The common name “miller moth” refers to any moth species that is abundant in or around the home. In Colorado, miller moths are most problematic in eastern parts of the state, and the most common miller moth is the adult stage of army cutworm (Euxoa auxiliaris). These moths have a wingspan of 4-5 cm (1.5-2 inches) and are typically gray or light brown. They also have wavy markings of dark and light shades on the wing, as well as a distinct kidney-shaped pattern on the forewing.
- Moths and butterflies are insects in the order Lepidoptera. With around 180,000 species found worldwide, this is the second most diverse insect order after the beetles (Coleoptera).
- Miller moths are the adult stage of an important agricultural pest called army cutworm. The moths are considered a nuisance in and around households.
- The most severe miller moth nuisances in Colorado are generally observed in eastern parts of the state.
Army cutworm moth. Miller moths are the adult stage of army cutworm. These moths are considered household pests, especially during their annual migration westward as they seek out flowering plants at higher elevations. Image credit: Joseph Berger, Bugwood.org
Army cutworm moth. Image credit: John Capinera, University of Florida, Bugwood.org
Life history and habits
All moths undergo complete metamorphosis, and have four life stages: egg, larva, pupa, adult. Miller moths lay eggs in weedy areas including wheat, alfalfa, turfgrass, or other areas of thick vegetation in the late summer and early fall. Army cutworm caterpillars (larvae) emerge a few weeks later and begin feeding on broad leafed plants or grasses, depending on availability. They continue feeding through the winter when temperatures are high enough, eventually pupating in soil the following spring beginning in March – May. The exact timing and duration of the pupal stage is also dependent on temperature.
Adult moths emerge 3-6 weeks later and migrate west to the mountains where they feed on nectar and rest in sheltered areas. The migration lasts around five to six weeks; however, the moths are only a nuisance in households for two or three weeks during their migration. These moths also make a return trip east from the mountains to the plains in the fall.
Adult miller moths are an important food source for animals like birds, bears, and bats. Bats use echolocation to pinpoint their prey during flight. In response to certain acoustic frequencies, moths will instinctively make evasive maneuvers to avoid predation. This behavior can be exploited to increase the capture rate of traps (see management section below).
Army cutworm caterpillar. These are considered agricultural pests. Image credit: Frank Peairs, Colorado State University, Bugwood.org
These moths are susceptible to few insecticides, and those that are killed will quickly be replaced by new moths as they continue migrating west to higher altitudes.
Certain landscapes provide more nectar and shelter for miller moths than others, which means they will naturally be more attracted to the area. Plants frequently visited by miller moths for food include cherries, lilac, cotoneaster, horsechestnut, raspberry, Russian olive, and spirea. Plants that provide shelter for these moths include cotoneaster shrubs, spruce, and pine trees.
Nocturnal moths like these are attracted to light, therefore minimizing the amount of light around homes will draw in fewer moths. When possible, light bulbs emitting yellow light are preferable to others as insects do not see yellow light as well as other colors. Any cracks around doors and windows should be sealed with caulking. Old screens, doors, and vents should be repaired or replaced.
Moths inside the home can be dealt with by vacuuming, trapping, or swatting. Traps can be made by filling buckets with soapy water and placing them under light bulbs. It is important to mix the water with soap or detergent as this decreases the surface tension of water and makes it more difficult for moths to escape. The capture rate of these traps can be increased by jingling keys, dog tags, rattling coins, or crumpling empty soda cans since these noises startle the moths and send them into flight.
Cranshaw, W. 2014. Miller Moths. Colorado State University – Extension. Available https://extension.colostate.edu/topic-areas/insects/miller-moths-5-597/
University of Nebraska-Lincoln. 2020. Spring Miller Moth Invasion. University of Nebraska-Lincoln: Institute of Agriculture and Natural Resources. Available https://cropwatch.unl.edu/2020/spring-miller-moth-invasion
University of Nebraska-Lincoln. 2013. The Miller Moth Returns – UNL CropWatch. University of Nebraska-Lincoln: Institute of Agriculture and Natural Resources. Available https://cropwatch.unl.edu/miller-moth-returns-unl-cropwatch-may-3-2013#:~:text=Moths%20emerging%20in%20Nebraska%20tend,time%20will%20extend%20their%20stay.