Armyworms taking aim at farmer’s fields

Shelby Varner
K-State Research and Extension

MANHATTAN  – The destructive – though rarely seen – armyworm has taken its voracious appetite to many Kansas farm fields this fall. 

The Fall Armyworm is in its larval stage from 14 to 22 days.

Kansas State University crop entomologist Jeff Whitworth said many Kansas farmers are reporting sightings of the small worm, which feeds on turf grasses, vegetables and other plants when other food sources become scarce. 

“It has been 14 or 15 years since we’ve gotten this many reports and seen this much worm infestation in the state of Kansas,” Whitworth said. 

He said there have been reports of armyworms, true armyworms, fall armyworms and beet armyworms. 

“It started last February and March with army cutworms,” he said. The armyworms started mostly in eastern Kansas, affecting brome and wheat, before heading west in June and July.  

“The armyworms actually prefer grasses (such as) corn, sorghum, wheat, fescue or brome.”

The fall armyworm prefers soybeans and alfalfa, though they will also infest other crops. 

In brome, armyworms typically chew only the green parts and don’t leave behind toxins. For that reason, Whitworth said they usually don’t kill the grass. But, as the brome starts to die-off, the worms begin finding other green things to feed on.  

“(About that time), the wheat will start growing and the worms will start feeding on the green growth of the wheat,” Whitworth said. “They don’t care whether it’s domestic wheat that we planted or volunteer that just came up on its own. They will infest anything green that they can utilize as a food source.” 

Seed treated with insecticides likely will not affect the caterpillars, so Whitworth does not recommend using that for treatment.  

“The best thing is to manage them by planting as late as you possibly can,” he said. 

Armyworms take 4-5 days to pupate, then will emerge as a moth. Then, they mate, lay eggs and then take another 4-5 days to lay eggs.  

“They look for sites with greenery because that’s what the larvae need to survive on,” Whitworth said. 

When large infestations of worms are found in alfalfa, Whitworth recommends cutting it (if the plan was to cut it within the week), rather than treating it.  

He said that in several alfalfa fields that had worm counts of 4-5 per square foot, the fields were swathed instead of sprayed. In all cases, the worms did not return. Birds helped in these fields by gorging on the worms after the foliage was cut, which helped expose the worms.  

For more information and assistance with worm infestations, contact your local extension office.