By Whitney Cranshaw and Frank Peairs, Colorado State University Cooperative Extension Entomologists
Miller moth is the term given to any type of moth that is particularly abundant in and around homes. In the eastern half of Colorado, the common miller is the army cutworm, Euxoa auxiliaris.
The wings of all moths are covered with fine scales that easily rub off. These scales reminded people of the dusty flour that covered the clothing of the miller. The caterpillar stage is a typical cutworm. In high populations, however, they have the unusual habit of banding together in army-like groups and may be seen crawling across fields or highways in large numbers.
The 1 1/2 to 2-inch wingspan of the army cutworm moth is typical of the size of many other cutworms found in the state. It is generally gray or light brown and has wavy dark and light markings on the wings. The wing patterns of the moths are quite variable in color and markings.
Many of the moths that fly at night have specialized eyes that increase the light reaching the light- sensing receptor cells. In the base of the miller-moth eyes are a series of thread-like trachea that carry oxygen. These are pale colored and reflect light, giving the appearance of glowing. There also are colored pigments in the eye, which may give an iridescent color to the light.
The army cutworm has an unusual life history. Eggs are laid by the moths in late summer and early fall. Most eggs are laid in weedy areas of wheat fields, alfalfa fields or other areas where vegetation is thick. The eggs hatch within a few weeks and the young caterpillars feed. The army cutworm spends the winter as a partially grown caterpillar and resumes feeding the following spring. At this time, the cutworms may damage crops, including alfalfa, winter wheat (after the broadleaf weeds nearly are gone) and gardens. They become full grown by mid-spring, burrow into the soil and pupate.
About two to three weeks later, the adult miller stage of the insect emerges. Spring flights of miller moths, moving east to west across the eastern half of the state, originate from moths that developed across eastern Colorado and probably as far as the border areas of Wyoming, Nebraska and Kansas. They fly west and ultimately settle at higher elevations. There they spend one to two months feeding on nectar. During this time, they are in reproductive diapause and do not lay eggs. In late summer, they return east to lay eggs and repeat the cycle.
No one is sure why army cutworms, the "Colorado miller," migrate to the mountains in the summer. One likely explanation is that the mountains reliably provide an abundance of summer flowers, a source of nectar they need as food. In addition, the relatively cool temperatures of the higher elevations may be less stressful to the moths, allowing them to conserve energy and live longer.
During outbreak years, miller-moth flights may last five to six weeks, generally starting in late May or early June. However, they tend to be most severe for only two to three weeks. Return flights in late summer are usually less spread out. However, since many of the moths die during the summer, the return flight is less obvious.
The army cutworm also can be found in western Colorado. Presumably, it also follows sources of nectar plants, although it does not occur in the nuisance numbers found in eastern Colorado.
Population density varies from year to year. Dry periods will help shorten our misery, as the millers will move on through the area more quickly and not stay to linger over the many flowers seen in the spring of a wetter year.
Miller Moths In the Home
Miller moths avoid daylight and seek shelter before daybreak. Ideally, a daytime shelter is dark and tight. Small cracks in doorways of homes, garages and cars make perfect hiding spots. Often, many moths may be found sheltered together in particularly good shelters.
Miller moths between the coils of a garden hose
At night, the moths emerge from the daytime shelters to resume their migratory flight. Since cracks often continue into the living space of a home (or a garage, car, etc.), a wrong turn may lead them indoors, instead of outside.
Although there is still debate among scientists on this point, most believe moths are attracted to the light because they use the moon to help orient their flights. Such distant points of light allow the insects to fix their flights by maintaining a constant angle to the light source. Artificial lights confuse the insect response, since these lights are so close (an unnatural situation). Trying to maintain the flight angle to these close light sources cause the insects to spiral to the source.
Are miller moths harmful? The caterpillar stage of the army cutworm is sometimes an important pest for crops in spring. However, the adult-miller stage is primarily a nuisance--albeit a considerable nuisance at times. Moths in the home do not feed or lay eggs. During the migratory flights, the moths do not produce or lay eggs. Furthermore, the caterpillar stage would not survive on household furnishings or other foods in the home.
Moths in the home will eventually either find a way outdoors or die. When large numbers do die in a home, there may be a small odor problem (due to the fat in their bodies turning rancid). Also, unless they are cleaned out, the old moths may serve as food for carpet beetles and other household scavengers.
Probably the greatest damage by millers is lost sleep, when they are flying about the room and the (needless) worry they may cause some harm.
Moths that have recently emerged from the pupa produce a reddish-brown fluid that often is deposited on windows, walls or other areas where the insect rests. This is called meconia and is the waste product stored during pupal development. Meconia is primarily proteinaceous and is usually not difficult to remove. Follow normal fabric-care instructions on clothing. Spray-and-wash type household cleaners can remove the spots from walls and other surfaces.
The caterpillar stage of the army cutworm has many natural enemies. Predatory ground beetles and many birds eat cutworms, and the larvae of various flies and wasps develop within and kill the caterpillars. Adult millers may be eaten by bats or even many birds when the millers are forced to fly during the day.
The number of miller moths in late spring is related to the number of army cutworm caterpillars which occurred earlier in the season. Outbreaks of the army cutworm usually are followed by large flights of millers.
Many things can influence cutworm outbreaks. Extremely cold winter conditions may kill many caterpillars, and when few moths are present, low numbers of eggs may be laid. The effectiveness of natural enemies, such as ground beetles and parasitic wasps, help regulate numbers of cutworms. Plowing fields where cutworms have laid eggs kills many.
In the Home
Before miller moths start to fly, try to seal any obvious openings, particularly around windows and doors. Also, reduce lighting at night in and around the home during flights. This includes turning off all unnecessary lights or substituting non-attractive yellow lights. Although the moths avoid daylight, they are attracted to point- sources of light at night.
Once in the home, the best way to remove the moths is to swat or vacuum them or to attract them to traps. An easy trap to make is to suspend a light bulb over a partially filled bucket of water. Moths attracted to the light often will fall into the water and be killed.
Insecticides have little or no effect in controlling millers. The moths are not very susceptible to insecticides. Furthermore, any moths killed will rapidly be replaced by new moths that migrate into the area nightly.
Photos: Judy Sedbrook
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010