WEST LAFAYETTE, Ind. – It’s not too late for farmers to protect their crops or limit the damage from what’s described as an unprecedented invasion by the fall armyworm.

Reports of fall armyworms in unusually heavy numbers have come from states as far north as Kentucky, Indiana, Ohio and Michigan.

The species migrates from the south where they thrive in the warmer climate but their late summer numbers to the north this year are extreme.

“We’ve not seen anything like this before,” said Christian Krupke, a field crop entomologist at Purdue University.

Krupke said the fall armyworm likes to eat the leaves on many kinds of vegetation but prefer young plants, leaving crops like hay and late planted soybeans especially vulnerable.

Once the leaves are gone, the plants are no longer able to generate the energy from the sun they need to survive.

Fall armyworms depending on density and size can decimate a field practically overnight before marching in a group to the next and closest food source, he said. 

Krupke said armyworms begin as larvae and turn into caterpillars before going underground to transform into moths.

He said signs of a bumper crop in the Midwest emerged about a month ago when fall armyworm moths in large numbers were detected coming up from Kentucky. In response, Purdue issued an advisory to farmers to scout for the predators in their fields. Armyworms also like many grasses and other plants such as alfalfa, corn, beets, cabbage and onions.

Krupke said late planted crops like corn and soybeans are too mature this time of year to attract many of the species and not every grower of still young plants is going to be impacted.

However, Krupke said the chances of infestation are “decent” and producers especially vulnerable to the species right now should pay close attention to their fields. “Anyone that is growing forage, particularly alfalfa, would be prudent to have a look,” he said.

Krupke said an insecticide applied at dusk or dawn is very effective in killing armyworms while they’re young. Controlling them is more difficult, though, as they become larger and more resistant to the chemicals, he said.

The Midwest invasion is already easing up in the more southern areas. Krupke expects the threat further north to start diminishing in mid-September when the species, after reaching maximum growth, burrows into the soil to become a moth.

He said the threat is further reduced as more hay gets brought in from the fields, reducing a major food source for the species.

Krupke said another wave in the fall is likely from the next crop of moths coming up from the ground depositing larvae.

The fall armyworm numbers won’t be nearly as great because their food supply will be further reduced and other factors like cooler weather in what’s a subtropical species.

Krupke with certainty could not point to a specific cause to explain the much higher numbers in this part of the country. He said the boom might have something to do with the direction of the wind patterns, though, after they rise above the surface as moths.

“A lot of their flight is passive. In other words, they get up in the jet stream and land where they land,” he said.

According to researchers at The Ohio State University, fall armyworm caterpillars vary in color from greenish to tan to dark brown with stripes along the body.

A good identifier is an inverted white “Y” shape behind the head.