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Saltmarsh caterpillars hit Mid-South

With saltmarsh caterpillars posing a threat to seedling cotton in the Mid-South, crop consultants have been scurrying to find a solution that will bring this heretofore little-known pest under control.

One new product that proved to be very effective is Intrepid insecticide, a molt-accelerating compound (MAC) from Rohm and Haas. Like its predecessor, Confirm insecticide, Intrepid causes lepidoptera larvae to undergo a lethal molt, according to Rohm and Haas scientists.

Once the pest has ingested the material, it stops feeding immediately, and complete mortality occurs within three to four days. It is active only on lepidoptera and is not harmful to beneficial insects, workers or the environment.

“Intrepid is about the only thing that's working on them very well,” said Charles Denver, a crop consultant with Agriliance Technical Services, in Dermott, Ark., who has used Intrepid to control saltmarsh caterpillars on more than 1,000 acres of cotton.

According to university experts, saltmarsh caterpillars, which have been causing a problem around the borders of cotton fields, can cause considerable damage to newly emerged cotton plants.

“If the plant is at the two-leaf stage or smaller, the growing terminal is likely to be damaged, demanding treatment,” said Blake Layton, Extension cotton entomologist for Mississippi State University. “If it's at the four-leaf stage or beyond, the worm may eat several leaves, but usually won't damage the growing terminal.”

Gus Lorenz, Extension entomologist and IPM coordinator for the University of Arkansas' Cooperative Extension Service, said Intrepid insecticide has been “highly efficacious” in controlling saltmarsh caterpillars in southeast Arkansas, which is bearing the brunt of the current infestation.

“It's very economical to the grower,” said Lorenz. “Even at the full use rate of 4 ounces, it only costs about $6 per acre.”

With its long residual control, Intrepid continues to kill worms feeding on treated foliage for 14 days, according to Lorenz. “After the worms are treated, there continues to be an influx from the edges of the field where there may be a ditch, a levee or a tree line,” he noted. “So having something out there that provides some residual has been a big bonus for growers.”

In a small plot trial conducted in Greenville, Miss., in mid-May, MSU's Layton discovered that Intrepid insecticide is a cost-effective means of controlling saltmarsh caterpillar when applied at the rate of 4 ounces per acre.

Two days after treatment, Intrepid provided 74 percent control of saltmarsh caterpillars compared to 84 percent control with Fury and 77 percent control with Orthene.

“Most of the worms in the Intrepid-treated plot had ceased feeding by the second day, but some of them were still crawling around, so we had to count them,” explained Layton. “However, by the fifth day, the Intrepid-treated plots had the lowest number of live worms — or 84 percent control.”

By the fifth day, Fury insecticide, a pyrethroid, achieved 77 percent control when applied at the rate of one gallon for every 33 acres, and Orthene insecticide, an organophosphate, registered 60 percent control at the rate 0.75 pound per acre.

“The most cost-effective treatments would be Intrepid or Fury,” said Layton. “We've had a number of complaints that pyrethroids are providing less than acceptable control, but in most cases, this was at the lower rate, so if growers use full or high label rates, they should be okay.”

However, university experts are reluctant to recommend early-season use of broad-spectrum insecticides, such as pyrethroids and OPs, because of the negative impact they may have on beneficial insect populations.

“We like to stay away from broad-spectrum chemistries early in the season as much as we can,” explained Eugene Burris, an entomologist with Louisiana State University Ag Center's Northeast Research Station. “At this time of year, Intrepid is a good option because it's softer and safer for beneficials.”

Saltmarsh caterpillars had not been a problem in Mid-South cotton until they showed up close to harvest last season.

“The only thing we could kill them with was the little bit of Intrepid we could get our hands on,” said Denver, who used the material under a Section 18 emergency label late last season.

Denver speculated that the saltmarsh caterpillars that showed up last year overwintered, resulting in the early-season attack this spring. “There are a lot more of them around than usual,” he noted.

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