Tent Caterpillars : A Pest Or A Boon?

Different instar stages of the Western tent caterpillars
Western Meadowlark may eat tent caterpillars and the pupae.
White setae or bristles on the segments of the Western tent caterpillars
Ash tree branch encased in webbing with leaves eaten by tent caterpillars.
Thick web like strands form the tent structure that shelters the caterpillars.
Alder tree showing tent caterpillars.

August 31, 2021

In this time of drought and heat there are some strange things happening in the trees. White web “tents” are in many kinds of trees, and it looks like large spiders have been at work. Several friends have called and asked me what these spidery webs are all about. On the mitigation lands there are at least four tree species that have the web tents on them. If you look closely, you will see that each large web structure is filled with caterpillars, not spiders. They are Western tent caterpillars, Malacosoma californicum.

There are three main species and several subspecies of tent caterpillars on the west coast. Several subspecies do not build large web tents but may have small webs, or hardly any at all.

The adult moth of the tent caterpillar is brown, yellow, tan or gray with two lighter or darker lines crossing the body. Their wingspan can reach from 3.5 to 5 centimeters. The larvae or caterpillar can also come in different colors. They can be black, grey, or white with an orange stripe running longitudinally across the body. There are blue-white lines on each segment oftentimes with very long hairs or setae extruding from their bodies. They are quite gregarious and after hatching will find each other to form colonies of 350 or more caterpillars to spin their silken tents of protection. This congregating behavior is unusual for caterpillars. The tents are large enough to cover entire branches, especially on smaller trees, and are constructed so that the larvae get the warmth of the morning sun.

The caterpillars live in their tent houses for five to six weeks, growing from tiny larvae to caterpillars four to five centimeters long. The tents provide shelter, protection, and a molting site. Late-stage larvae become more solitary feeders and no longer use the tent. They then form hairy white cocoons that can attach to twigs or lie in the leaf litter, which I have been finding on the ground under the trees. It takes another two to three weeks before the adult yellowish-brown moth will emerge and begin looking for a mate and a tree to lay eggs for next year’s larvae.

Even though this is a particularly good year for the Western tent caterpillars, I do not see them causing damage to any of the trees they are living in. They can be found on black walnut, oaks, willow, alder, ash trees, cottonwoods, birches, aspen, madrone, fruit trees, ceanothus, coffeeberry, redbud, toyon, apple, almond, apricot, cherry, prune, plum, currant, and roses. The tree species in Little Lake Valley that I have seen the web caterpillars in are black walnut, willow, ash, and alder. The Pacific tent caterpillar, Malacosoma constictum, is only found on oak trees. I have yet to see these caterpillars on the oaks of Little Lake Valley.

These tent caterpillars have evolved with our trees and are considered natural pests when there is a population boom. Their populations are cyclic, with large numbers some years and almost nonexistent the next. This is because they have many natural predators including the tachnid fly, that parasitizes the caterpillars. This fly lays its eggs on the caterpillars, whose larvae hatch and then eat the live caterpillar. The caterpillars are also eaten by numerous bird species, predacious beetles, and parasitic wasps. Small mammals eat the nutritious pupae as do larger mammals like bears. The caterpillars are also devastated at times by outbreaks of bacterial or viral diseases. According to some studies there are several benefits to the trees that harbor tent caterpillars. If defoliation happens early enough, the tree is stimulated to produce a second set of leaves that may be more resistant to further defoliation. The leaves that are eaten and pass through the caterpillars’ bodies emerge as little pellets, or caterpillar poop that can break down easily, returning nutrients to the forest floor and in turn to the trees! There are so many amazing food cycles out there we are just beginning to understand. With climate change added to the picture it will be interesting to watch how this natural phenomenon is affected.