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Miller moth mania: What you need to know about the migration as it hits the Front Range

miller moths
Posted at 11:51 AM, May 15, 2023

DENVER — You have probably noticed them in your doorway, along windows, in the nooks and crannies of your car. Or maybe you've had to chase one around your house.

The miller moths have returned to the Front Range.

"Whenever the miller moths come around, there’s always a minute of mass panic," said Shiran Hershcovich, a butterfly scientist — or lepidopterist — at the Butterfly Pavilion in Westminster. "But actually, I find them to be a comforting sign of spring."

Across much of the Rocky Mountain west, miller moths — named because their scales are reminiscent of grain millers and flour dust — are the adult stage of the army cutworm, Euxoa auxiliaris, which migrates from the eastern plains to the mountains every spring.

Let's break down their lifecycle: It starts when the moths lay eggs in eastern Colorado, Wyoming, Nebraska, Kansas and Oklahoma. During the winter and early spring, the caterpillars feed on crops and garden plants. If a winter is particularly cold and wet, many of the caterpillars will die and the cutworm outbreak is reduced. The adult form of the army cutworm — the moth — consumes nectar in the late spring through the early fall as they migrate to higher elevations to seek those flowering plants. They spend five to six weeks traveling, typically starting in mid-May. In the early fall or late summer, they migrate back to lower elevations and the surviving moths lay eggs. During this reverse migration, most people don't notice the moths as much.

"Their population numbers might be smaller," said Lisa Mason, horticulture specialist with Colorado State University Extension. "I mean, there's a lot of risks, you know, flying all the way up to the mountains."

One of the places they travel through is Colorado and some years, they can become a serious nuisance. And no, they're not deterred by our recent rainy weather.

They tend to spend the daylight hours hiding in any crack and crevice they can find. Along the Front Range, this includes places like doorways, garages and around cars.

miller moths

Come sunset and the evening hours, they are attracted to light sources and use the moon for navigation. A lit-up house is a stimulating alternative for them to check out and can disorient them, so Mason and Hershcovich recommended reducing the light inside a home at night.

Inevitably, moths will make their way into homes, one way or another. Luckily, they do not reproduce during this time in their lifecycle and aren't harmful to people, but an unexpected moth encounter can still be startling and irritating. Mason said there's a few different things to do when this happens, but warned that there's no magic trick to make them disappear.

First, you can try to catch and release them.

"It's going to be a lot of trial and error to try and catch it," she said. "Other people will use like fly swatters — that can be a good way. Or they'll vacuum them up."

You can also set a trap by suspending a light bulb over a bucket partially filled with soapy water.

Mason said it's perfectly safe for pets, especially cats, to hunt the moths.

Outside, bats and birds also eat the moths. If you provide best practices and a good home for bats, like using bat boxes, your yard could attract more of the predators, which eat not only moths but mosquitoes and other pests.

Mason said residents can rest assured that any moth that gets stuck and lost in a home will likely end up dead on a windowsill.

If you're like the two reporters behind this story, you may also be wondering exactly what role these miller moths play in the ecosystem.

Mason and Hershcovich patiently explained that in addition to being prey for other animals, including grizzly bears in Yellowstone, they are also pollinators, which are critical for human survival.

"They are an important food source for animals all around us," Hershcovich said. "The animals we love to see, like birds, even bears, rely on them for a food source. They’re essentially protein snacks on wings.... They also feed on nectar, so as they go visiting from bloom to bloom, they do the important work of pollination.”

If you're really annoyed by them, we have some good news for you: Miller moths tend to only stick around for two to three weeks before continuing west.

But Hershcovich said they are butterfly cousins, so they do not have to be scary, confusing or intimidating.