Army Cutworm Research

Exotic cheatgrass fuels wildfires that weaken or kill native plants on intermountain rangelands. A native insect can help us reestablish desirable native plants in areas now dominated by cheatgrass.

Native army cutworms (Euxoa auxiliaris) eat exotic cheatgrass and the mustards (Brassicaceae) that often grow with it. During their periodic outbreaks, the larvae create widespread cheatgrass die-offs in low, dry areas in the intermountain West.

Cheatgrass die-offs near Golconda, NV in 2003 (L), and near Bruneau, ID in 2014 (R).

Most cheatgrass seeds sprout as soon as conditions are right, depleting the soil  “seed bank.” When army cutworms eat all the seedlings, the seed bank isn’t replenished. Most seeds that don’t germinate the first year are killed by fungi or eaten before the next growing season. We can capitalize on this temporary dearth of cheatgrass seed by reseeding die-offs with native perennial species. This will give the sown plants a head start on cheatgrass.

Extensive die-offs first appeared in 2003, during a widespread army cutworm outbreak. The outbreak and die-offs occurred after:
1. a dry winter followed by a dry summer created egg-laying sites,
2. heavy late summer rain germinated cheatgrass for larvae to eat,
3. a large flight of adult moths laid eggs in fall, and
4. a second dry winter helped larvae survive to eat cheatgrass seedlings.
My 2004 research poster described how army cutworm outbreaks can lead to cheatgrass die-offs.

Army cutworm outbreaks are most likely after a year of dry weather is broken by September rain, followed by a large flight of miller moths, and a dry weather through January.

I recognized the same conditions again in January 2014 and found army cutworms in cheatgrass die-offs in late February in Owyhee County, southwest Idaho. My research paper documented larval damage to cheatgrass, mustards, and native shrubs, and followed the subsequent recovery of vegetation.

Army cutworms also defoliated native sagebrush (Artemisia) and saltbush (Atriplex) in Owyhee County, ID in 2014.

Army cutworms create cheatgrass die-offs while hiding in plain sight. They spend the winter hiding in the soil or under cowpies during the day and eating seedlings at night. The larvae find and eat all seedlings within an area. They pupate in late spring and emerge as grayish-brown miller moths. The moths eat nectar and follow the blooms of flowering plants up to mountains for the summer.

Miller moths’ tiny wings carry them on impressive journeys. Moths emerging in the Great Plains fly through Colorado’s Front Range on their way to high peaks in the northern Rockies, where grizzly bears wait to feast on their fat-filled bodies. Miller moths emerging in the Intermountain West seem to spend summers in nearby mountain ranges. “Blizzards” of moths summered in Great Basin National Park after army cutworm outbreaks and cheatgrass die-offs in 2014, and black bears munched on millers in the Jemez Mountains of northern New Mexico in 2003.

I’ve studied these intriguing insects and the cheatgrass they love to eat since 2003. My “trapline” monitors fall miller moth numbers in southwest Idaho, where sharp-eyed residents help watch for winter army cutworm outbreaks and spring cheatgrass die-offs. Contact me if you’d like to help.

When we understand army cutworms enough to predict their outbreaks, we’ll know when and where to look for die-offs. When conditions that lead to army cutworm outbreaks occur though the end of January, it’s time to start looking for larvae and die-offs.