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Primary pests changing in South Carolina

EDITOR'S NOTE — In this and the next several issues, Southeast Farm Press will take a look at cotton insect control trends and recommendations from the various states in the region. Presentations in this series will come from Extension entomologists in Virginia, North Carolina, South Carolina, Georgia and Alabama.

The list of primary insect pests on South Carolina cotton has changed considerably in the years since boll weevil eradication, says Mitchell Roof, Clemson University Extension entomologist.

“Our major insect pest problems now are bollworms, stink bugs and thrips. In the ‘old days,’ the list included bollworms, boll weevils, budworms and thrips - stink bugs weren't even in the picture. Thrips — considering the money our growers have spent controlling them — must now be considered a major player,” said Roof at the Southeastern Cotton Conference in Raleigh, N.C.

The Boll Weevil Eradication Program, says Roof, has put South Carolina cotton producers in a better position to manage insect pests. Another factor in recent insect control changes, he adds, is the advent of Bt cotton varieties.

“Bt cotton made up about 75 percent of our acres in South Carolina this past year,” he says. “In 2002, we're expecting that 75 to 80 percent of our cotton will be planted in Bt varieties. Back in 1999, we were as high as 83 percent,” says Roof.

These recent changes in South Carolina cotton production have raised concerns about possible pyrethroid resistance, he says. “We first noticed pyrethroid resistance in South Carolina in 1996, down in the Hamptom County area in the Savannah Valley. In 1997, we found a couple of field failures in Orangeburg and Calhoun counties. And, in 1998, we found five farms with field failures, and what appeared to be resistance to bollworms.

“Since 1998, we've seen no problems. None were reported in 1999, 2000 or 2001. We haven't seen the resistance development that we initially expected to see,” notes Roof.

Growers have delayed the development of resistance through cultural practices such as reducing the number of pyrethroid applications, says Roof. “And by growing Bt cotton, we're spraying a lot less and putting much less pressure on the insects to develop resistance.”

The stink bug, he says, has developed into a major insect pest of South Carolina cotton since the eradication of the boll weevil.

“Stink bugs seldom were a problem before boll weevil eradication because our boll weevil sprays kept them under control. During the 1990s, stink bugs became a problem throughout South Carolina. Mild winters and the introduction of Bt cotton in 1996 have enabled stink bugs to expand their numbers.”

In the cotton insect loss report presented at the Beltwide Cotton Conference, stink bugs are listed as the third most damaging insect pest in the Cotton Belt. Losses to stink bugs, says Roof, came primarily in the Southeastern states. In 2001, stink bugs infested an estimated six million acres of cotton in the United States.

“It appears the stink bug is moving with the eradication program and becoming more of a problem in the Mid-South. In 1992, about 300,000 cotton acres were treated for stink bugs. And in 2000, 1.4 million acres were treated for the pest. The problem is expanding, and in 2000 we estimated that 153,000 bales were lost to stink bugs. This compares to about 20,000 bales lost in 1992. It's a growing problem, and one that will be with us from now on.”

Many avenues of research need to be explored in an effort to control stink bug damage on cotton, says Roof. One of these is scouting techniques, he adds.

“There are no great breakthroughs in this area. We currently use boll damage as our scouting technique, and our threshold is based primarily on boll damage. In South Carolina, our threshold is 15-percent damaged bolls, using quarter-sized bolls. But we need more information on thresholds, as this past year has shown us. We also need to continue to look at different insecticides and at host preferences.”

In tests conducted in 2000 and 2001 with USDA, researchers in South Carolina looked at the effects of nitrogen rates, planting dates and varieties on stink bug damage. Three planting dates were used — April 15, May 1 and May 15 — while nitrogen rates of 40, 80 and 120 pounds were used in the trials, says Roof.

Varieties planted included DPL 5415, 5690 and Bt varieties DPL 458 and 655. Researchers sliced open quarter-sized bolls to determine if stink bug damage was present in the cotton.

“We didn't see any real difference when comparing nitrogen rates. We did see a trend of more stink bug damage at the 80-pound nitrogen rate, but it wasn't significant.”

More stink bug damage was seen in the Bt varieties, says Roof, because fewer pyrethroids were applied. But no trends were identified between varieties and stink bug damage, he says.

The only significant stink bug damage trend was seen in the planting date trials, says the entomologist. “In the 2001 boll damage ratings, we saw much less damage on the later planted cotton. Our earlier planted cotton had the most damage.”

The trials also showed no correlation between stink bug damage and cotton seed rot, he says.

In reviewing observations from last year's cotton crop, Roof says that hard lock doesn't always accompany stink bug damage. “We saw cotton last year with stink bug punctures, but the locked bolls opened normally and eventually fluffed out.

“Some of the things we saw this past year — very dry conditions and bolls starting to open up — make us start to question our thresholds for stink bugs. We might have to start thinking of these in a different way depending on weather conditions and other factors.”

Turning to thrips control in South Carolina, Roof says most growers use an average of five pounds or better of Temik. Higher rates might be due to nematode problems in the field, he says.

“We're seeing more interest in seed treatments, and an increased use of Telone. But our tests generally have shown that seed treatments don't provide adequate control of thrips in South Carolina. We usually recommend Temik even where growers use Telone. But we're continuing to look at seed treatments and hoping we can utilize them in certain situations, such as late planting.”

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