What are miller moths, where do they come from, and what is the best way to control their presence inside a house?

Gardens & Landscapes June 29, 2010|Print
"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 and parts of Wyoming, the common "miller" is the adult stage of the army cutworm, Euxoa auxiliaris. All moths have characteristic fine scales that cover their wings. These scales easily rub off and remind people of the dusty flour that covers the clothing of one who mills grain, hence the name, "millers." The army cutworm in its caterpillar stage is a typical cutworm. In high populations, they have the unusual habit of banding together in army like groups and may be seen crawling across fields or highways in large numbers. A few other regional insects share this habit of forming large bands, including Mormon crickets, true armyworms, and forest tent caterpillars. Army cutworm moths are generally gray or light brown with wavy dark and light markings on the 1½- to 2-inch wings. The wings are quite variable in pattern and color but all have a distinctive kidney shaped marking on the forewing. In Colorado, spring migration flights move westward, originating from eastern Colorado and border areas of Wyoming, Nebraska, Kansas and Oklahoma where the army cutworm is also present. 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 begin to feed. Army cutworms spend the winter as partially-grown caterpillars, and resume feeding the following spring. At this time the cutworms may damage crops, including alfalfa, winter wheat (after the broadleaf weeds are mostly gone), and gardens. They become full grown by mid spring, burrow into the soil and pupate. Between three to six weeks later, the adult "miller" stage of the insect emerges. The moths then migrate and ultimately settle at higher elevations. There they spend a few months, feeding on nectar and resting in sheltered areas, such as under the rocks of talus slopes. Although no one is sure why they migrate to the mountains, one likely explanation is that the mountains provide an abundance of summer flowers as a good source of nectar they need as food, and the relatively cool temperatures of the higher elevations may be less stressful to the moths allowing them to conserve energy and live longer. During this time they are in reproductive diapause (inactivity), and do not lay eggs. In late summer, they return to the lower elevations to lay eggs and repeat the cycle. During outbreak years, miller moth flights may last five to six weeks, generally starting the last week of May or early June; they tend to cause most nuisance problems for only about two to three weeks. Some suggestions for controlling miller moths are to seal any obvious openings around the home where they can hide, particularly around windows and doors. This should be done before the miller moth flying season. Reducing night lighting in and around the home, including turning off all unnecessary lights or substituting non attractive yellow lights, will help reduce their numbers around the house. Once miller moths are in the home, the best ways to remove them are to swat them, vacuum them or attract them to traps. An easy trap to make is to carefully suspend a light bulb over a bucket or tub partially filled with soapy water. Some wetting agent, such as soap or detergent must be added to the water or many moths will escape. Moths attracted to the light will fall into the soapy water and be killed. Jingling keys can sometimes dramatically speed the capture rate when using the soapy-water trap. Insecticides have little or no place in controlling millers because the insects are not very susceptible to insecticides. Furthermore, any moths killed will be rapidly replaced by new moths migrating into the area nightly. For additional information on miller moths, see the tip sheet, Questions and Answers about Miller Moths.