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Fall armyworms ravage fields, could produce 2nd life cycle

Caterpillars are invading states where they are normally not an issue.

In the 25 years Chris DiFonzo has been with Michigan State University, she’s never seen the massive infestation and damage caused by fall armyworms until this year.

DiFonzo, a field crops entomologist, says some of her colleagues are saying it’s been more like 50 years since the tropical insect has shown up ravaging fields. And it’s not just in Michigan — Ohio, Pennsylvania, Virginia, Indiana, Kentucky also reported major outbreaks.

“Whole fields were gone in a couple of days,” she says. “They have mowed down entire alfalfa fields, leaving only the stems. We normally don’t find fall armyworms in Michigan to any degree. If they do make it this far north, it’s the end of the growing season, and it’s either too late or the numbers are so small it’s of no consequence.”

In addition to alfalfa, armyworms were reported in mixed stands of alfalfa and grass, oats, teff, sudex, and home and golf course turfs.

Fall armyworms are typically found in tropical areas, such as Mexico, Puerto Rico, Florida and Texas and other Southern states. They do not withstand cold climates.

DiFonzo hypothesizes moths may have been carried on a weather front in early to mid-August and dumped over the area. 

Some growers tried to spray, but by the time the damage was detected, the larvae were already large and sprays had little impact, she says.

The damage is done at this point, as the caterpillars, which can reach 1.5 inches, have fattened up, moved on in their metamorphosis and are now pupating underground, says Curtis Young, entomology specialist with Ohio State University and Extension educator for Van Wert County.

So, is that end of it? Both Young and DiFonzo are uncertain. Could the pest produce another life cycle by hatching as a butterfly and laying more eggs producing more fall armyworms before winds of winter roll through?

“That is certainly a concern we are watching very closely,” Young says. “We will continue to run our pheromone traps to catch a new emergence — if it happens. We’re not sure at this point since we haven't had tremendous experience with it.”

DiFonzo thinks the moths may emerge because September is still pretty warm. “But my gut tells me they will not make it through another cycle because we’re talking the third week of September, and the weather is getting colder,” she says.

However, the potential for another round is a possibility. “If the moths emerge in time, we could potentially see further damage in things like cover crop fields, winter wheat fields and in our alfalfa fields again,” Young adds.

The first signs

The phone calls concerning fall armyworms started pouring in Aug. 26, Young remembers, as he had just started placing pheromone traps to monitor moths the week before.

“I was apparently about two weeks too late based on the infestations that just suddenly exploded on the scene that day, which is the same day I had gone back to check my traps for the first time,” he says.

Young had three traps out across the county in northwest Ohio. One trapped 299 moths, another had 470 moths, and the last one topped out at 523 moths in a week's time. “So yeah, I was flabbergasted when I opened them up,” he says. “The calls kept coming in about fields looking fine, and two days later are stripped clean.”

While the current outbreak appears to be the grass strain larvae, it favored alfalfa hay, Young says. “That’s where we saw some of the worst impact, which caught us off guard because alfalfa is not a grass, it’s a broadleaf plant,” he says. “It took us some time to discover they were in the alfalfa, eating the alfalfa, and not just grass plants interspersed.”

While growers reported losing fields almost overnight, DiFonzo says the caterpillars were probably there for two weeks beforehand, but were not big enough to show damage from the road. “Growers need to scout their fields — walk them on a routine basis,” she says.

Insecticide treatments were attempted on a number of fields, sometimes with disappointing results, Young says. “But they had to attempt to do something so they didn't completely wipe out the alfalfa fields, and potentially if they finished off with the alfalfa, there was also the possibility they could move over into soybean fields,” he explains. “Thankfully, we didn't see a lot of that happen.”

Because the fall armyworms essentially provided a late cutting, there is some concern if stands will have enough time to replenish the root system before winter sets in, Young warns. To add insult to injury, even though there wasn't really anything left behind for harvesting as hay, “the stem still had to be mowed to re-stimulate growth again in the plant,” Young says.

If temperatures continue to go down, it will slow the rate of metamorphosis, pushing the adult moth stage back, says Young, who predicts it could be 10 days to two weeks before moths emerge.

“We don't know for sure what to expect since we haven't had this experience in the past, or at least in the recent past,” Young says. “It's a wait-and-see situation and as soon as we know — as soon as we start picking up moths in our traps again — we will get the word out. That's about all we can do at this stage; it's a learning experience even for us.”

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