Expect an abundance of miller moths this year, say CSU entomologists

miller moth

An adult army cutworm, or miller moth. Photo credit: Whitney Cranshaw/Colorado State University

If the swarms of flying insects infiltrating doorways, backyards and gardens the last few weeks are any indication, miller moth season in Colorado is here.

Following four straight years of below-average numbers for the winged creatures, the insect should be noticeably more abundant in 2020, according to Colorado State University entomologists Whitney Cranshaw and Frank Peairs, both professors in the Department of Agricultural Biology and CSU Extension specialists.

“Flights of the moths crossing through into eastern Colorado on their annual migration to the mountains first became noticeable the first few days of May, much earlier than 2019, when noticeable numbers were first observed in late May,” the insect experts write in a fact sheet for CSU Extension.

Familiar to native Coloradans, “miller moth” is the common name for the adult stage of the army cutworm, a.k.a. Euxoa auxiliaris. They originate from fields across eastern Colorado and adjacent areas of western Kansas and Nebraska on a yearly westward migration, ultimately arriving in the mountains to spend the summer, and returning to the plains in September. Peak moth flights may last five to six weeks, generally starting the last week of May or early June. Already, the moths have damaged crops in Colorado, Nebraska and Kansas.

If Front Range residents are swatting more moths than usual this year, they have relatively low moisture conditions in fall, winter and spring to thank, according to the CSU entomology team. It’s been drier this year than last, reducing the number of blooming flowers. Plus, a freeze in mid-April killed blossoms from plants that would normally be in peak bloom in early to mid-May. With fewer flowers available to the insects during their migration, they tend to concentrate in areas with larger numbers of flowering plants – usually, irrigated landscapes, including yards and gardens.

Annoying as they can be, the moths won’t stick around forever. Flight activity of the moths will accelerate as the night temperatures warm. A warning, though: Cool, wet periods might make the migrating moths stay a while.

And just to confuse matters, Cranshaw adds, there is another moth event happening along the Front Range now – the adult alfalfa webworm, a smaller, mottled-gray moth that is very visible by day when walking across a yard.

Miller moth fast facts

Why are they called “miller moths?”

Fine scales that easily rub off cover their wings. The scales reminded people of the dusty flour on the clothing of grain millers.

What was the heaviest moth flight in recent history?

Since at least the early 1980s, the heaviest, most prolonged flight of miller moths across eastern Colorado occurred in 1991.

What eats them?

They have many natural enemies, including predatory ground beetles, hunting wasps, and many birds. Grizzly bears in Yellowstone are known to feed on large numbers of the fat-rich moths they find under loose rocks.

What are those swallows doing circling my car?

A commonly observed phenomenon during miller time is swallows – which love to eat miller moths – congregating at road intersections. This can occur because moths that had sought shelter in car chassis are warmed by the engine and emerge while the cars are idling at a stop light.

How do the moths get inside a house?

They avoid daylight and seek shelter before daybreak. Small cracks in doorways, garages and cars are perfect hiding spots. At night, they emerge to resume their migratory flights and to feed.

Why are moths attracted to lights at night?

Moths, and many other flying insects, are confused by low-point light sources because they naturally use the moon or stars to orient their flights.

larval stage of miller moth
Caterpillar stage of army cutworm, a.k.a. miller moth. Credit: Whitney Cranshaw

Are miller moths harmful?

The caterpillar stage can be a pest for crops in the spring, and they can kill seedling plants. Mostly, the adults are just a nuisance. Moths in the home do not lay eggs or feed on household furnishings or food. If large numbers die in a home, there may be a small odor problem due to the fat in their bodies turning rancid. Large numbers can leave spots on walls from their meconia, which is waste stored during pupal development.

How do I control miller moths in my home?

Seal obvious openings around windows and doors. Reduce lighting at night in and around homes. An easy trap to make is to suspend a light bulb over a bucket partially filled with soapy water (always use a grounded plug and extreme caution with any electrical device near water). Moths attracted to the light often fall into the water and are killed. Moths are not very susceptible to insecticides.

What is extreme fear of moths called?


More info

Much more information can be found at the CSU Extension fact sheet on miller moths.