Prepare for battle

“How many of you dealt with armyworms this past year?” asked Shannon McClintock, MFA staff agronomist, as he began a presentation to more than 200 attendees at MFA’s Winter Agronomy Training in January.

Nearly every hand in the room shot up.

And they weren’t alone. In 2021, growers experienced the worst invasion of fall armyworms in some 30 years. Battalions of these devastating pests ate their way through forages and row crops across MFA territory, often obliter­ating entire fields seemingly overnight.

“I’ve never seen anything like it,” MFA Senior Staff Agronomist Scott Wilburn said. “We’ve had armyworm infestations before, but they’ve always been very isolated. They were all over the place last year.”

In late August, Wilburn got a call from local farmer Kevin Davis who wanted to know what was happening in some alfalfa on his farm in Mexico, Mo. One look, and Wilburn knew. True to their name, fall armyworms had marched across the four-acre field, stripping the leaves from nearly all the plants.

“One day that field was fine, and the next day it was gone,” Davis said. “You could see the armyworms crawling all over the ground. They left the stems, and that was about it.”

Experts say a perfect storm of conditions contributed to the onslaught. Fall armyworms are not cold-tolerant, so they usu­ally overwinter in South America or the southernmost regions of Texas and Florida. Each spring, the adult moths emerge from the soil and make their way north by flying into the jet stream or other storm fronts. The pests tend to drop out of the sky in random locations, which makes it hard to predict where they’re going to be.

But last year’s warm, mild winter caused more fall army­worms to survive in typically colder climates. This led to an earlier infestation than usual, catching many farmers off guard. 

What’s more, fall armyworms feed late at night or early in the morning, so they can be difficult to detect, especially the younger larvae that eat very little. Most of the food in their life cycle is consumed in later growth stages when the pests are particularly voracious.

“Over 75 percent of the armyworm’s feeding actually comes in the last in-star, which is roughly two to three days of its life,” McClintock said. “That’s why it seems like it happens overnight. By the time we realize there’s a problem, the damage is done.”

Will such an assault happen again this year? McClintock said there’s no easy answer.

“It depends,” he said. “There are so many factors involved in whether or not we’ll actually see a large outbreak like we did last year.”

Weather is certainly one of those factors, and much of the U.S. experienced another mild winter, according to the National Oceanic and Atmospheric Administration. In March, the agency reported that the average temperature from December through February in the contiguous states was 34.8 degrees—about 2.5 degrees above average. Missouri was even warmer, with an average temperature of 35.2 degrees, 3.1 degrees higher than the winter of 2021.

Typically, fall armyworms don’t show up in MFA territory un­til June or July, McClintock said. Damage may initially appear as drought stress. Fall armyworms spread quickly throughout pastures and crop fields, so scouting is the best way to detect their presence and determine their threat. Fall armyworms do not feed during the heat of the day, so scouting in early morning or evening is the best way to find them. Birds or other predator activity can also be an indicator of infestation.

Fall armyworm caterpillars range from shades of brown, gray, green or yellow-green. McClintock said their most distinguishing characteristic is a white inverted Y between the eyes and three white stripes behind the head. Develop­ment from eggs to full-grown lar­vae often takes two to three weeks, at which point the caterpillars will burrow into the soil to pupate and emerge as adult moths 10 to 14 days later. The life cycle begins again, and multiple generations can occur each year.

Because armyworms feed rapidly, it is important to treat severe infestations with an appropriate insecticide as soon as possible to avoid further damage. However, there are some im­portant considerations before treatment, McClintock warned.

“There were a lot of reports last year that insecticide appli­cations were being made, but worms weren’t dying,” he said. “There’s a few theories why. One thing, we had high popula­tions in each field and multiple generations at a time. Second, application timing is huge. Because the larvae actually go below the residue to cool off during the heat of the day, you probably won’t get good coverage if you spray during that time. Also, fall armyworms resistant to pyrethroids have been discovered, so we could have some possible resistance issues.”

