by Andrea Williams
This time of year, there’s a lot of perfectly legal tent camping going on around Mt. Tam. Our native tent caterpillars are recreating in the oaks, and it’s fun to see!
We have three tent caterpillar species in California, probably all of which can be found on the watershed (I just haven’t seen the forest tent caterpillar yet). The most obvious is the western tent caterpillar (Malacosoma californicum), which spins the most complete tents, where the caterpillars hang out, eat, and molt. Their covering of fine orange bristles over a black base gives them a cinnamon hue, but like the other two species they do have some blue and white speckling as well.
While western tent caterpillars are attractive, I must confess to finding the Pacific tent caterpillars (Malacosoma constictum) much cooler—maybe it’s the blue racing stripes down the sides, offset with white tufts, and just a hint of orange in the central black stripe. These caterpillars spin a more rudimentary tent which they just use to change (molt their skin, that is); they feed in communal masses outside the tents.
The forest tent caterpillar (Malacosoma disstria) doesn’t even make a tent, but spins silken mats on branches or trunks where a bunch of them will group together to rest or molt—the equivalent of just tossing a tarp on the ground to sleep. These dark blue caterpillars have a striking white keyhole pattern down their backs flanked by wavy red-brown lines.
Another noticeable caterpillar on our oaks is the California oakworm, which later in life becomes the California oakmoth (Phrygandia californica). These caterpillars are also mostly black, with creamy stripes and some blue-green and reddish mottling; but unlike the tent caterpillars, the oakworms are hairless with a large, round, orange-brown head. In a good year for them, they can completely defoliate a coast live oak (Quercus agrifolia).
Like most moths, the caterpillars are much showier than the adults. Both young and adults are a couple of inches long, but where the larvae are all blacks, blues, rust and white with patterns and bristles, the adult color ranges from cream to buff to cinnamon-sugar with a couple of paler or darker diagonal lines on the wings. The moths are a nutritious treat for insect-eating birds such as black phoebes (Sayomis nigricans). While the damage from these native caterpillars certainly looks bad, the oaks can easily shake it off and hungry caterpillars have never been known to kill a tree. They just camp out for a few weeks in the spring!Much of this information is distilled from a fabulous and useful publication from the U.S. Forest Service’s Pacific Southwest Research Station, “A Field Guide to Insects and Diseases of California Oaks” (General Technical Report PSW-GTR-197 July 2006). It is available for free online, although if you can find a printed copy it’s much nicer!