Miller moths, which hatch from the army cutworm, are expected to make a rare mass appearance in Southwest Colorado this month. Although the moths are harmless, the larvae can be devastating to crops./Photo Todd Newcomer..

Though it may not be biblical in proportion, an unprecedented swarm should be blowing into the Durango area in coming weeks. Local and regional officials are bracing for a widespread infestation of miller moths, and while they may be unpleasant, the insects are totally harmless, having already done their damage.

“Miller moth” is a name given to any type of moth that is particularly abundant in and around homes. In Colorado and parts of New Mexico and Arizona, miller moth refers to the adult stage of the army cutworm. In the fall, army cutworm eggs are laid in weedy areas of wheat fields, alfalfa fields or other areas where vegetation is thick. The eggs hatch within a few weeks, and after spending winter as a partially grown caterpillar, an army cutworm resumes feeding the following spring. At this time, the cutworms can severely damage crops, including alfalfa, winter wheat, grassland and vegetables. By mid-spring, the worms burrow into the soil, and three to six weeks later the miller moth emerges.

Feeding exclusively on nectar, they migrate to higher elevations for the summer. After a few months feeding and resting in sheltered areas, they return to lower elevations, lay their eggs in the fall and repeat the cycle.

Miller moths are a recurring problem on the Front Range, where large flights of the bugs move from the wide pastures of the Eastern Slope through cities and up into the mountains. Traditionally, Colorado’s Western Slope has been relatively immune to the nuisance, largely because of less abundant farmland and more severe winters. However, this year is shaping up to be a serious exception.

Bob Hammond, agent with the Colorado State University Tri-River Cooperative Extension in Grand Junction, said that local residents should brace themselves. Over the course of the winter and spring, he looked at army cutworm populations all over the Western Slope and said the numbers were staggering.

“We had an outbreak of army cutworms, huge numbers like we’ve never seen before on the Western Slope,” Hammond said. “Economically, the hardest hit areas were Montezuma County and Dolores County where the cutworms wiped out dryland winter wheat and irrigated alfalfa.”

Elaborating on the intensity of the outbreak, Hammond added, “Over near Dove Creek, we would count hundreds of cutworms per square foot.”

Hammond said that while the moths may seem bad now, their numbers will only intensify. “We’re probably only a third of the way through the emergent cycle,” he said. “We’re all going to be seeing high numbers, and they’re going to be increasing over the next couple weeks. I would bet if you go over to the Cortez area, there are billions of them right now.”

Kevin Mallow, agriculture and natural resource specialist for the La Plata County Extension Office, said that army cutworms were rampant south of Durango this winter and spring. “We will have a miller moth problem for sure,” Mallow said. “Everywhere south of Durango, crops were hit hard by army cutworms.”

Mallow said that currently miller moths are thick south of Durango and in certain areas of the city, and when they start migrating north, they’ll affect more areas.

“Right now, they’re hanging around where they hatched,” Mallow said. “You’ll start seeing them in swarms as they move up into the high country.”

Weather will determine when the miller moths decide to migrate, according to Whitney Cranshaw, a professor of entomology at Colorado State University. Once full emergence has occurred, hot and dry weather tends to accelerate the flights to the mountains. Conversely, cool, moist conditions and extended plant bloom keep millers in place. Cranshaw said that spring rains have made flowering plants more abundant and consequently the moths’ activity is currently more spread out.

The reason why the Western Slope is expected to experience such an intense cycle of miller moths remains largely unknown. Hammond and Mallow credited the mild winter for the survival of the cutworms. However, miller moths have historically been next to nonexistent in this part of the state.

“We haven’t had an event like this in recent memory,” Hammond said. “As far as I know, the last big year for miller moths was in the early 1970s.”

Hammond continued: “The Front Range goes through this frequently because they’re drawing moths from all the fields of the East Slope. But the West Slope’s moths are generated locally, and historically we haven’t had the big flights. We just don’t have the same volume of pasture.”

Cranshaw said he doesn’t know why the miller moths are posting such high returns particularly in unusual places like La Plata County. “It beats me,” he said. “A lot of people blame it on the winter, but nobody really knows. They’re highly cyclical, and they survived well for one reason or another.”

Cranshaw, Mallow and Hammond all stressed that the miller moths are purely a nuisance and pose no threat to humans or their clothing. The insects can be common inside homes because they avoid daylight and seek shelter before daybreak, frequently in the tight and dark cracks of doorways, garages and cars. At night, the moths emerge from their day-time shelters to resume their migration to higher ground and to feed. Cranshaw noted that moths do not feed in homes or lay their eggs. Moths in the home will eventually either find a way outdoors or die without reproducing.

“Probably the greatest harm is having them die in your house,” Cranshaw said.

In addition, controlling the moths can be futile. The insects are largely immune to pesticides, and fresh waves of moths are usually right around the corner.

Hammond said that patience can be the best way to deal with miller moths. “There’s not a lot you can do,” he said. “You can suck them up with a vacuum, but there will be just as many the next day.”

And Hammond added that the end is not far off.

“Typically, they’re gone by the end of June,” he said. “At least, I hope so.”