Army cutworm moth. Photo by J. Capinera, insectimages.org
(Click on the following link to listen to an audio version of this blog … Its miller time
“Miller moth” is a generalized term given to moths that enter human habitations often in great numbers. There are many miller-moth species found all around the world. The feature of these house-invading moths that has generated the sobriquet “miller” is the tendency of these species to shed scales inside of the houses they have entered. The scales are white and powdery and resemble the flour that a miller might generate from grinding grain.
In western North America the most common miller-moth is the army cutworm moth (Euxoa auxilaris). In the Fall, adult army cutworm moths lay eggs a centimeter or so deep in the soil of agricultural fields (especially alfalfa and winter wheat) and rangelands throughout several High Plains states (eastern Colorado, western Nebraska, Kansas, Oklahoma and Wyoming) and also in New Mexico and Arizona. These eggs quickly hatch into tiny, sand-grain-sized caterpillars which begin to feed (primarily during the day at first) on the foliage of any still-standing vegetation.
Army cutworm moth caterpillar. Photo by F. Peairs, forestryimages.org
This foliage consumption continues through the winter and accelerates as days begin to warm in the early spring. Feeding shifts over to a nocturnal cycle as temperatures warm. The caterpillars reach their full size in mid-spring and then burrow back into the ground to pupate. These caterpillars are capable of doing considerable damage to both alfalfa and winter wheat. Very cold winter temperatures, though, especially in years without significant, ground insulating snow cover, can kill large numbers of these caterpillars. Adult moths emerge in May or June depending on the relative warmth of the early spring season.
Adult, army leafcutter moths are 1.5 to 1.7 inches long and are typically a dark, grey-brown in color. They also have a distinctive “kidney-bean” shaped pattern on the edges of their forewings. These emerging adults are capable of flying 100 miles or more from their pupation sites toward the higher, western elevations of the Rocky Mountains. In these mountain ecosystems there will be abundant summer wildflowers that will provide the moths with sufficient nectar for their growth and survival and also with enough energy to ensure their return, late summer or early fall migration back to the eastern fields and rangelands where they will lay the next generation of eggs.
Here is a brief video of swarming miller moths (bravely filmed by D. Sillman)
In wet years army cutworm moths are less of a nuisance to humans because there are abundant nectar-rich, wild, flowering plants available to the moths. The hungry, migrating moths, then, are less likely to seek human-irrigated landscapes (like yards and gardens) for their nectar meals. With a reduced draw into areas occupied by people, the moths, then, are less likely to accidently enter houses and other buildings.
Miller moth. Photo by D. Sillman
The timing of the moth migration flights is greatly affected by seasonal temperatures. When it is warmer than average the army cutworm moths begin their migrations in early May. When it is cooler than average, though, the moths may not begin their migrations until well into June. The moths follow the sequential, temporal blooming pattern of nectar producing plants all the way up into the mountains. The migration typically takes six weeks from start to finish.
The migrating moths fly westward by night and then hide in narrow, dark refuges during the day. The night migrating moths are easy, protein-rich prey for hungry bats. During the day, birds pick off resting moths from outer walls of buildings or startle them up from their refuges. We have watched our backyard flocks of house finches and house sparrows feast on the army cutworm moths. Both the fuinches and sparrows have also been actively tending to their first nestlings and fledges of the season. The arrival of the moths was perfectly timed to give them a rich food supply with whch they could nurture their offspring.
Bears up in the high country eat these moths in huge numbers. Grizzly bears in Yellowstone eat up to 40,000 moths a day in the summer! The bears forage for moths by turning over rocks and licking up the hiding moths before they can fly away.
Army cutworm moth. Photo by D. Sillman
The day-time refuges for the moths are very diverse and seem to be quite opportunistically selected. Crevices around doors and windows, protected spaces behind or under flower pots and trash cans, tiny spaces under the uplifted bark of a tree, gaps between bricks and stones and more can serve as hiding spots for one or a hundred of the resting moths. Cars and trucks also have abundant spaces within which resting miller moths can spend the daylight hours. During a large miller moth outbreak, cars idling at a red light in an intersection often have clouds of miller moths flying out of the now stationary vehicle-spaces. Over the past week, Deborah and I have seen large numbers of swallows flying around the intersections here in Greeley. We speculate that they are there to harvest the bounty of the vehicle transported miller moths.
Unfortunately, when these moths select a crevice around a window or door there is an increased chance that they may accidently enter the house or other type of building when that window or door is opened. The moths do no actual damage in a house. They don’t lay eggs that might hatch into problem caterpillars, they don’t consume woolens, carpets, drapes or stored foods. They may leave their distinctive, white, scale dust behind on surfaces, and they may also secrete a reddish-brown, waste fluid called “meconia” on walls, windows, counter-tops or furniture. This meconia, though, is water-soluble and is relatively easy to clean up. Dead miller moths (they cannot survive inside a house for very long), though, may serve as food sources for destructive house pests like carpet beetles.
These army leafcutter “miller” moths are very resistant to insecticides, strongly attracted to lights (especially white lights), and flee from sounds (even sounds as simple as keys jingling or hands clapping). One of the best ways to avoid getting a houseful of miller moths is to turn off outside (and sometimes even inside) lights. Use of yellow lights is said to decrease the attractive influence, but our observations do not support this.
My daughter and son-in-law noticed that the miller moths that invaded their house here in Greeley did not go near their LED-lit, indoor, hydroponic garden. We speculated that the blue shifted LED lights might not have been a powerful enough attractant for the moths, but it could also have been that the repulsive effect of the of the sound of the vibrating water pump in the garden overcame any noticeable light attraction.
Army cutworm moth. Photo by W. Cranshaw, Wikipedia Commons.
We have had hundreds of miller moths inside our house over the past week. One evening, I lured the moths out of the darkened house and out into the brightly lit sunroom. I sealed them off in the sun room until the next morning when my grandson, Ari, and I used a broom and a long Pampa grass stalk to push the moths out the open door into the back yard. It was a very successful “moth roundup,” although the next night there were as many moths inside the house as before. Deborah and I have been sitting in the dark these past few nights trying to lure any miller moths in the house to our nightlight illuminated bathroom. We then shut the moths into the bathroom and shoo them out an open window in the morning.
The moths are annoying, but we are trying not to kill too many of them. Helping them on their way and on into the beaks and jaws of their many waiting predators is a much more satisfying activity! They will come back through here on their Fall migration when they fly back to their egg-laying fields, but, thanks to all of the moth predators on their routes and in their summer ranges, there will hardly be enough of them left to be noticed!
This Spring, moth-tsunami is a wonder of nature! I just wish there weren’t quite so many inside my house!