• By Vicky Boyd,
The U.S. Environmental Protection Agency has agreed to separate Section 18 Emergency Crisis Exemptions for Intrepid 2F insect growth regulator for use on fall armyworms in Arkansas and Mississippi rice.
The action comes after pyrethroids, which were the standard armyworm treatment, lost much of their effectiveness against the pest this season. The only alternative is Dimilin, which has an 80-day preharvest interval, essentially making it a nonplayer for all but young rice.
University of Arkansas Extension entomologist Gus Lorenz said he has never seen armyworm infestations this bad and on as broad a scale as this season.
“I’ve been doing this since the ‘70s, and I’ve never seen anything like this,” he said. ”It’s also really bad in the beans. There’s a lot of pasture damage with multiple applications, and this can get expensive on the hay crop. It’s just every crop.”
Bad in Mississippi too
Mississippi State University Extension entomologist Jeff Gore said the situation in his state isn’t quite as bad, but worm numbers are still high.
“The thing that makes it bad is it’s something we don’t spray a lot for every year,” he said. “But this year we have sprayed a large percentage of our rice in the last week.”
The problem in Mississippi started in June with the first generation. Some growers who applied pyrethroids did not achieve effective control, but they were still early enough they could come back with a tankmix that included Dimilin.
The situation grew worse about 10 days ago with the second generation of armyworms, Gore said. By then, the rice was too far along for Dimilin’s use.
“We’ve had several cases where guys have sprayed the highest labeled rates of pyrethroids, and in some cases they got what I wouldn’t say was good control but enough control they didn’t lose a lot of foliage and would suffer economic damage. But a lot of guys left a lot of larvae after that pyrethroid spray.”
That prompted Mississippi to seek a Section 18 exemption.
Fall armyworm thresholds
University of Arkansas Extension entomologist Nick Bateman said Extension is sticking to its fall armyworm treatment thresholds for the state’s rice crop. They are:
• For May and June plantings, armyworms should be treated when defoliation exceeds 40% at 5-6 tiller and 20% at green ring.
• Higher yield loss has been observed for later plantings. Late season, treat when armyworms are present and head cutting is occurring.
MSU Extension considers fall armyworm an occasional pest and has this threshold for the state’s rice:
• Treat when you find an average of five or more worms per 10 sweeps or when you see considerable damage.
With the lateness of the rice crop, Lorenz said growers shouldn’t wait if they are nearing the threshold.
“With as late as this crop is for these guys, we can’t afford any more delays, so we want to be a little pushy on it and a little quicker on the trigger to spray,” he said.
Bateman said he also understands deciding to treat can be a judgment call, depending on growers’ individual situations, and some may decide to spray at below-threshold levels.
“They’re looking at more worm numbers, and I don’t blame them with the numbers we’re seeing this year,” Bateman said, adding he’s heard of populations of five to 10 per sweep or 50-100 per 10 sweeps.
“If it’s 10% defoliation but I’ve got six to eight worms hanging off the plant, can I spray at 10% or wait until it meets the defoliation number?” he said he’s been asked.
Intrepid 2F application recommendations
Intrepid 2F from Corteva Agriscience is labeled at a rate of 4 to 8 ounces of product per acre, with a maximum of 16 ounces or two applications per season. The minimum retreatment period is 10 days.
Bateman said in most cases, growers will see good control with the 4-ounce rate, and it will safely provide seven to 10 days of residual. The product targets larvae and doesn’t affect adult moths. The preharvest interval is 14 days.
Intrepid 2F begins to work after worms ingest it. As an insect growth regulator, it initially causes worms to stop feeding in two to four hours. Significant death will be visible within 48 hours.
Whether growers will have to retreat will depend on the upcoming moth generation.
“We haven’t seen this third generation come through yet,” Bateman said. “All of our current problems are based on the second generation.”
Although Intrepid 2F is labeled for use on a broad array of crops, it is not labeled for rice. That’s why Arkansas and Mississippi sought Section 18 exemptions for its use on rice.
Crisis Use Exemption
All applicators must have the approved Section 18 Crisis Use Exemption Intrepid 2F label in hand and must follow all label recommendations and restrictions.
These crisis use exemptions only cover use on rice in Arkansas and Mississippi for now and do not apply to use on rice in other states. They also only apply to the Intrepid 2F formulation. These exemptions are effective July 28.
“The specific exemption is still under review at EPA, but application can be made under the crisis exemption,” Lorenz said.