Spring, summer, or fall – you’ve probably seen your share of one or more of the three species of tent caterpillar that are common in this area. The cyclical populations of fall webworm, eastern tent caterpillar, and forest tent caterpillar have of all peaked dramatically over the last few years, bringing them squarely into the public eye.
Contrary to popular opinion, none of these tent dwellers is the gypsy moth, an imported pest that doesn’t build a tent and whose numbers are currently relatively low. Instead, all three of our local tent caterpillars are natives, and though they have distinct markings, the easiest way to tell two of the hairy beasts apart is by the time of year when you see their tents. Despite its name, the third type of tent caterpillar doesn’t build a tent at all.
The fall webworm is in evidence now, and it was the first to exhibit a population boom, beginning way back in 1985. From 1998 to 2002, fall webworm webs were glaringly obvious at this time of year, though the relatively low number this year suggests that the outbreak may be winding down.
Fall webworms build large, unsightly tents high in the branches, usually in roadside trees or trees at the edge of fields. The silken tent starts out being tiny, covering just a leaf or two when the caterpillars are newly hatched. As they feed and grow, so does the tent, and after molting five or so times and reaching a length of one inch, the caterpillars are in housing that is sometimes three feet high and is filled with dead leaf skeletons, droppings, expired family members, and shed skins.
Eastern tent caterpillars are the scourge of spring, and their webs are completely unrelated to those of the fall webworm. This past spring, eastern tent caterpillars seemed set to take over the world and managed to totally strip many of their favorite host trees, which coincidentally are favorites of people, too. They prefer members of the rose family, especially wild cherry and apple trees, often found along roads and around houses, where the caterpillars’ messy living quarters and early spring defoliation are all too obvious. They rest, digest, and molt in their tent, but, unlike the fall webworm, they venture outside it to feed. Eastern tent caterpillar populations appear to be at or near their cyclical peak.
This summer, the third of our hairy beasts – the forest tent caterpillar – was also on the rampage, and if this outbreak follows the pattern of past ones, next year there will be even more of them. In June, tree crowns in some areas were tattered and thin, and the continual rain of caterpillar droppings took the pleasure out of strolling through the woods. This species too has silky capabilities, but the caterpillars just construct a mat, not a tent.
There must be a reason why an insect would call attention to itself by building a highly visible structure, and indeed, the coziness of a caterpillar tent contributes to the insect’s success. In either May or September, when it’s chilly, these animals fare the best and gain the most weight when it’s 77-86 degrees Fahrenheit, considerably above the average outdoor readings in spring and fall. Just as in a greenhouse, the temperature inside a tent caterpillar tent is typically 35-45 degrees Fahrenheit warmer than it is outside. Plus, the metabolic heat produced by a family of 200-300 entwined larvae adds to the warmth.
A defoliated tree or forest is truly a gruesome sight, and it’s hard to believe that the trees will eventually recover. But they usually do. In the case of the fall webworms, currently visible from our highways and front porches, the leaves they are destroying have already provided the tree with food. Next year’s buds will open pretty much as usual. Eastern tent caterpillar and forest tent caterpillar attacks will only kill if a tree if it gets nailed in several consecutive years. Most trees will produce a new set of leaves a few weeks after the annual caterpillar outbreak is over.
At least 16 other species of Lepidoptera (moths and butterflies) have a similar strongly cyclic pattern, in which the outbreaks occur at somewhat predictable intervals. Predatory wasps and diseases help end outbreaks, but these cycles are very difficult to disrupt. Over three to four years, even when hit by insecticides or frost, populations will be high over a broad region, sometimes over most of the U.S. and Canada, and then will collapse completely for many years, only to dramatically rise again, almost as though they were on a schedule. In recent years, all three native tent caterpillars have been easy to find, making it a good time to get acquainted with our most common hairy caterpillars.