Tuesday, June 11, 2013

Know Your Pests: Forest Tent Caterpillar

The Forest Tent Caterpillar, Malacosoma disstria (sometimes "disstrium"), is not common in Virginia or Maryland. For us it's more of a curiosity than a pest. "Look, Mommy, is that a tent caterpillar?" The answer is yes and no; it's in the same genus, different species. Both moths and caterpillars look alike in some ways and can be confused by people who are not familiar with them, but are easy to tell apart.

The most obvious difference between Forest Tent Caterpillars and Eastern (or Western) Tent Caterpillars is that the Forest Tent Caterpillars do not actually build tents. They do sometimes spin mats or sheets on which they can rest. They are gregarious, and sometimes cuddle together, but seem more independent of their family groups than Eastern Tent Caterpillars.

Another difference is the coloring of the caterpillars' final coats. For all Malacosoma species, there is some variation among individuals. Eastern Tent Caterpillars have a solid, narrow white stripe down the center back. Forest Tent Caterpillars have a row of white dots. The amount of grayish-blue in the caterpillars' coats varies, but nobody describes Eastern Tent Caterpillars as "blue." Some people, notably Margaret Atwood, do describe Forest Tent Caterpillars as "blue," because blue is the predominant color of some specimens.

This set of pictures shows both the darkest type of Forest Tent Caterpillar, with quite a strong resemblance to an Eastern Tent Caterpillar, and the palest "blue" type:


Spots at the ends of the body can suggest eyes, although a caterpillar's working eyes are closer to its mouth and not usually seen by humans...


As this Wikipedia page shows, in addition to predominantly black and predominantly blue, some Forest Tent Caterpillars are predominantly brown...


(I question reports that the caterpillars can be four or five inches long. Although I've seen few of them firsthand, the normal size they reach before pupating is about two inches. Like Eastern Tent Caterpillars, some may be all of 6 cm and some only 4 cm...but four inches, 10 cm, sounds as if somebody measured a caterpillar while it was molting.)

Like Eastern Tent Caterpillars, Forest Tent Caterpillars are a native species that successfully resist most predators and are held in balance by a tendency to overpopulate an area until they become vulnerable to plagues. This boom-and-bust life cycle can be a nuisance to humans. Although the animals aren't common in most parts of North America, in a few places they have overpopulated to the point where they killed their host trees by repeated defoliation. The caterpillars can also become a nuisance when there are too many of them; in some places it's been claimed that their crushed bodies gummed up train wheels on railroads.

The easiest way to control this species is the same technique that works with Eastern Tent Caterpillars. Host trees are deciduous; egg masses are easy to find on the ends of twigs. Forest Tent Caterpillars prefer forest trees like oak, maple, and aspen to fruit trees like apple and cherry. Egg masses of both species look very much alike. Prune the infested twigs in winter, before the eggs hatch, and there will be no need to kill the caterpillars.

Like Eastern Tent Caterpillars, Forest Tent Caterpillars don't sting or bite, but a few people are allergic to their soft fur. As with the hair of most animal species, including humans, the chances of allergy reactions are greater when the hair stays in contact with more sensitive skin for longer periods of time in warmer weather. (Human hair can also cause contact dermatitis in humans; wearing a "hair shirt" or vest was a medieval torture technique favored by penitent monks.)

Forest Tent Caterpillars are usually seen in spring. When I've seen them it's always been two or three weeks after the Eastern Tent Caterpillars have gone into pupation. (Usually in late May. The timing of this series is getting away from me. I apologize.)

In the Midwest, Forest Tent Caterpillars have become such a serious pest that they've even been nicknamed "armyworms." In the East "armyworms" means a different nuisance species. Nevertheless:


(Note: Although this seems to be a legitimate U.S. government web site, something about it seems to clash with some, not all, of the computers I've been using...so beware. If your browser claims it's a security risk, you're not missing a great deal. Most of the basic facts about this species are at the other web sites linked above.)

In some times and places it may be worth trying the various harmless icky things these web sites mention people having done to control this species: whack them with sticks, drown them in soapy water (the fur is water-resistant, and in warm weather tent caterpillars enjoy paddling in cool water), squirt them with Deet or alcohol. However, even when they're threatening to defoliate trees that are barely recovering from late frost or previous defoliation by other insects, this web site does not recommend poisoning the land just to kill tent caterpillars.