Trapping Miller Moths in Military Reserve, Boise

Miller moths and their army cutworm larvae are well known east of the Rocky Mountains. Although the insects are not as common west of the Rockies, they occasionally reach high numbers. I suspect that army cutworms were at least partly responsible for the “disappearance” of nearly a million acres of cheatgrass in the Intermountain West in 2003. Counting miller moths each fall can tell us how many army cutworms we might have the next spring. If we knew when to expect an outbreak, we could be ready to reseed the bare areas with more desirable plants.

Miller moths and army cutworms east of the Rockies

Miller moths are unwelcome spring and fall visitors to the Front Range in Colorado. Although the insects are familiar pests when they cluster around lights and invade homes, few residents are aware of the impressive trip these tiny creatures make.

The moths that move through Denver and Fort Collins each spring hatched in the soil of the Great Plains the previous winter. The larvae spent the following months hiding in the soil during the day and feeding above ground at night. When active, the larvae eat young plant leaves--sometimes, down to the ground. Emerging fields of wheat, and other plants, are just the right height for the hungry “armies.” After the larvae reach full size, they pupate in the soil. When they emerge as miller moths, they are ready to migrate west.

Although the moths sometimes linger along the Front Range for weeks, their summer home is high elevation slopes of the Greater Yellowstone area. The insects feed on nectar at night and hide among the rocks of cool talus slopes during the day. The moths’ habit of congregating in certain areas makes it easy for grizzly bears to find one of their main foods. At least two popular books have described grizzlies rolling over rocks to feast on miller moths: Cold Case, by Stephen White and Blood Lure, by Nevada Barr. When the weather cools, the moths return to the Great Plains to lay eggs that will hatch the following winter.

Miller moths and army cutworms in the Intermountain West

The winter of 2002-03 was unusually warm and dry around Winnemucca, NV. That spring a BLM employee noticed bare areas where he expected to see cheatgrass. He called the new USGS plant ecologist in Boise to ask what she thought it was. I told him I didn’t know, but I’d take a look.

I didn't find any answers by looking at the miles-wide bare areas. A rancher I stopped to ask said that army cutworms were responsible. He saw larvae “eating every green shoot” on a warm January evening and took some of them to the USDA, where they were identified as army cutworms.

Despite the eyewitness account every entomologist I told the story to laughed: army cutworms couldn’t eat all the plants in an area. It took four months to find another witness, a researcher in northern Utah who saw larvae eating cheatgrass in his field plots. Finally, I found entomologists who had seen army cutworms devouring cheatgrass and young crops in western Colorado and northern New Mexico. I concluded that the rancher in Winnemucca really had seen army cutworm eating cheatgrass.

I suspect that army cutworms were at least partly responsible for the “disappearance” of nearly a million acres of cheatgrass in the Intermountain West in 2003. (Full disclosure: other researchers scoff at the idea.) I hope to learn more about these insects, their distribution, and their migration patterns west of the Rockies.

Although we haven't had a big army cutworm year since 2003, the insects are still present in the Intermountain West and will increase again with the right conditions. As our weather becomes warmer and more variable, the chances of another warm, dry winter increase. Counting miller moths each fall can tell us how many army cutworms we might have the next spring. When we see a lot of miller moths in the fall and then have a warm, dry winter, we should start looking for larvae in early spring.

Trapping Miller Moths in Military Reserve, Boise

If we knew when to expect an outbreak of army cutworms, we could be ready to reseed the bare areas with more desirable plants. The seeded plants would have a head start on the cheatgrass and it would be easier for them to grow into a healthy stand.

I trapped miller moths in Military Reserve, in the foothills of the Boise Front, this fall. Pheromones--scents that female moths make to attract males--lured male moths into the traps.

Julia Grant, Boise's Foothills And Open Space Manager, is collaborating in this work. Julia is using weed-eating goats and kids in soccer cleats to manage weeds and restore burned areas at Military Reserve, one of Boise's open space reserves.