Caterpillarmageddon 2025
The Western Tent Caterpillars are back. Again.
Story and photos by Minesh Bacrania
It's springtime in Los Alamos - the daffodils and irises are blooming, the trees are leafing out, the people are sneezing, and those weird silk nests of wriggling caterpillars are popping up in trees all over town. Boomtown can't help with the yardwork or allergies, but we did contact Los Alamos arborist Miguel Hoffman to learn more about the (perhaps misunderstood) creatures.
Social caterpillars
The creature in question is the Western Tent Caterpillar (Malacosoma californicum). At the beginning of spring, just as new leaves began to emerge on the trees, the caterpillars hatch from eggs laid the previous summer, then come together in groups to build silk nests in the crooks of trees for protection and warmth.
At first, they spend their days hiding in their protective nests during the day, and leave the nest at night, as a group, to feed on leaves. As the larvae grow larger, they work collectively to enlarge their silk nest, and eventually also begin to venture out alone to feed at all times of day.
After a month of feeding (around mid-June), the caterpillars leave their nest and spread out individually to find a solid surface on which to attach themselves — rocks, tree trunks, walls and the like — and hole up in their individual cocoons to transform into adult moths. The adult moths hatch a couple weeks later.
The moths' only goal in their short four-day life is to find a mate and lay eggs. They don’t even take time to eat. The larvae develop in these eggs for a few weeks and then enter a state of dormancy until the following spring when the cycle repeats itself.
Are the caterpillars or moths dangerous to humans and pets?
Dangerous, no. But many people might consider them to be a nuisance, particularly when they're all spreading out looking for a place to cocoon up. Hoffman says he’s even heard stories about the roads in the Jemez Mountains becoming slick from squished caterpillars.
The voracious Western Tent Caterpillar will do a quick hit on the first new leaf growth of the season but rarely damage the leaf stalks (petioles) or the buds (stipules) from which new leaves can regenerate. Aspen trees even have chlorophyll in their bark that allows them to photosynthesize and regenerate.
This defoliation is a relatively minor stressor for healthy, established trees, but can be detrimental to younger trees, or trees that are already stressed due to drought and disease. Keeping trees well-watered and healthy will go a long way toward helping them withstand an attack.
What if they've overstayed their welcome in your yard, house, or trees?
Hoffman recommends disrupting caterpillar groups and nests with a stiff brush or a stream of water from a garden hose. Branches with individual nests can be pruned, but this can do more damage than the caterpillars themselves since it eliminates the ability of that branch to regrow its leaves.
Biological insecticides — specific strains of Bt and Spinosad — can also be effective against the caterpillars early in the season, when they first hatch and begin building nests. It’s important to choose caterpillar-specific strains of these treatments so as not to interfere with populations of bees, moths, and other pollinators. Chemical sprays can also be effective but are far more likely to indiscriminately harm other beneficial insects.
Are they prey for anything?
The eggs and larvae are prey for birds, parasitic wasps, flies, and beetles.
What's the black stuff in the tents?
The technical term is “frass,” which is more commonly known as “poop.”
Other local caterpillars
In Los Alamos, there are two other common caterpillar species.
Fall webworms behave similarly to tent caterpillars. They are smaller and, as the name implies, emerge later in the season. In the fall, trees are usually done producing and storing energy for the winter, so the defoliation is typically not as hard on trees as it is in the spring. However, as with Western Tent Caterpillars, repeated outbreaks can eventually take a toll on tree health.
Douglas fir tussock caterpillar outbreaks are common around Northern New Mexico in the summer. Along with weakening their host trees and making the susceptible to other stressors such as drought and bark beetles, their hairs can provoke an allergic reaction (tussockosis) in people and animals. This danger even resulted in the closure of picnic areas around Hyde State Park in Santa Fe. Hoffman feels that “these are the worst” of the lot.
Some other resources for more information:
USDA’s Forest Insect and Disease Leaflet on Western Tent Caterpillars
New Mexico Energy, Minerals, and Natural Resources Department’s 2023 state forest health report.
Our neighborhood is covered with these web tents and caterpillars. Thank you for this article. Very helpful to understand more about them.
Very interesting!