ARROW ROCK, Mo. – It took just two days for caterpillars to destroy 10 acres of Duane Brune’s alfalfa.

Brune, who runs Pin Oak Farms in New Haven, said he gets four or five pedals every time he puts his foot down.

“It was literally an army of worms crawling across the ground,” he said.

Indeed. The caterpillars are the larvae of the fall armyworm moth, a planetary crop invader. The annual toll of their attacks is at least $300 million for farmers in the US, and billions of dollars around the world.

But now University of Missouri scientists are on the verge of a new frontier in pest control: They’re filling fields with a chemical — not a pesticide — that replicates the moth’s pheromones, overwhelming its senses and preventing it from mating, essentially using the insect’s own biology against it. The system could reform pest control in the US, and be even more useful in countries where subsistence farming is common and access to genetically modified crops is not.

And the research comes at a critical time. It’s possible that climate change could boost armyworm attacks, scientists fear, enabling them to cause more widespread damage just as the insects adapt to common weapons in a farmer’s arsenal, such as genetically modified crops and pesticides.

“There’s a whole new playing field now,” said Kevin Rice, a Mizzou Extension entomologist who is leading a trial of the new system.

Here’s how it works: Researchers hammer grids of sticks into cornfields. The sticks hold strips of factory-made pheromone, which slowly escapes into the air above the corn. And when moths land, hoping to mate, they are overwhelmed by the lure instead. The strategy, called “mating disruption,” can prevent the moths from producing crop-devouring caterpillars, the larval stage of the insect.

Mating disruption isn’t a new tool—the romantic peril of moths is a growing business.

Oklahoma-based Trécé Inc. make mating disruption products that work for other moths. The number of new farmers buying the systems is growing 30-40 percent each year, said Danielle Kirkpatrick, the company’s technical support coordinator.

Provivi, a Los Angeles company that makes similar dispensers, launched a fall armyworm system in Mexico in 2020. The company says the product was used by 1,000 growers in its first year.

But the Mizzou researchers, using Trécé stakes, are among the first to try the setup on fall armyworms in the U.S.

There are still some hurdles to clear in the Missouri trial. Farmers may not want to plant hundreds of sticks as the scientists did, Rice said. And they won’t buy the dispensers if they are too expensive.

Still, the goal, he said, is to establish proof of concept for Trécé’s system and then make it cost-effective for farmers.

“We did it at the maximum level, and we know no producer would ever do that,” Rice said. “But if it doesn’t work at that level, well then it’s time to move on.”

If it does work, it might make a dent in attacks that destroy fields, and farmers’ profits.

The army is gathering in the south

Fall armyworms, distinct from the true armyworm, are cold-blooded, so they spend the winter in warm southern Texas and the tip of Florida and then ride wind currents north to farmers’ fields beginning in the spring. They drop from the sky “like paratroopers” onto crops to produce offspring, Rice said.

The caterpillars grow to 1.5 inches long, green-and-brown soldiers that emerge under the cover of darkness to feed. They eat more than 80 types of plants.

Once the armyworms have consumed a field, they march en masse to the next available target, giving them their distinctive name.

Andy McCorkill, a field specialist in livestock for the University of Missouri Extension and a farmer in Dade County, estimated that he had a platoon of armyworms 300 feet wide and a quarter mile long in one of his pastures of Johnsongrass, a type of hay , attacked, last year.

“They had every leaf of it eaten down to the ground,” he said.

The larvae can completely defoliate fields, or damage crops just enough to lower yields and make plants more vulnerable to other disease attacks. And they can be especially difficult to kill because larger caterpillars are more tolerant of insecticides.

Eventually they bury themselves underground to turn into a moth. The adults emerge from the soil and start the whole process again, jumping frogs north to higher latitudes.

The march is seasonal: it stops when winter descends to freeze out the army.

But last year armyworms rode wind currents further north and ate crops for much longer than they usually do. They took advantage of optimal weather conditions to mount an invasion that entomologists say was the worst since the 1970s.

