Fall Armyworms threaten Arkansas Crops EPA Steps in

Arkansas rice farmers watching fall armyworms obliterate their fields can now fight back, with the federal Environmental Protection Agency on Wednesday granting a crisis exemption enabling growers to use a needed insecticide.

Arkansas entomologists have submitted documentation to obtain a Section 18 exemption to use Intrepid on rice. The crisis exemption allows use of Intrepid beginning July 28. Intrepid is labeled for use on other crops, but not rice.

“The specific exemption is still under review at EPA, but application can be made under the crisis exemption,” said Gus Lorenz, extension entomologist for the University of Arkansas System Division of Agriculture.

“This is the biggest outbreak of fall armyworm situation that I’ve ever seen in my career,” Lorenz said Wednesday. “They’re in pastures, rice, soybeans, grain sorghum. It’s epic.”

Intrepid is a growth regulator that’s approved for use in just about every other row crop but is not labeled for use in rice.

“This armyworm thing started about three to four weeks ago,” he said. “It’s continued to build from that time. It’s from the Bootheel of Missouri down to Louisiana.”

Lorenz said he received a call from a producer in “south Arkansas, that they’d eaten his bermudagrass pasture to the ground. It was a 30 to 40 acre pasture. And he wasn’t even calling about the pasture. He was calling about his rice crop. He said his rice was being eaten to the ground.”

“Fall armyworm is a particularly voracious caterpillar,” said Jarrod Hardke, extension rice agronomist for the Division of Agriculture. “They have a tendency to surprise us because adults lay very large egg masses but the earliest instar larvae eat very little. It’s not until they get older and start to spread out that they consume most of the food in their life cycle.

“This is why we go from zero to TREAT seemingly overnight,” Hardke said.

Typically, armyworms can be managed well using pyrethroids, but Lorenz said “when this outbreak first started, we got reports out of Texas and Louisiana that they weren’t getting control. We’re getting failures.”

Lorenz said he and colleagues ran some quick tests, spraying this year’s armyworms with pyrethroids “and we got 48 percent control.”

In cattle-heavy parts of the state producers use another insect growth regulator called Dimilin to manage armyworms, but in row crop country, “they just don’t carry it. It’s just not available,” Lorenz said.

Fellow extension entomologist Nick Bateman said, “another problem with using Dimilin is the pre-harvest interval. The pre-harvest interval on Dimilin is 80 days which will lead to major harvest issues.”

Fall armyworms will move up and down plants throughout the day, so larvae that are low in the canopy may not come into contact with the insecticide until they move to the upper portions of the plants. Sometimes, this may not occur until 2-3 days after an application. Using higher rates in larger rice will help ensure that there is enough residual insecticide remaining to provide effective control.

As a general rule, most larvae will consume a large percentage of the foliage they are going to eat in their lifetime within the last few days of the larval stage. Waiting a few days to spray so that the insecticide can be tank mixed with a fungicide or so the spray will be better timed to also control stink bugs may be a producer’s best bet.

If scouting reveals primarily small larvae (<1/2 inch), remember that development of larvae is highly dependent on temperature, so significant defoliation can occur fairly rapidly this time of year. If you are going to wait a few days, keep a close eye on those fields and don’t let the larvae get too big.


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