A mature brown colored miller moth. Photo by Robert Dryja

A black bear foraging for moths in a felsenmeer on Redondo Mountain, New Mexico. Photo by Jonathan Coop

By ROBERT DRYJA
Amateur Naturalist

A small moth may be seen in May. It is dark brown in color and called a miller moth. These moths may aggregate in buildings if they can crawl through a door or window. Their goal is to find shelter in a dark area during the day away from sunlight. They will return to flying out-of-doors when night comes again, if they can find their way out of a building. Except for congregating in large numbers, this moth does not seem that interesting. Butterflies are far more colorful and easily seen during the day, flying from flower to flower.

However, the life cycle of a miller moth is remarkable. Eggs are laid at the end of summer in the grasslands of eastern New Mexico and Colorado. Pupae (small caterpillars) emerge and live on plant leaves growing close to the soil surface. The pupae stay below the surface during the day and come up to feed during the night. The pupae are half grown when winter arrives. They remain inactive below the surface until spring. Fully grown pupae then become flying adult moths in early summer.

Adult moths migrate westward, flying toward the Rocky Mountains. This is a distance of approximately 200 miles and may take four to six weeks of flying. The 200 miles represents a straight line and the moths flies at night. The elevation changes from about 3,800 to 10,400 feet. This is a change of more than a mile. Remarkably the miller moth has a sense of direction. It can adjust its flying when the wind changes direction and would push it off course.

This travel is done by a creature that is about one inch long with a wingspan of one and half inches. Its brain is the size of large dot. The goal of the moth is to follow blooming plants that emerge at later dates with the increasing elevation. The miller moth spends the remaining summer months feeding while on a high mountain. It flies back to lower eastern New Mexico or Colorado as autumn approaches. A miller moth therefore may fly a total of 400 migratory miles during its life. A new generation of eggs are laid to start the next life cycle. A female moth may lay 1,000 to 3,000 eggs.

Flowers decline in number as summer approaches its end. A miller moth therefore stores fat during the summer to prepare for it fall flight. Up to 72 percent of its body may be composed of fat. It becomes a good source of food for many kinds of animals. These range from birds to bears. What! Bears eat moths?

Bears take advantage of the moth’s behavior. This is aggregating into groups during the day to rest, waiting for night fall. This is done in locations that are dark and shady. High mountain sides may have a particular type of formation called a felsenmeer. The formation is a slope composed of large rocks lying together with few, if any, plants growing among them. The space between and below the rocks provides the shade and darkness that the moths want. Hundreds of miller moths may be together. A bear simply has to push rocks apart to reach them.

What a life cycle. Be one of thousands of eggs laid in the fall. Fly hundreds of miles in the early and late summer. Fly upwards for more than mile as part of migrating. Sleep in a rock field that may be two miles above sea level. Develop a massive amount of body fat during the summer. And then end up as a meal for a hungry bear.