Worldwide, fall armyworms feed on more than 300 species of plants. Grasses are the preferred food source, but McClintock said all major crops in MFA territory are potential hosts.

Corn

Fall armyworms feed in the whorl, giving the plant a buckshot appearance. Refuge acreage or conventional corn is at most risk, McClintock said. Planting Bt corn with above-ground stacked traits is the best management tool. Good weed control is also important, especially grasses, a favorite target for armyworms.

“We’re not going to recommend insecticide applications unless we find armyworms in conventional corn or early in the season on traited corn,” McClintock said. “If corn was planted in late May or early June, and we have a bad enough infestation, we can see them clip off those plants and damage young stands.”

Cotton

In cotton, early damage will be in the squares and blooms; later, the worms will actually bore into the bolls. If blooms are being clipped or squares being damaged, treatment may be necessary.

“Typically, there are other insects, such as thrips, that can be a problem around the same time,” McClintock said. “Armyworms usually aren’t a big threat in cotton because they’re being taken care of with other insecticide applications.” 

Forages

Because pastures and hay fields usually aren’t scouted on a reg­ular basis, fall armyworm infestations can sneak up on forage producers, McClintock said.

“Usually when we find out there’s armyworms—at least in the first pasture of the season—it’s too late for that field,” he said. “Extra bird activity can be an indicator, but the worms have to be a certain size before they’re going to attract the birds. And they will have done a lot of feeding by then.”

When scouting pastures, avoid the hottest part of the day, McClintock said. Treatment is necessary when four half-grown or larger worms are found per square foot.

“Scouting will require more than just walking and looking down,” he said. “Get down on your hands and knees and move some residue around to look for them.”

McClintock said many growers question whether army­worm-infested forages should be sprayed or cut for hay. Factors such as worm size, population and how quickly hay can be harvested and stored must be considered.

“If we’re at or close to threshold and know it’s going to be more than five days from the time we put the hay down to baling it, you probably need to make the application, wait the preharvest interval and then cut it,” McClintock said. “The worms will continue to feed while the forage is mowed for hay, and damage to the regrowth can set back the longevity of your stand.”

Rice

McClintock said research finds rice can withstand quite a bit of armyworm defoliation without yield loss. Young rice, unless it’s completely defoliated, tends to regrow with no significant setbacks.

“Similar to wheat, if heads are being clipped and armyworms are present, I’d recommend an insecticide application,” he said.

Soybeans

While fall armyworms can damage soybeans at any stage, later-planted and double-crop fields are most at risk, McClintock said. Early infestations can lead to stand loss, and the pests tend to clip blooms and pods, which can reduce yield.

“During the vegetative stage, the threshold for treatment is 10 to 50 worms per 25 sweeps,” McClintock said. “During the reproductive stages, if blooms are being clipped, application is required. Typically, this can be done when we’re spraying for other lepidoptera like podworms and stink bugs.”

Wheat

Fall armyworms can be the biggest threat to early-planted wheat in September and October when the plants are young, McClintock said.

“When larvae are small, damage will show up as a window­pane effect,” he said. “Excessive damage won’t be obvious until the worms get bigger, which is obviously too late. If more than 25% of the field has that windowpane damage, that’s when we would make an application. In late spring or early summer, if heads are being clipped, and we find armyworms, we also need to spray.”

While no one yet knows if fall armyworms will be such an extensive threat this year, McClintock encourages growers and crop scouts to be on the lookout for a repeat attack. The Uni­versity of Missouri sets out traps every year to track fall army­worm movement and population. Counts can be found online at ipm.missouri.edu/pestmonitoring/faw/.

“From an agronomy standpoint, we’re trying to be proactive and not reactive when it comes to fall armyworms,” McClintock said. “I think it’s important to make sure we’re at threshold before spraying to help manage resistance. At the same time, we want to be vigilant with our scouting and be prepared to treat if necessary. If we start seeing fall armyworms, they’re going to keep coming, and they’re not going to disappear tomorrow.”

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