Katelyn Kesheimer, an Auburn University extension entomologist, said a warm winter allowed more of the cold-blooded animals to survive and gave them a “jump start” on their spring migration. Record spring rains made plants grow faster and greener, creating more attractive landing targets for mature moths.

“It was just a fantastic buffet of whatever they wanted to eat,” Kesheimer said.

Climate change is expected to make the warm winter and stormy spring of 2021 more common, she said, leading to more frequent severe outbreaks.

Some research also predicts that rising temperatures may allow the armyworms to spend the winter further north as tropical temperatures become too warm for them to survive. And they can produce more generations per growing season because heat can speed up their life cycle, researchers found.

Rice, the Mizzou scientist, said it’s a combination that promotes destruction.

“It’s kind of like investment banking,” he said. “If you start with a higher number and you have multiple generations per year, you’re going to have higher populations in the area.”

Growing seasons are also lengthening with rising temperatures, meaning caterpillars may be able to stay longer once they arrive in the north.

And that is not all. The tools that farmers rely on to control fall armyworms may become less powerful.

There is research showing that the insects can develop resistance to insecticides, and even a type of genetically modified crop called Bt corn. The corn is a widespread pest control tool, planted in more than 80% of all US corn area, which produces toxic proteins that kill the insects.

“We just need more tools in the toolbox against this scourge,” Kesheimer said. “Because a little bit of a lot of things is going to be a lot better than one thing that they’re going to get resistant to or eventually just find a way around.”

Crash into the game

The Missouri scientists are waving pheromones, a chemical female moths produce like a beacon to attract mates, as a new tool to fight armyworms.

Female moths have glands that emit the pheromone, a compound specific to the species. They pump the pheromone into the air at night, and male moths use pheromone receptors in their antennae to detect the chemicals and find the female. Then they mate. Females can produce up to 2,000 eggs in their five-day lifespan.

The researchers’ experiment was designed to stop the moths’ romance: Plastic pheromone strips are attached to wooden posts in the ground with a tie clip. The strips release clouds of pheromones so intense that the males cannot identify a mate.

“They can’t talk to each other because it’s just mass confusion,” Rice said.

Early one warm morning last month, the scientists met at the corner of a cornfield owned by Legacy Ridge Farms just northwest of Arrow Rock, one of their test sites, and hatched a game plan. The heat index was almost 100 degrees.

Technicians Justice Hennemann and Jack Tallmage walked out of sight with a surveyor’s wheel through barn rows of corn to measure the length and width of each section of the field.

They returned their measurements to Kelsey Benthall, a Missouri PhD student who co-led the project with Rice, who calculated the measurements the team would use for the site’s three 18-acre treatment plots.

“Whenever I signed up for this, I didn’t know I’d be doing maths in 100 degrees all over again,” Benthall said.

She designated one piece of field, separated from the others by a barrier of trees, as a control plot and mapped two other tests. They put in 640 single-dispenser stakes, plus 80 towers — multiple dispensers in a ring around a central pole — a new system the researchers are testing.

Extra humidity greeted the team every time they dove into the cornfields to plant a stake. Sweat glistens on the foreheads.

The researchers also added a series of glue traps in each plot, which attract the fall armyworms using the same pheromones as the sticks, then trap the moths inside. Each week, Benthall will travel back to the corn to count the number of armyworms they catch in that lure.

They hope they don’t find many in the pheromone fields. This would mean that the male moths have as much trouble finding the attractants as they do their romantic partners.

Farmers are not uniformly convinced.

Brune, the Pin Oak rancher, said a mating interruption system might not be worth it for his operation because he only sees outbreaks every four or five years. Cost and the concentration of dispensers needed per hectare will play a large role in his decision.

But Derek Davis, the owner of Legacy Ridge, said he would consider the pheromone stakes — if the trials are successful.

Finding ways to control insects in organic and non-GMO fields has always been a challenge, he said.

“Anything we can learn here, not just for our own farm,” he said, “we can help other producers with those challenges.”