Army Cutworm Research

Rangeland managers and researchers were mystified in 2003 by kilometer-wide holes in the blanket of cheatgrass (Bromus tectorum) that usually covers large areas in the Intermountain West. Where did the cheatgrass go?

Cheatgrass is an exotic, Eurasian annual grass that sneaks in around our native perennial grasses and shrubs. When cheatgrass dries, its thin leaves and narrow stems are perfect fuel for wildfires.

Our native shrubs and perennial grasses are damaged and killed by fire. Cheatgrass plants, on the other hand, die at the end of the growing season, after producing seeds. These seed move in to burned areas while the native plants are still recovering. The new stand of cheatgrass is fuel for the next wildfire. And the one after that.

When I visited Winnemucca in June 2003, I found former cheatgrass stands looking more like parking lots than rangelands. After walking and scratching my head for a couple days, I stopped at a nearby ranch to ask about the cheatgrass.

Rancher Jim Christison saw “millions” of larvae “eating every green shoot” one January night, east of Winnemucca. Jim took photos, collected larvae, and asked an entomologist what they were. They were army cutworms (Euxoa axiliaris).

Army cutworms are native larvae that eat introduced plants, both crops, such as wheat and mustard, and weeds, such as cheatgrass. The larvae grow into miller moths. Moths from the Great Plains fly to the Yellowstone area for the summer. Grizzly bears search for the fat-filled moths in talus slopes, where they send their days. We don’t know where moths from the Intermountain West spend their summers.

But in 2003, the first entomologists I asked about Jim’s story had never seen army cutworms eat cheatgrass. They couldn’t imagine the larvae could consume enough seedlings to create large bare areas.

That September, I found Bob Hammon, with Colorado State University Extension in Grand Junction, Colorado. Bob had seen more millers than usual the previous fall. In the spring, he watched army cutworms consume crops and rangeland plants—including cheatgrass. Bob’s observations from Grand Junction were the backstory to Jim’s discovery near Winnemucca.

I shared what I’d learned about army cutworms and cheatgrass die-offs at the Society for Range Management annual meeting in early 2004. Rangeland managers were as skeptical of the story as the first entomologists had been. I’ll just have to wait for the next outbreak and see what the army cutworms eat.

While I waited and watched, I wrote about The Cheatgrass that Wasn’t There in my column in Rangelands magazine, a publication of the Society for Range Management.

In January 2014, I recognized in southwest Idaho the conditions Bob Hammon had seen in Colorado in 2003. I emailed “Wanted” posters and residents of Owyhee County reported seeing larvae in February.

I wasn’t surprised when the larvae ate cheatgrass and created cheatgrass die-offs. But I was shocked when they then went arboreal and defoliated native shrubs. I talked about the army cutworm outbreak at the 2017 Society for Range Management annual meeting.

My recentRangelands research report describes the 2014 army cutworm outbreak, cheatgrass die-offs, shrub defoliation, and subsequent vegetation recovery. Contact me for a copy of the article for research or education.