Armyworms may march into your fields


WEST LAFAYETTE, Ind. – A spring armyworm infestation of near historic proportions could stretch well into July, when another armyworm species begins arriving and feeding on corn, say Purdue University entomologists Larry Bledsoe and John Obermeyer.

If the scenario plays out, it could be double trouble for farmers or no problem at all, Bledsoe said.

Much will depend on nature’s own control measures in reducing both armyworm species.

“It’s too early to tell whether the spring, or “true,” armyworms will overlap with the fall armyworms, Bledsoe said.

“We’ve got unusually large true armyworm populations, and now we’re awaiting the appearance of the fall armyworm,” he said. “Our anticipation is that natural controlling agents will keep damage below economic impact levels.

Too many to count.

“This is speculation, because we’ve never dealt with armyworm numbers this high before.”

True armyworms are active from May to early July. Fall armyworms appear in early July and are present until frost. But this is no typical armyworm year.

Waves of true armyworm moths swept across Indiana this spring. Eggs from the first armyworm flight hatched in May, and larvae ate their way from southwest to northeast Indiana.

Some cropfields and pastures were so badly damaged no green vegetation was left. A second generation armyworm has been flying throughout the state. The moths are laying eggs, which could hatch by month’s end and begin the process all over again. Larval-phase feeding could go on for a full month afterward.

Fields are next.

A true armyworm consumes about 80 percent of its food in its final three to five days as a caterpillar. If the crop is gone and the larvae are still hungry, they’ll move as a unit to the next grassy field.

Some Indiana counties are registering true armyworm moth populations off the entomological chart. A trap in Clinton County caught 4,208 moths in a five-day period. Bledsoe described the number as “phenomenal.”

“We haven’t seen outbreaks of this magnitude since the 1950s,” he said.

Not in Midwest – yet.

Fall armyworms have yet to arrive in the Midwest. The pests seek very late planted corn on which to lay their eggs. During July and August the developing larvae feed in the whorls of corn, eventually moving to the tassels and ears as they emerge.

Natural forces may spare farmers a dual armyworm attack, Obermeyer said. “We’ve noticed a disease picking up in the armyworm population,” he said.

“Once the weather gets hot and humid this disease should flourish and the armyworms will not. We are also aware of parasitic wasps and flies killing armyworms.

“All of these natural enemies of the armyworms are sitting and waiting like vultures for the eggs to hatch.”

Check your corn.

Farmers should inspect their fields regularly for armyworm activity, Bledsoe said. Not all fields with feeding larvae are candidates for pesticides, he said.

“If you see armyworms in corn, you’ve got to calculate how much yield you expect to get, the cost of pesticide application and how you’ll apply it,” he said.

“One of the things that makes treatment difficult with standing corn is you would almost need a helicopter or plane to treat the field.”

The winter wheat crop is safe from future armyworm damage.

“Armyworms should leave the wheat crop alone,” Obermeyer said. “It’s too yellowed or browned up for them to lay eggs on. They aren’t going to be attracted to it.”

Learn more.

More information about armyworms and pesticide options is available online at


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