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.
Cheatgrass is an exotic Eurasian annual grass that sneaks in around our native perennial grasses and shrubs. When cheatgrass dries, its narrow leaves and stems are perfect fuel for fire.
When I visited Winnemucca in June 2003, I found former cheatgrass stands looking more like parking lots than rangelands. The annual grasses and mustards were gone, but the perennial grasses and forbs were thriving. After walking around and scratching my head for a couple days, I stopped at a nearby ranch and asked what they knew about the missing cheatgrass.
The corral I pulled up to belonged to Jim Christison’s mother. She sent me up the road to talk to her son. Jim told me he’d seen “millions” of larvae “eating every green shoot” one January night. He took photos, collected larvae, and asked an entomologist to ID them. Jim had found army cutworms (Euxoa axiliaris).
Army cutworms are native larvae that eat introduced crops—wheat and canola—and exotic weeds—cheatgrass and mustards. The larvae grow into miller moths, which eat nectar. Moths from the Great Plains fly to Yellowstone for the summer, where they’re an important food for grizzly bears. We don’t know where moths from the Intermountain West go in summer.
That September, I ran into Bob Hammon, with Colorado State University Extension in Grand Junction, Colorado. Bob had seen more millers than usual the previous fall. The moths found lots of bare ground for egg laying, due to a recent drought. The drought had been broken by a late summer rain that germinated winter annual plants for the larvae to eat. Then the dry winter of 2002—2003 kept larval diseases low.
I shared what I’d learned about army cutworms and cheatgrass die-offs at the Society for Range Management’s annual meeting in early 2004. Rangeland managers hadn’t seen army cutworms eat cheatgrass. They were as skeptical of our story as the first entomologists had been. I realized I’d have to wait for the conditions Bob described and see if they led to another outbreak. If there were another outbreak, I could see what the larvae ate.
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.