Of all the years for the aphid fungus to be late arriving in Mid-South cotton fields, 2006 was not the year. The fungus, Neozygites fresenii, typically shows up in Mississippi, for example, in late June or early July and causes cotton aphid populations to “crash” across entire fields, according to Extension entomologists.
As of July 14, however, the fungus had been confirmed in only two counties in the southernmost portion of the state, leaving Extension specialists and university researchers scratching their heads.
The reason for their concern? Farmers in Arkansas, Louisiana and Mississippi appear to be battling aphids that may be resistant to the neonicotinoid class of insecticides, the chemicals that have become the workhorses for cotton aphid control in the region. So the fungus is sorely missed.
“I've been getting a lot of calls about not being able to control aphids,” said Angus Catchot, Extension entomologist with Mississippi State University, who spoke at the Mississippi Farm Bureau's Summer Commodity Conference in Jackson. “This started in Louisiana about a month ago. Then it moved into Mississippi.”
Catchot said he was hearing reports that farmers in Louisiana were spraying with chemicals such as Trimax, Centric and Intruder, products growers have been using for a number of years.
“Those applications would knock the populations down by 80 percent to 85 percent,” he said. “But five to seven days later, aphids would boil back up behind them.”
The Louisiana Department of Agriculture and Forestry requested and received a Section 18 emergency exemption for carbofuran or Furadan on cotton aphids from EPA in mid-June. Louisiana's Section 18 exemption has now expired.
“In the last seven days, we've been seeing the exact, same thing in Mississippi,” Catchot said. “We spray them, we knock them down and five to seven days later they're back up to treatable levels.
“We walk these fields four to five days out — I've walked a bunch of them — and we're seeing pockets of 2 to 3 feet of row that are covered in aphids from the top to the bottom. It almost looks like you missed them.”
Mississippi State Extension specialists collected aphids from two such locations near Tchula, Miss., in the south Delta and took them to the USDA Southern Insect Management Research Unit at Stoneville, Miss. Researchers also took samples from locations in Washington County, Miss., that had not been sprayed.
Jeff Gore, research entomologist with the USDA's Agricultural Research Service, conducted tests on the two Mississippi samples and a collection from Arkansas for different rates of Intruder. (Gore reported on the findings in an article in the July 14 edition of the Delta Farm Press.)
The label rate of Intruder produced 98 percent mortality in the aphid sample from Washington County. But the mortality from the same rate in the collections from Tchula, Miss., and Arkansas ranged from 4 percent to 16 percent for the highest rate of Intruder or 0.05 pound of active ingredient per acre.
“Based on these lab tests, I'm convinced we're seeing aphids that are resistant to the neonic insecticides — Trimax, Centric and Intruder,” he said. “We're killing off the susceptible insects. The ones that are resistant are the ones that are blowing back up.”
Unlike other insect species, female aphids don't need males to reproduce, he said. “Female aphids are just spitting out clones of themselves. So if you have a resistant aphid, that's what's going back out in that field. I'm convinced we're seeing that now.”
Catchot suggests growers facing that situation consider applying Carbine, a new insecticide with a slightly different mode of action, for the second application on cotton aphids. The Mississippi Department of Agriculture and Commerce has also requested a Section 18 exemption for Furadan.
When Catchot spoke at the Farm Bureau meeting in Jackson on July 6, farmers were close to the window for arrival of the aphid fungus. He said he had sent samples from several counties to the University of Arkansas for testing for the presence of Neozygites fresenii. All of those samples came back negative.
“I would try to wait for the fungus, especially if you have irrigation water,” he told members of the Farm Bureau's Cotton Committee. “I would try to live with the aphids as long as you can. Hopefully, the fungus will come in and get them off your back.
“Short of that we don't have a lot of true options,” he said. “Bidrin has been used in the past. This year, Bidrin at 0.5 pound per acre didn't kill aphids in some areas. In others, it did.”