While forsythia’s bright yellow blossoms are the harbinger of spring, they also mark the emergence of an impressive defoliator, eastern tent caterpillar. Since last summer, this herbivore has survived as eggs deposited in masses on small branches of cherry, apple, and crabapple trees. This noisome creature made its appearance in mid-March, about the time my mason bees emerged from their nurseries. Tiny caterpillars hatched from Styrofoam-like egg masses which each contained as many as 300 eggs.
In chilly weather caterpillars hunker down near their egg masses awaiting the appearance of nutritious young leaves. Larvae build small silken tents over the egg mass and the surrounding branch. From this bivouac, they move along silken trails to the newly expanded leaves on which they feed. Trail marking chemicals called pheromones are deposited by the caterpillars as they return to their tent after dining. These trail markers assist nest mates in locating delectable clusters of leaves. As larvae grow during April, their tents expand. Eventually tents are constructed in the crotches of large branches or where large limbs branch from the trunk.
On the silk trail, sated caterpillars returning to the tent pass hungry ones heading off to dine on tender leaves.
Eastern tent caterpillars are a rather chummy lot. Brothers and sisters from the same egg mass and nearby egg masses often participate in group activities such as communal foraging and enlargement of their remarkable tent. Tents help caterpillars conserve heat and elevate their body temperature for more rapid growth and development during chilly spring days. Their silken homes may also provide protection from predatory or parasitic insects. These hairy caterpillars have voracious appetites and they can strip even large trees of all their leaves.
After the caterpillars have completed development, an exodus occurs from the tree and larvae wander the land seeking protected spots beneath logs, leaves, stones, and under man-made structures to spin yellowish or white silken cocoons. Adult eastern tent caterpillars emerge as moths from their cocoons in June or July, mate and lay egg masses on small branches of rosaceous trees such as cherry, apple, and crab apple. These eggs house the next generation of caterpillars that will emerge with the bloom of forsythia next spring.
How do you know if eastern tent caterpillars threaten your trees? The best predictor of a problem this year may be if you had a problem last year. The images in this Bug of the Week came from a small stand of wild cherry trees that are perennially infested with eastern tent caterpillars. If you had a cherry, apple, or purple plum with tent caterpillars last spring, now is an excellent time to carefully inspect pencil-sized branches for egg masses and tiny silken webs. Egg masses look like hardened foam collars covered with a shiny varnish-like material, which completely encircle twigs and small branches. The egg masses with attendant caterpillars are easily removed with a pinch of the fingers, or if you are a bit squeamish about touching bugs, simply get out your nippers and prune them out, and then seal them in a trash bag (don’t toss them in the woods as the caterpillars may simply crawl into the nearest host tree and continue to develop). As the tents enlarge and get moved to the crotches of the tree, tents and their inhabitants can be removed with a gloved hand on a cool or overcast day (when the caterpillars tend to stay in their nests rather than going out to feed), placed in a bag, and destroyed. The old-school remedy of “burning them out”, though dramatic, went out with the storming of Frankenstein’s castle. Flames are very damaging to the bark of a tree and should not ever be used.
Pruning eggs masses and small tents from prized trees and relocating them to feral cherry trees may help protect your trees and preserve these morsels for hungry birds.
However, if you have a fondness for letting Mother Nature take her course and can tolerate the presence of the caterpillars, another option may appeal to you. One observant colleague watched as small wrens plucked juicy caterpillars from their tent. Caterpillars are an important source of protein in spring for birds as they develop eggs within their bodies, and later when eggs hatch and their hungry broods need fresh meat. She also observed these cheery birds gathering silk from the tent, perhaps for use in nest construction. If you want to protect your prized trees from defoliation by tent caterpillars and assist your resident birds, simply prune out the infested branches, tents and all, and relocate them to a nearby feral cherry. They will likely establish a new nest and hopefully become bird food.
Trees small and large may be festooned with tents and totally stripped of leaves. While trees may recover and produce a second flush of leaves, repeated defoliation surely takes a toll on these trees. If you have a tall tree from which you cannot safely remove eggs or tents, you may want to seek the help of a professional certified arborist. For “do-it-yourselfers”, insecticides labeled for use against caterpillars and containing the active ingredients Bacillus thuringiensis (Bt) or spinosad will provide excellent control of these tiny leaf eaters. As with all insecticides, be sure to read and follow directions and cautions on the label and take special care if plants are in bloom and beneficial pollinators are present. Entomologists believe that eastern tent caterpillar populations run in cycles. After a few years of caterpillar abundance, natural enemies such as predators, parasites, and pathogens often reduce tent caterpillars to innocuous levels. So, go out to the garden and have a peek at what these interesting herbivores are up to this week.
The wonderful book “The Tent Caterpillars” by Terrence Fitzgerald and “Managing Insects and Mites on Woody Landscape Plants” by John A. Davidson and Michael J. Raupp were used as a references for this episode.
For more information on eastern tent caterpillars, visit the following web sites: