet, this past year, we found resistance in Marion County. Resistance also may be increasing in budworms."
To help prevent further problems, growers are advised to follow a resistance management strategy, he says. "Bt cotton should be planted where there have been control problems. Growers should continue to use pyrethroids where there have been no problems. But I’ll qualify that by saying that no pyrethroids should be sprayed in June — that’s the general philosophy in South Carolina — no pyrethroid before it’s time, and June is before it’s time."
If a pyrethroid fails, growers should switch to a material such as Larvin, Tracer or Steward, says Roof. If pyrethroid failures were experienced in previous years, growers should start off with Larvin, Tracer or Steward, he says.
Growers also should avoid using low rates of pyrethroids, notes Roof.
"Another recommendation is to aim low, because they’re riding Shetland ponies. In other words, do the best job possible of getting insecticides where they’re needed at the right time. React quickly, be timely with your applications, use adequate amounts of water and pressure, and use the proper nozzles."