Tents in the Mojave: Western tent caterpillar, Malacosoma californicum

Tents in the Mojave: Western tent caterpillar, Malacosoma californicum


Within silken tents adorning desert shrubs, beautiful western tent caterpillars huddle for warmth and protection from predators.


Last week we visited industrious harvester ants in the Mojave Desert. While patrolling these arid lands, we met a plant guru and long-time resident of the Mojave who shared a tale of colorful caterpillars embedded in silken webs on a beautiful native shrub called desert almond, Prunus fasciculata. Desert almond was flora incognita to a Bug Guy from the East Coast, but our desert naturalist shared that fruits of desert almond are highly attractive to many resident birds and rodents and edible to humankind as well. Native Americans of the region, including those of the Cahuilla Nation, consider them a delicacy.

Cousin of the western tent caterpillar, the pretty eastern tent caterpillar is active in March and April in the mid-Atlantic region.

With a vague notion of what desert almond plants looked like and a strong desire to taste their curious fruits and see caterpillars, we set out across the boulder-strewn moonscape in search of caterpillars and almonds. In less than a mile, we stumbled across a sweet smelling, tightly branched shrub festooned with silken webs. As the late afternoon sun warmed their tents, dozens of beautiful western tent caterpillars snuggled together amidst the unseemly remains of pelletized former meals. Excrement of insects is politely known as frass. These leaf-munching tent-makers bore a striking resemblance to their cousins, eastern tent caterpillars, which we visited in previous episodes.  Western tent caterpillars, as their name suggests, occupy western regions of North America ranging from Alberta, Canada to the southern deserts of California and Arizona. Winter is spent as eggs deposited by the hundreds in a dark Styrofoam-like mass encircling a small branch. With the warmth of spring, eggs hatch and tiny caterpillars dine on tender leaves and build their characteristic webs. In addition to desert almond, western tent caterpillars dine on other members of the rose family, including cherries and apples, and also on alders, oaks, poplars, and willow. From their silken bivouac, larvae move along branches to the newly expanding leaves to feed. As larvae grow during spring, they enlarge their silken tents. Tents help caterpillars conserve heat as the writhing mass of caterpillars elevates the temperature inside the tent. Higher temperatures accelerate growth and development of caterpillars on chilly spring days. Their silken homes may also provide protection from predatory or parasitic insects.

The starkly beautiful Mojave Desert is home to a rich diversity of plants and animals. Desert almond shrubs, kin to our domestic almonds, cherries, plums, and peaches dot the landscape. Western tent caterpillars festoon branches of desert almonds with silken webs. A peek inside a tent reveals a messy scrum of wiggling larvae amidst strands of silk and pellets of excrement called frass. Outside the tent mature caterpillars cruise branches in search of fresh leaves.

Cherry trees can be completely defoliated by eastern tent caterpillars in some years.

Near the end of larval development, caterpillars go solo and forage on their own. With growth complete, caterpillars pupate within the confines of their tents or wander from plants seeking protected spots beneath logs, leaves, stones, or human-made structures to spin yellowish or white, silken cocoons. While the desert almonds we visited in the Mojave seemed not to be overwhelmed by western tent caterpillars, other species of tent caterpillars, including the forest tent caterpillar and eastern tent caterpillar, can be problematic when cyclic, outbreaking populations strip trees of foliage. In a previous episode, we provided some easy ways to deal with tent caterpillars when they get out of hand on trees and shrubs in residential landscapes.

Although our quest for western tent caterpillars was a success, the chance to dine on desert almonds was, well, a failure. We arrived in the desert just a bit too early in the season and desert almonds still in bloom had not yet produced fruit. Of course this provides a perfect excuse for a return visit to this remarkable realm.      


“Western Tent Caterpillar” by the USDA Forest Service and “Population fluctuations of the western tent caterpillar in southwestern British Columbia” by J. H. Myers were consulted in preparation of this episode. We thank Dr. Shrewsbury for spotting and wrangling western tent caterpillars and Robin and Max at the High Desert Eden for sharing their tent caterpillars with Bug of the Week.

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