It’s Only Natural: The Good, the Bad and the Ugly
By Linda Childs
Take a walk or drive through the Norfolk countryside this season and you’ll find the trees bedecked in glorious autumn color, and not so glorious web-like bags of dead leaves and live caterpillars. While perfectly timed for Halloween, one wonders what havoc these nests wreak upon the host trees and how quickly they will spread.
Last year, the small cherry trees along Station Place were essentially defoliated by a spring infestation. In order to avoid similar destruction to my crabapple tree, I dusted a small nest in May with a biological control known as Bt (Bacillus thuringiensis var kurstaki). Bt is a nontoxic, inexpensive biological control available locally at Ward’s Nursery in Great Barrington. The Bt dusting stopped the critters dead in their tracks. I then took it upon myself to treat every tent I could reach in the trees that line Station Place, with the same satisfying results.
Now, as the seasons change again, it seems these ugly nests are reappearing everywhere. I contacted Norfolk’s Tree Warden, Star Childs, for his assessment of the situation and learned there are three types of caterpillars successively chewing up the tree canopy, not just the one I suspected.
The defoliation I witnessed the past two springs is the work of the Eastern Tent Caterpillar (malacosoma americana). Their larva strips critical foliage from wild cherry and flowering fruit trees for three to four weeks after bud break. What we are presently witnessing in the canopy is the work of the Fall Webworm (hyphantria cunea), which builds its nests on the distal ends of branches and the Forest Tent Caterpillar (malacosoma disstria), which spins silken mats on tree trunks and large branches. Both can be seen in many different varieties of our fruit, shade and woodland trees.
Since this is the season when trees naturally give up their leaves, the damage done by autumn caterpillars is minimal. According to Childs, the tents are full of NPK-rich caterpillar “frass” which, when washed down by the rains of autumn, fertilize the host tree beneath its canopy. Childs advises to let nature take its course.
Both Eastern and Forest Tent Caterpillar moths lay their eggs over the winter in shiny, grey-black, Styrofoam-like masses, which encircle smaller branches. A single inch-long mass holds over 400 eggs. When the branches are bare, these egg bands are easy to spot and prune out.
In May of next year, as the Japanese quince, flowering cherry and bridalwreath begin to bloom, keep an eye on the south-facing aspect of your maples, crabapples, chokecherry and oak trees. As the tiny black caterpillars hatch, they migrate out in late morning to eat new leaves and return to the tent at night. The best time to eradicate them, by dusting the tent with Bt, is in the early morning. Within a few days, the tent turns brown and it will eventually drop off with the spring rains.