Denver, CO

After mild winter, dry spring, expect summer miller moth invasion

Claire Cleveland

By Claire Cleveland / NewsBreak

(Denver, CO.) Miller moths are out in force this year, buzzing around porch lights and darting through intersections to avoid swooping birds.

The populations of these moths, which are the adult stage of the army cutworm, fluctuate every year depending on a constellation of factors. The preceding winter and spring are essential for the fuzzy, dusty invertebrates. This last winter was mild, and the spring was dry, which means lots of miller moths, according to Shiran Hershcovich, lepidopterist manager at the Butterfly Pavilion.

“Army cutworms will spend late fall, early winter as caterpillars and a mild winter means a high number of them are actually surviving all the way through to the spring,” Hershcovich said. “So right now, when they are performing that incredible metamorphosis process and emerging from their cocoons as miller moths we're seeing high numbers.”

The dry spring gives way to what experts call the “oasis effect.” The moths are hungry and looking for blooming flowers to feed on, but the dry weather means those flowers are primarily in artificially irrigated areas, like green spaces and gardens, which is why people tend to see high numbers of them around their homes.

The miller moth tendency to congregate around homes and porch lights can be a nuisance for some, especially when they get into homes and hide in linens or leave their signature brown marks on walls and furniture -- a defense mechanism to convince predators not to eat them. Hershcovich hopes people can shift their attitude from annoyance to appreciation, or at least acceptance, of the flighty creatures crucial to local ecosystems.

“When they come in in huge numbers through that migration, they are actually some of the biggest food stores for some of our favorite animals, including migratory birds, and even the Yellowstone bears,” she said. “I like to think of them as flying almonds or Hazelnuts. They are high fat, high protein, calorically dense, and nutrient rich. So they're actually very, very nutritious for all of these other animals that we love to see.”

Hershcovich encourages people to “internalize the wonders of invertebrates.” She says they can be cool and weird in the best way.

“I think if we start being invertebrates from that curiosity perspective, as opposed to they're annoying and they're flying at my face, we can really shift our perspective,” she said. “I do like to encourage everyone to just stare at a moth for five minutes, and your life might be changed. It may not, but it might increase your tolerance level, and if that feels like too high of a barrier to entry, visit the Butterfly Pavilion and learn about some of the amazing things that invertebrates can do both in terms of body plan and in terms of survival strategies.”

Miller moths start their short lives as army cutworms on the Eastern Plains of Colorado, where they feed through the winter and early spring. Then they pupate or create a pod known as a chrysalis and then start the process of growing into their mature moth form. After that, they start to migrate West, stopping along the Front Range before moving to high altitudes for the summer.

“Any survivors from the summer season will make it back down to the east to lay their eggs and continue the cycle again,” Hershcovich said. “So there is one generation each year.”

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Claire Cleveland is a Denver-based freelance writer with a background in health and science reporting. She's covered the pandemic extensively and local news in Colorado. Previously, Claire was a reporter and producer for Colorado Public Radio.

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