Also called the miller moth, this moth ranges across the western U.S. and into Canada and east into the Midwestern states. This is the most common cutworm in the high plains and it is the species that has caused our “plague” of moths this spring. Our overabundance of moths is not unique. The Colorado State University Extension service reports that environmental conditions have resulted in moth outbreaks throughout the eastern Rocky Mountains and high plains this year. A recent visit to a friend’s house near Carrizozo also showed an accumulation of hundreds of Army cutworm moths dead and scattered throughout the house. The CSU Extension service state that a lower than normal winter precipitation in 2020 compared with 2019 caused a decrease in wildflower abundance in non-irrigated lands on the prairies of eastern Colorado, western Kansas and Nebraska, and eastern New Mexico. This, combined with a larger than average population of cutworm larvae, has resulted in the adult moths migrating to higher, cooler and more moist environments, and congregating in areas where flowering plants are more abundant.
During later summer and fall a female may lay from 1,000 to 3,000 eggs in vegetation in prairie habitats at lower elevations. Caterpillars overwinter in the ground and feed voraciously in early spring before pupating in the soil. Adults emerge in April through early June, migrating to higher elevations and cooler temperatures of mountainous regions, and continuing further development during the summer where they feed on nectar of such flowering plants as fruit trees, lilac, spirea, and roses. They also seek out moist shaded conditions, explaining why we find them clustered in the shaded moist vegetation and under the cover of other objects outside our dwellings. They also are able to enter the shelter of our homes and buildings through small cracks or openings, perhaps attracted by light. The adults are not harmful and do not damage clothes and other fabrics but are great nuisances due to the large numbers that show up every day. This spring has been a particularly bad one. As fall approaches the adults will migrate to lower elevations where they lay their eggs.
At lower elevations where eggs are laid, caterpillars feed on winter wheat and alfalfa crops, because these crops are growing in early spring when caterpillars are most actively feeding. They also cause damage to many vegetable crops, as well as nursery plants and juniper forests. In times of abundance cutworms can defoliate fields of mustards, cheatgrass, and Russian olive. resulting in areas devoid of grasses. They then move on to attack native shrubs and cause damage to them as well. Tilling crop fields in the fall can kill many of the overwintering caterpillars. Because from two-thirds to three-quarters of the adult body weight is fat, they are an excellent food source for birds and other carnivorous animals, even being taken by bears.
Our plague seems to be abating in the East Mountains as of the end of May. My home and yard have been filled with moths for the month of May but I notice that in recent days I’m suddenly finding very few new moths when I come home. Hopefully the moths are now moving on in their migrations and their 2020 invasion is nearing its end.
James Taulman is a semi-retired research wildlife biologist, having worked with the U.S. Forest Service research branch and taught zoology, ecology and other courses in several university positions. He is currently living in the East Mountains, and explores natural areas observing native wildlife and conducting independent research projects.