When the rangeland manager on the phone saw the first bare areas, he was surprised. When he found whole valleys without plants, he called the new plant ecologist at the U.S. Geological Survey in Boise. I said I didn’t know where the cheatgrass went, but I’d take a look.
I was looking for a fugitive with few friends. Weedy, nonnative cheatgrass usually blankets large swaths of the Intermountain West. This annual grass moves in after native plants are killed by fire, weakened by drought, or damaged by people or livestock. Cheatgrass sprouts in fall or early spring across the lowest, driest parts of the West. The young plants sink roots to forage for water and nutrients while many of our native plants are dormant. Before cheatgrass plants die in spring, each produces a bumper crop of seeds to continue the cycle the next year.
A few days after the alarming phone call, I stood in what had been rangeland. The area seemed to have been bladed for a parking lot. The few surviving plants were a short hike apart. The survivors were all native perennials--and they were thriving. The globemallow, indian ricegrass, and squirreltail were guzzling the water and nutrients left after the usual crop of cheatgrass failed to grow.
At the edges of the cheatgrass die-off, the ‘dozer driver seemed to have stopped for lunch and forgotten to come back. Miles of bare soil ended abruptly in normal-looking cheatgrass. The farthest edge of the dieoff ended partway up the toe slope of the nearby mountains, as if the ‘dozer driver had avoided driving on the steepest slopes.
I had a long list of ways plants can die; I needed help narrowing down my list. I stopped at the nearest ranch and asked about the missing cheatgrass.
The couple working in the corrals was younger than the century-old ranch, but they hadn’t seen a year without cheatgrass before. “My son says it’s army cutworm. He lives up the road,” said the woman, who wore a painful-looking shiner inflicted by one of the horses.
The son, Jim, described seeing insect larvae “eating every green shoot” on a warm, dry January evening. “It was warmer than today,” he said, as I shivered in the June dusk in his front uaof his house. Jim did what researchers do: he photographed the insects and took some to the local U.S. Department of Agriculture office. The entomologist ID’d them as army cutworms.
As I drove away, I congratulated myself on inquiring locally to solve the mystery in record time. Then I did what researchers do: I got a second opinion.
“Army cutworms??” My entomologist friend’s voice shot up in surprise. “No way. Army cutworms would never eat that much cheatgrass.” She vouched for the insects--they were innocent. I needed a third opinion.
The next entomologist laughed out loud. I went back to my list of Ways Plants Can Die.
Walking to and from work, or driving to remote field sites, I went over my mental list: Frost? Yes, cold air drainage might damage plants in the bottoms of the valleys and leave those on nearby slopes. Fungal root disease? Probably not during a warm, dry winter, as fungi need abundant moisture. And how could a fungus kill ALL the plants in an area? And why would it suddenly stop killing plants at the edges of patches?
Months later, a second witness came forward and fingered army cutworms. A researcher in northern Utah had seen larvae destroying his cheatgrass experiment.
Finally, that fall, I found an entomologist in western Colorado who had seen army cutworms devouring cheatgrass and crops in his area. The outbreak of larvae hadn’t surprised this witness, as he’d noticed more miller moths than usual the previous fall. He knew they would lay eggs that would hatch into army cutworms.
Although few people know army cutworms in the Intermountain West, residents east of the Rockies know both larvae and adults all too well.
Between its summer wildfires and winter blizzards, Colorado’s Front Range suffers semiannual plagues of miller moths. Billions of the insects invade in spring and late summer as they migrate from the Great Plains to and from high peaks of the Rocky Mountains.
The moths annoy local residents by loitering around lights, invading homes, and defecating on walls. Media describe “squadrons” of moths “attacking” and “dive-bombing” people. The insects’ unpleasant habits make it hard to appreciate the impressive journey these tiny creatures complete: on a two-inch wing span, miller moths make a thousand-mile round trip on their summer vacation.
The moths that fly through Denver and Fort Collins each spring hatched in the soil of the Great Plains the previous winter. The army cutworm larvae hide underground during the day and come out to feed at night. Sprouting fields of wheat, and other crops, are just the right height for the hungry insects.
The cutworms' nocturnal habit makes it hard to catch them in the act of destroying crops. The young larvae are so tiny that it's hard to see them at all--until it's too late. The insects earn their moniker when they reach 1½ inches and are mostly jaws. After consuming all the food in an area, the army marches off in search of more. The destruction ends when the troops stop, drop, and pupate in the soil. They emerge several weeks later as miller moths.
Although a long, wet spring can persuade the moths to linger and feed along the Front Range for weeks, their summer home is high elevation slopes around Yellowstone Park. The insects feed at night and congregate among the rocks of cool talus slopes during the day. The gatherings of moths are grizzly bear banquets. By eating as many as 40,000 moths a day, the bears get up to half their yearly energy from the maligned miller moths.
In late summer, the surviving moths turn their back on the Rockies and head back east to the Great Plains. Or do they all? Do any of the Yellowstone moths fly west? Or do "our" miller moths spend their summers in talus slopes closer to home?
Farmers in the Great Plains check their fields for army cutworms, homeowners along the Front Range recalk their windows before the miller moth migration, and Yellowstone’s bears count on the plump insects to get them through the next winter. But the army cutworm outbreak of 2003 was a surprise attack in the Intermountain West. We don’t know exactly why there were so many larvae that year or even where the adults spent the previous summer.
I blame army cutworms, perhaps working with a pathogen accomplice, for the disappearance of hundreds of thousands of acres of cheatgrass in the Intermountain West in 2003. The weather was perfect for the insects, the cheatgrass disappeared when the larvae were feeding most voraciously, and their need to stop eating and pupate could account for the sharp borders of the bare areas. (Full disclosure: some other researchers scoff at the idea.)
We haven't had a big army cutworm year since 2003--or seen big cheatgrass dieoffs. But the insects are still here, waiting for their next opportunity. Our warming climate could give them many opportunities in the future.
Army cutworms need both a mild winter and lots of eggs to reach impressive numbers. That means lots of miller moths have to return from their summer journey. When we see high numbers of miller moths in late summer, followed by a warm, dry winter, we need to start looking for army cutworms on our rangelands. When we see bare areas where we expect to see cheatgrass, it’s time to get out the seeders and reseed those areas with native perennial plants. Without competition from cheatgrass, the seeded plants will be able to establish vigorous stands that can hold the line against cheatgrass.
In August and September, I trapped and counted miller moths in the foothills of the Boise Front. Pheromones--scents that female moths make to attract males--lured males into the traps.
When I catch lots of moths in fall and we have heavy late summer rain followed by a dry winter, I'll start looking for army cutworms and watching for bare areas. The next time someone calls with a missing cheatgrass report, I'll start with my prime suspect.
