We have arrived at our Littleton, Colorado, farm during the peak of the annual "miller moth" invasion. These army cutworm moths, move through the Front Range corridor each June, on their way to a summer in the mountains. There they enjoy milder conditions and feast on the nectar of numerous wildflowers.
By late August, the moths head back to the High Plains where they lay their eggs in weedy vegetation along streams and fence lines. The adults die as autumn progresses but the eggs hatch into caterpillars, which feed on grasses and broad-leafed weeds. Overwintering in the larval form, the caterpillars resume feeding as the weather warms and often move as "armies" across the prairie landscape. In early May, the cutworms burrow into the soil, pupate for a few weeks and emerge as adult moths.
Migrating toward the mountains, the moths travel at night and seek refuge in narrow crevices (tree bark, human structures) during the day. Attempting to resume their journey at dusk, many become trapped in homes, heightening the illusion of a moth invasion. Numbers vary from year to year, depending on the severity of the previous winter and on other factors that affect the survival of larvae. In addition to the malice of frenzied homeowners, the moths face predation by birds, bats and amphibians as they migrate to and from the high country. Since many die in the mountains and the return flight is less concentrated in time, Front Range homeowners do not experience a second invasion in late summer.
