Armyworms have come to Iowa. Fall armyworms might show up in the state on any given year, but Rebecca Vittetoe, field agronomist with Iowa State University Extension and Outreach, said this is an unusual year for armyworms in Iowa.

Armyworms don’t overwinter in Iowa, but the moths will fly up from Texas and Florida and reach Iowa during the growing season. Vittetoe said the plethora of armyworms this year isn’t unique to Washington County or Iowa, but to the entire Midwest. She said it’s possible that the hurricanes in the south have helped push the armyworms to Iowa.

“We see different jet steams that can move moths up farther north,” Vittetoe said. “We’ve got other pests like black cutworms and potato leafhoppers that travel that way. Those are pests that don’t overwinter here but, depending on where the wind brings them, they show up here.”

Armyworms are defoliators that feed on leaf tissue.

“Oftentimes, you’ll see a nice green lawn or a hayfield that’s nice and green and then you go back and there’s just little stems left,” Vittetoe said. “Armyworms aren’t going to eat the stem part, but they’ll eat the leaf tissue around that. With grass in a lawn, they’ll just eat it down. They can cause a lot of defoliation damage.”

The size of armyworms can tell us where they are in their lifecycle. If they’re smaller than three quarters of an inch, they still have a lot of feeding to do, but once they get past an inch or an inch and a half, they’re going to pupate soon and turn into moths.

Knowing how to treat armyworms depends on where they are in their life cycle.

“If [farmers] find them when they’re three fourths of an inch or smaller and they’re also seeing more than two or three per square foot, we would consider spaying a foliar insecticide to kill them so they don’t cause more damage,” Vittetoe said. “If they find them when they’re about an inch and a half, they’ve basically done their damage and we just let nature take its course. If we catch them when they’re on the bigger side, they get harder to kill and most of the damage has already been done.”

When it comes to homeowners who find armyworms in their yards, but Vittetoe said that by the time most homeowners discover them, most of the damage has already been done.

“One thing homeowners want to think about is doing something to control the eggs. People might find egg masses on the side of their house or a lawn shed, or I’ve got some on my mailbox,” Vittetoe said. “Spraying an insecticide won’t do anything on the egg masses. We would recommend going out with some soapy water and scraping or brushing away those egg masses.”

The good news, Vittetoe said, is that the armyworms don’t overwinter in Iowa, so as soon as we get a killing frost, we don’t have to worry about armyworms anymore. But we might see another generation of armyworms before the frost comes.

“There’s a small chance we could see a second generation this year,” Vittetoe said. “They have about a 30-day life cycle. I first started getting a lot of inquiries about them a week to two weeks ago, so if we think about 30 days out from there, it’d be early October. So, there’s a chance we may see some in early October, it wouldn’t surprise me if we do. Nothing really changes except that if temperatures are colder, they’re probably going to be moving slower and we might not see the damage happen as quickly.”