Amazing Desert Moth

by Ron Harton

I had not climbed to the top of 11,941 foot Mt. Jefferson in the Alta Toquima Wilderness of Central Nevada to look for moths, but that is where they found me. They found me while I sought relief from the hatches of flies and mosquitoes swarming on the flat top of the south summit of Mt. Jefferson. I had climbed to the mountain without a tent, expecting the mid July weather to be dry and mild. I did not think I would hit the peak hatch for mosquitoes and flies, and the most amazing flight of moths I have ever seen.

I had found a perch a few feet down the side of the mountain to watch the last colors of dusk and perhaps catch a breeze that would drive away the insects. Suddenly a literal river of moths began to race over me, moving with the breeze, but much faster. Thousands and thousands of moths swept over, around, and into me. They seemed to be emerging from the rocks all around me and migrating en masse to some unknown location. One landed in my coffee mug, and I pulled it out. It was a plain silvery gray moth with a one-inch body and wing span of about 2 inches. After being pelted with moths for ten minutes, I retreated to my sleeping bag on top of the mountain. The moths were there, too, and I fell asleep listening to moths striking my bag like hailstones. When I awoke a couple of hours later, the moths were gone. The next morning, I expected to find dead moths littering the ground. I did not find any. They were completely gone. But they came back the next night with a repeat performance.

When I arrived home, I tried to find out some information about the moth mass emergence that I had observed. I discovered that there is still much to be discovered about moths, particularly moths in the desert. Moths that damage agricultural products are well documented, but moths in remote areas are still in many ways unknown. There are very few moth identification books in print, but there are over 10,500 identified species of moths in North America. This seems to be yet another area of research where Desert Survivors can add valuable data. I could not positively identify the moth I saw. It seems to be some type of Ethmia, perhaps Ethmia monticola that does live in that region of Nevada.

Perhaps the most famous desert moth is the yucca moth. It is a small white moth, easily overlooked, but absolutely essential to the ecology of the desert. The yucca and the yucca moth are necessary to each other. The yucca plant needs the moth to pollinate its flowers to reproduce by seed, and the yucca moth caterpillars only eat yucca seeds. The yucca moth pollinates the flowers in the evening from dusk to midnight. The moth gathers pollen on specially adapted parts near the mouth called palps. The moth forms the pollen into a ball, which is carried to other flowers and inserted into the stigma of the flowers. When we see yucca pods on a yucca plant, we know that yucca moths have been busy there because the yucca is fertilized in no other way.

The moths mate inside the yucca flowers, then the female lays her eggs in the base of the flower. The hatching of the moth eggs and the development of the yucca pod are synchronized, so that the hatching caterpillars can begin to eat the seeds. The females know somehow not to lay too many eggs in one flower, so the seeds will not all be eaten. The pod continues to grow and develop many viable seeds around the munching caterpillars. When the caterpillars are mature, they chew through the surface of the pod and spin a silken thread to drop to the ground. They crawl away from the plant and burrow into the ground to create a cocoon. They emerge as moths the next year. However, if conditions are not favorable, they may remain longer in their underground burrow.

In fact, a species of yucca moth, Prodoxus y-inversus has the longest reported diapause, or waiting period due to environmental factors. Pods of Yucca baccata containing the prepupae stage of the moths were collected in Nevada and stored. Adult moths emerged 19 years later. In a natural setting, Prodoxus y-inversus have been recorded to emerge en masse after 16 and 17 years in diapause.

Although the life cycles of the yucca and yucca moth have been extensively studied, there are still many unanswered questions. For example, gardeners far away from normal yucca habitat often plant yucca. However, yucca moths frequently appear to pollinate them. Where do the moths come from, and how do they find the plants?

Another interesting desert moth is the creosote bagworm, Thyridopteryx ephemeraeformis. Not many things eat the creosote bush, but the creosote bagworm find its leaves appealing. The bagworm has a wide ranging appetite, feeding on over 128 species of plants including cedar, juniper, pine, oak, locust, and willow species in addition to the creosote bush. Bagworms may be found on other species in the desert, too.

The creosote bagworm is easy to find. It weaves a silk cocoon an inch to two inches long into which it incorporates creosote leaves for camouflage. The bag looks like a slender pendant of leaves hanging from the branches of the bush. The larvae of the moth emerge from the bag in spring and begin to create their own bags. The bag is designed like a sleeping bag. The larva’s head and legs are free, so it can crawl around the bush and eat the leaves. If the larvae eat all the leaves on a bush, they will crawl on the ground to a new creosote bush.

In late summer, the mature larva attaches the bag to a branch, seals the bag, and pupates into an adult. About a month later, the inch long adult male moths emerge from their bags. The adult females, however, never become moths with wings. The adult female bagworm is a worm-like creature. She remains inside the bag all her life. The male moth flies around to find a female in her bag. After mating, the female lays between 500 and 1,000 eggs in her own bag and then she dies. The eggs spend the winter in her bag to hatch and emerge in the spring. If you find a bagworm bag on a creosote bush in the winter, it may contain a dead female and hundreds of eggs.

The life cycles of moths are fascinating and in many ways still mysterious. There is so much to be discovered. I wish I had taken better notes of the mass emergence I witnessed on Mt. Jefferson. I’m better prepared for the next time, if there is a next time. Whenever I see a moth now, I wonder about what mysteries its life cycle holds.


“Bagworms.” Missouri Botanical Garden., 1998.

Jaeger, Edmund C. The California Deserts. Stanford: Stanford University Press, 1965.

“Longest Diapause.” University of Florida Book of Insect Records. Gainsville: University of Florida Department of Entomology and Nematology, 1998.

Moths of North America. Northern Prairie Wildlife Research Center, 1998.

Pellmyr, Olle. “Yucca Moths: Prodoxidae.” Vanderbilt University, 1998.

Ramsay, Marylee and Schrock, John. “The Yucca Plant and the Yucca Moth.” The Kansas School Naturalist. Emporia State University. May 1992.