Farmers battling back against armyworm infestations

Colleen Kottke
Wisconsin State Farmer
Armyworms may be little but the damage they cause to crops adds up to big economic losses.

In all the years he has been crop farming, Leroy Dassow says he's never seen anything like the damage that armyworms have inflicted on his crops up in Taylor County.

"I depend on selling my corn and soybeans to make a little profit, but there's not going to be any profit this year," said the frustrated Medford farmer.

Dassow isn't the only farmer who has been waging war against the hungry pest. Bryan Jensen, Integrated Pest Management specialist for Cooperative Extension and UW Horticulture professor says he's been getting plenty of emails and phone calls from farmers who describe damage ranging from moderate damage of leaf tissue to extreme defoliation in corn plants.

Armyworms are often go unnoticed in fields until injury is severe. Larvae feed primarily at night and may cause damage before being detected.

"Just the name armyworms conjures up images of defoliated corn fields. Well, this summer they are living up to their names in isolated fields," he said. 

Jensen says that Wisconsin farmers appear to be in the middle to the end of the second generation (sometimes called the summer generation).

"The corn is tall and unless you're actively scouting, populations can 'magically' appear," he said.

Like Dassow, many farmers wonder whether the mild winter had any bearing on this year's population of armyworms. That's not the case, Jensen says, noting that armyworms do not overwinter in Wisconsin and migrating adults (moths) depend on the winds to usher them up into the state.

"They aren't strong fliers and are assisted by weather fronts," he said. "We evidently received the brunt of the migration."

Migrating adults usually arrive in waves during spring. Jensen said the spring migrants are attracted to corn fields with weedy grass escapes, rye cover crops and those which are no-tilled into alfalfa.

"These field conditions area where I would initially concentrate my scouting efforts in seedling corn," Jensen said. "The second generation, which we are having problems with now, is attracted to grassy weeds that have escaped control and corn. It's hard to predict where they will be so I advise farmers to scout all acres in July and early August."

Jensen says wheat and other small grains are also at risk until harvest.

Armyworm larvae feeding on wheat heads.

Scouting mission

Medford Cooperative Ag Services Manager Dan Tanata says he has been fielding many calls from farmers concerning armyworms and advises them to get out and scout their fields.

"The best time to look for activity is in the early morning or early evening. You won't see them during the day as they're burrowing in the ground or hiding under debris out in the fields," Tanata said.

Armyworm larvae vary in color from dark greenish-brown to black. On each side, there are long, pale white, orange, and dark brown stripes along the length of the abdomen. Mature larvae are approximately 1 ½ inches long. The head capsule is yellowish brown with a brown network of veins, giving it a mottled appearance.

Armyworm larvae may also be distinguished by a dark band on the outer side of each proleg. Armyworm pupa stay in a brown earthen shell just below the soil surface. The armyworm moth, approximately an inch long with a 1 ½ inch wingspan, is tan to light brown, with a tiny white spot centered on each forewing. Eggs, which resemble small white globules, are laid in rows or groups on leaves of host plants.

The complete lifecycle of the armyworm is around 21 days.

To treat or not to treat

Feeding activity in corn fields above the ear zone is particularly of economy concern, Jensen says.

In the event signs of armyworm feeding are observed - holes in corn leaves - farmers should check five sets of 20 plants at random, recording the number of damaged plants and the number of worms per plant. 

Armyworms consume leaf tissue of corn plants. Feeding is usually confined to leaf margins, but in some instances, larvae may strip the plants entirely of leaf tissue. Corn generally recovers from damage caused by moderate infestations if the growing pointhas not been injured.

Determining the threshold when it's economically feasible to treat corn fields are as follows: spot treat, if possible, when you find two or more armyworms (3/4 to one-inch or smaller) per plant on 25 percent of the plants, or one per plant on 75 percent of the plants. Treating in small grains is suggested if there are three or more armyworms per square foot (farmers are advised to watch for head clipping).

"Finding the worms while they are still small and before damage to corn becomes severe, increases the value of control," Jensen said. "Young worms are also easier to control than those reaching maturity."

When making treatment decisions, Jensen advises farmers to think about the damage they can prevent rather than focusing on how much damage is currently there.

"Large larvae will be feeding for a much shorter period of time," he said.

Tanata says Medford Cooperative has been trying to control armyworm infestations by spraying insecticide.

"(Spraying early) has some effect but there isn't a product that's great with (controlling armyworms)," said Tanata, estimating the cost at around $10/acre. "We haven't seen them like this in a long time. My advice to farmers is to get out there and scout your fields and spray quickly if they need to."