Questions and Answers about Miller Moths

by Whitney Cranshaw and Frank Peairs
Colorado State University Extension Entomologists

Prediction of Nuisance Problems with Army Cutworm (aka "Miller Moth") for June 3, 2009

Flights of miller moths (army cutworm) began in the first half of May - a bit earlier than in some years - and some have already made it to their summer quarters in the high country. But many remain at the lower elevations and still have to make their annual trek from the plains to the mountains.

Despite some isolated reports of large numbers of moths, 2009 should be a pretty “normal” for miller moths. A couple of moths will likely be observed most days and there may be a spurt or two following periods when flights accelerate – usually during warm, calm nights. Numbers likely will be above what has occurred in the past few years, which have been below normal. However, they most definitely will come no where near the the historic highs of 1990-1991 when it was typical to encounter scores of moths in homes, vehicles, and buildings on a daily basis from early May through mid-June.

There are several reasons for this prediction:

  1. The number of moths that survived last summer and were around in September to lay eggs were not very high;
  2. There have been few reports of cutworm problems in crop lands of eastern Colorado; the caterpillar stage of the miller moth (army cutworm) is the most common cutworm found in winter wheat, alfalfa, gardens and lawns in spring and their numbers are a pretty good predictor of later adult "miller moth" numbers;
  3. A good chunk of the winter wheat has been sprayed with insecticide to kill Russian wheat aphid and these applications would have incidentally whacked most cutworms in those fields;
  4. Spring rains are allowing the growth of flowering plants throughout the region, providing abundant nectar sources for the moths to visit. This will prevent an "oasis effect" we see during dry years when few flowers are produced so the moths concentrate on the flowering plants in irrigated yards/gardens.

On the other hand, a continuation of cool and overcast weather will slow the migration to the mountains, causing the moths to settle with us for longer. Regardless, they will be with us for several more weeks, as some larvae are still around that have yet to pupate - which must occur before the miller moth adults are present at all.

And there will be some properties that will have more miller moths than others. The greatest risk factor associated with having above average miller moths in one’s house is the presence of abundant flowering plants in the yard during the migration. But ultimately they all will find their way to the mountains, which usually happens when we get some hot dry weather and the Russian olive bloom has past. (Russian olive is the #1 best plant to have around if you like miller moths in your house.)

What are miller moths?

'Miller moth' is the term given to any type of moth that is particularly abundant in and around homes. In Colorado and parts of Wyoming, New Mexico and Arizona the common 'miller' is the adult stage of the army cutworm, Euxoa auxiliaris.

How did miller moths get their name?

Fine scales that easily rub off cover the wings of all moths. These scales reminded people of the dusty flour that cover the clothing of one that mills grain.

How does the army cutworm get its name?

The caterpillar stage is a typical cutworm. However, 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 cricket, the true armyworm, and forest tent caterpillars.

Are any other moths very common in Colorado?

Occasionally there are very large flights of other kinds of moths. Most common is the alfalfa webworm. Alfalfa webworm moths are about 1/2 the size of the army cutworm moth and hold their wings in a distinct V-shape when at rest. They are commonly seen about lawn areas and fly a considerable amount during the day time, unlike the army cutworm. Alfalfa webworms also do not have the nuisance habit of entering homes and cars. They are of minor importance as crop pests, as the caterpillars will eat alfalfa, beets, spinach and a few other early season crops.

What does the army cutworm miller moth look like?

The moth of the army cutworm is typical of the size of many other cutworms found in the state with a wing span of 1 and 1/2 to 2 inches. It is generally gray or light brown with wavy dark and light markings on the wings. The wing patterns of the moths are quite variable in color and markings, but all have a distinctive kidney-shaped marking on the foreign.

How do millers develop?

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 are mostly gone), and gardens. They become full grown by mid-Spring, burrow into the soil and pupate.

Between 3 to 6 weeks later, the adult 'miller' stage of the insect emerges. They 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. During this time they are in reproductive diapause, and do not lay eggs. In late summer, they return to the lower elevations to lay eggs and repeat the cycle.

Where do miller moths come from?

Miller moths are the migrating adult stage of the army cutworm. In eastern Colorado, spring flights move westward, originating from moths that developed across eastern Colorado and border areas of Wyoming, Nebraska, Kansas and Oklahoma where army cutworm also occurs.

Nuisance problems with 'millers' appears to be almost entirely limited to eastern Colorado. However, army cutworms also occur commonly in western Colorado. These similarly move to the mountains to spend the summer. However, they do not occur as such a serious nuisance as occurs along the West Slope, in part because there is area at lower elevations from which the cutworm populations develop.

Why do miller moths migrate?

Although no one is sure on this point, one likely explanation is that the mountains reliably provide an abundance of summer flowers, which are a good source of nectar that 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.

