Amateur Naturalist: The Annual Arrival Of Moths

Tent Caterpillars outside their tent in aspen tree. Note the partially eaten leaves. Photo by Amy Reeves

Tent caterpillars are covered with hair-like spines, unlike the caterpillars of miller moths. Photo by William Ciesla

By Robert Dryja
Los Alamos

Spring is moving along and summer is arriving. Plants started to grow leaves and flowers a few weeks ago in response to a combination temperature, length of day, and the passing of individual days. Remarkably, plants can sense a day by day cycle of day time light and night time darkness.

Plants also have been growing their seeds according to either one of two strategies. One strategy involves quickly growing many small seeds in the early spring and then disbursing them to the wind.

A few seeds may land in a good location and then have several months to grow. Siberian elm, aspen and dandelions are three kinds of plants that use this strategy. A second strategy involves growing a relatively few large berries and having them disbursed later in the summer. Nanking cherry bushes use this strategy. The large berry seeds have nutrients stored in them and so can support the growth of the seed even if it is not in an ideal setting for its early growth.

Leaves are a source of nutrient for plants, and also are a source of food for insects and animals. The relationship of aspen trees and tent caterpillars are an example. Aspen trees follow the strategy of growing leaves quickly in the early spring along with their seeds. With leaves emerging in the early spring, young dormant tent caterpillars become active and have more time over the summer to grow to adulthood. They make silken communal nests among branches and so are easy to spot. The hair-like spines may discourage birds looking for a meal but do stop them. Some birds have been observed cutting caterpillars in half with their sharp beaks and then extracting the innards for food, avoiding the spines.

Tent caterpillars spend much of the summer growing and storing nutrients for when they enter the pupal (cocoon) stage. The caterpillars mature over 30 to 42 days. They are easy to spot since they make a communal silk tent among the branches of aspen trees. The caterpillars enter the pupal stage for 12 to 18 days.  Adult moths therefore emerge after up to 60 days, well into summer, similar to the maturing of the berries of a nanking cherry.  A new generation of eggs are laid by the adult moths.  Larvae hatch 3-4 weeks later but remain inside the egg until the following spring.  The adult moths seen in one summer started as eggs laid in the prior summer.

The miller moth in contrast appears in the early summer and can become a nuisance around homes and of offices. It looks for small, dark crevasses in which to hide during the day. Doors and windows often have this type of space around their frames and provide an opening to the inside. The moths enter unseen at night when it is dark. When morning arrives they then fly frantically around and hit windows while trying to escape the daylight.

“Army cutworm” is the popular name of the caterpillar stage of the miller moth. Rather than living up in a tree like tent caterpillars, army cutworms live at ground level, eating grasses or small grains. A fresh green lawn with low growing plants is a good habitat for army cutworms. The historic park area adjacent to Ashley Pond is an example of such a setting. This helps explain the arrival of large numbers of adult millar moths inside the nearby buildings.

Army cutworms grow to partial maturity in the last half of summer and then become inactive for the winter. They then resume feeding when spring arrives. They are fully grown caterpillars by mid spring and burrow into the ground to pupate. In three to six weeks the adult millar moth emerges in the early summer. They may migrate to higher elevations, feeding on the nectar of flowering plants as they migrate. Miller moths in the eastern croplands of Colorado may fly hundreds of miles west to the Rocky Mountains. They then fly back to the croplands as summer begins to end and lay a new generation of eggs. Young cutworm caterpillars are like tent caterpillars. They begin growing in the late summer of one year and finish growing to adulthood when spring arrives in the next year.

Surprisingly, the smallness of the Miller moth does not protect it from being a food source for bears. Millar moths aggregate in large groups while in the Rocky Mountains. They sleep during the day in the dark spaces among rocks laying at the surface. Bears then prowl among the rocks, turning them over to find the moths to eat. It has been estimated that a single bear may eat as many as 40,000 moths in a single day.

Adult tent caterpillars moths, the lighter colored female and darker male. Photo by Jerald E. Dewey

An Army Cutworm caterpillar is a Miller Moth in its larval stage. Photo by Frank Peairs

An adult miller moth with a pattern of dots and bands on its wings. Photo by Robert Dryja