Whatever the reason, the army cutworm has a very different habit than most cutworms common in the region. After pupation, most cutworms remain in the soil for several months, until late summer and early fall, at which time the moth stage of these cutworms emerges. Other cutworms transform to the adult stage but then continue to develop and produce multiple generations per season.

How long do the spring flights last?

Typically, peak miller moth flights may last five to six weeks, generally starting the last week of May or early June. Often within this period they may be at peak numbers as nuisance problems for about two to three weeks. However, weather patterns greatly affect this.

There is variability in when the flights occur. Last year (2002) nuisance problems probably extended longer than at any other period of recent history - from late April through early July. During the 1991 outbreak high populations were present from early May through midJune. However, in 1990, a year that also was above average for miller flights, heavy flights were not noted until early June and lasted about a month. In 1997, flights were most noticeable during late May and early June.

During 1999, heavy flights were observed in the Arkansas Valley area in mid May. However, it was only in early June that significant activity was noted in more northern areas of the Front Range. Wet, overcast conditions apparently have prolonged peak migrations which are currently occurring at some of the latest dates of recent record.

The return flights (mountains to the plains) in early fall usually stretch over a shorter period. However, since the majority of moths die during the summer the return flight is less obvious.

Why were the flights so extended during 2002?

The drought greatly affected miller moth behavior in 2002. Probably only average or perhaps slightly above average numbers of insects were present. However, due to the drought their activity was much more concentrated to those locations were conditions were most favorable. Since miller moths feed on nectar and seek areas of relative cool and higher humidity this meant that they were found in the 'oases' of irrigated landscapes. Since there were very few blooms beyond irrigated sites and hot, dry weather persisted the miller moths tended to accumulate and persist, rather than move on to summer feeding areas in the mountains.

Survival in the mountains in 2002 appears to have be fairly good, based on the numbers of moths observed returning in September and October. Drought may have further affected behavior at this point with eggs being laid in sites that had received some recent precipitation or that were irrigated. This concentration of egg laying last fally may have contributed to some of the massive outbreaks of caterpillars observed in some locations during late winter and early spring 2003.

Why were the flights in 1991 so large and early?

Many areas in and around Colorado had extremely high army cutworm populations early in the season. This resulted in very large numbers of millers. This was particularly acute in southern Colorado and western Kansas.

Flights during the year were also initiated early, probably due to the warm February which allowed them to push development. As they arrived into populated areas there was an abundance of flowering plants in yards and gardens. Furthermore, May and early June weather tended to be cool and overcast. This combination of factors caused many of the migrating moths to settle along the Front Range rather than continue into the mountains. Populations 'piled up' until high temperatures and reduction in nectar sources occurred in mid-June.

In the three seasons subsequent to the outbreak, populations crashed. Cutworm activity in spring and nuisance miller moth problems were much less serious.

Why are millers more common some years than others?

The numbers of miller moths in late spring is primarily related to the numbers of army cutworm caterpillars which occurred earlier in the season. Outbreaks of the army cutworm are usually followed by large flights of miller moths.

Many things can influence cutworm outbreaks. Extremely cold winter conditions may kill many caterpillars. The effectiveness of natural enemies, such as ground beetles and parasitic wasps help regulate numbers of cutworms. Plowing fields where cutworms are developing kills many. Normal mortality of army cutworm following egg hatch is ca 99% - e.g., from an egg mass of 200 only two survive to reproduce. Small changes in the effects of natural enemies and other controls affecting caterpillar survival can lead to big changes in the numbers of adults present.

Miller moths may also concentrate around buildings more intensively during some years. The presence of flowering plants and local humidity conditions are suspected as being important in concentrations of miller moths.

Overcrowding of moths in their mountain shelter areas, including competition for nectar, likely contribute to their poor survival.

Do any other moths have a similar migratory habit?

One moth with a similar plains-mountain migration is the 'bogong moth' of Australia, Agrotis infusa. This species sometimes causes severe nuisance problems to population centers along its migration route, including the national capital, Canberra. Other moths with seasonal migrations include Euxoa sibrica, which migrates to the Alps of Europe and cutworms found in Africa (Spodoptera exempta) and Asia (Mythimna separata).

What eats millers?

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 birds when the millers are forced to fly during the day.

One commonly observed phenomenon involving birds are swallows concentrating at intersections where they feed on miller moths. (House sparrows and other birds also are found at these sites, feeding on wounded moths.) This likely occurs because many miller moths that had sought shelter in an automobile chassis and warmed by the vehicles engine emerge while cars are idling. Furthermore, many moths are released as drivers open windows at intersections to promote escape of moths that become active in the car.

Miller moths also may be fed upon by other wildlife as well. For example, they can be an important part of the diet of grizzly bears in the Yellowstone area, which feed on large numbers of the fat-rich moths that rest in large groups under loose rocks.

How do millers get indoors?

Miller moths avoid daylight and seek shelter before day break. Ideally, a day time shelter is dark and tight. Small cracks in the doorways of homes, garages and cars make perfect hiding spots. Often moths may be found sheltered together in particularly good shelters.

At night, the moths emerge from the day time shelters to resume their migratory flights and to feed. 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.

Is the army cutworm 'miller' the only common moth found in Colorado homes?

Although the army cutworm moth usually is the most common moth in eastern Colorado homes during the time of spring migrations, other moths are more common during other times of year.

The Indianmeal moth is the most important moth found in homes since its caterpillar stage can be a serious pest of pantry items such as nuts, dried fruit, grains, pet foods, and seeds. The Indianmeal moth is much smaller than the army cutworm miller with light brown wings that have darker tips.

Clothes moths also occur in Colorado. Fortunately they are very uncommon and most infestations occur after recent purchases of woolen goods from chronically infested areas (e.g., Mexico, Central America, South Pacific, etc.). Clothes moths are small, pale colored moths found most commonly among woolens and furs on which the caterpillars feed.

Of course, many other insects may become periodically more abundant and several other moths sometimes enter homes.

Why are moths attracted to lights at night?

Although there is still debate among scientists on this point, most feel that moths 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 they 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.

Why do the eyes of miller moths glow?

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 eyes of the miller moth there are a series of thread-like trachea, which carry oxygen. These are pale colored and reflect light, giving the appearance of glowing. There are also colored pigments in the eye which may give an iridescent color to the light.

Are miller moths harmful?

The caterpillar stage of the army cutworm is sometimes an important pest of crops in the spring. For example, during 1991 thousands of acres of alfalfa, winter wheat and other crops were treated with insecticides for army cutworm control.

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 nor lay eggs. Furthermore, they do not feed on any household furnishings or other foods in the home. Moths in the home will eventually either find a way outdoors or die without reproducing.

When large numbers 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, old moths may serve as food for carpet beetles and other household scavengers. These secondary insects may become problems in subsequent years.

Miller moths also may spot drapes or other surfaces, such as unfinished wood.

Probably the greatest damage done by millers is the lost sleep resulting from their flying about the room and the (needless) worry that they may cause some harm.

What are the spots they leave behind?

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 call meconia and is the waste products stored during pupal development.

In addition, miller moths are able to produce and excrete fluid for most of their adult life. This product is slightly acidic and is sprayed by the moth. Presumably the purpose of this is defensive, although it is not particularly irritating.

Miller moth spotting is usually not difficult to remove. Follow normal fabric care instructions on clothing. Spray and wash household cleaners can remove the spots from finished walls and most other surfaces. However, cleaning can be considerably more difficult when the spots have set on more porous surfaces, such as unfinished wood.

How does landscaping affect miller moth numbers around a home?

Landscape plants that provide food (nectar) and/or shelter for miller moths can increase local populations during migrations.

Some of the flowering plants most readily visited by miller moths along the Front Range include lilac, cherries, spirea, cotoneaster, horsechestnut, raspberry, and Russian olive. (Cherries and lilac are often blooming during early periods of moth migration; cotoneaster and spirea with peak migration periods; and Russian olive with late migrations.)

Dark dense plants will be used by miller moths for shelter. Cotoneaster shrubs, spruce and pines are among the plants most used by miller moths for shelter around homes.

Do noises affect miller moths?

Army cutworm moths are very sensitive to certain noises, making erratic flying movements in response. Among the sounds which elicit greatest response are jingling keys, dog tags, rattling coins, and crumpled pop cans.

The likely reason for this is that certain frequencies are produced to which the moths are sensitive. Many moths make evasive movements in response to frequencies used by bats during echolocation of prey. Since bats are an important predator of night flying moths, rapid evasive movements are a means of protection.

How can I control miller moths?

Before miller moths are flying 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.

Once in the home, the best way to remove the moths is to swat or vacuum them or to attract them to traps. A easy trap to make is to suspend a light bulb over a bucket partially filled with soapy water. (Always use a grounded plug and extreme caution when using any electrical device near water!) Some wetting agent, such as soap or detergent must be added or many moths will escape, the water beading readily off the scales of their wings and body. Moths attracted to the light often will fall into the water and be killed. Jingling keys or some other noise that induces evasive flight behaviors can sometimes dramatically speed the capture rate when using the soapy water trap.

Insecticides have little or no place in controlling millers. The moths are not very susceptible to insecticides. Furthermore, any moths killed will be rapidly replaced by new moths migrating into the area nightly.