Oftentimes we see caterpillars that produce large unsightly webs or “tents” in our deciduous trees and we think “uh-oh that doesn’t look very promising”. And you would be right, these caterpillars are larvae of different species of moths. They can be Tent caterpillars or Fall Web worm, they can multiply rapidly and can defoliate a large number of deciduous trees and shrubs in a short time. These caterpillars can be quite unsightly and start to cause real cosmetic distress to your trees. They usually don’t kill the trees but they will start to defoliate them, trees only die after multiple seasonal defoliation. The defoliation weakens the trees and leaves it predisposed to pests and disease. The good news is that their populations fluctuate and after a few years of high numbers, their populations usually drop.
There are several ways that we can differentiate who’s who in the caterpillar world; the caterpillar pattern appearance itself, location of feeding, location of the nest, and when they start to appear.
Tent Caterpillar
The Eastern tent caterpillar (Malacosoma americanum) and Forest tent caterpillars (Malacosoma disstria) are a pest native to North America. The eastern tent caterpillar has a white line down its back with light blue and black spots on its sides. The forest tent caterpillar has white footprint-shaped marks down its back and light blue stripes on its sides. They prefer fruit trees including ornamental crab apples, cherry trees, plum, and pears. In the Spring, they produce large unsightly webs or tents in the crotches of tree branches and feed on leaves outside of the tent. These webs are used to protect the caterpillars from predators and weather. They lay dark brown-gray egg masses containing up to 400 eggs, that they attach around the small twigs of trees and shrubs. Most species of tent caterpillars overwinter in the egg stage and hatch about the time that leaf buds begin to unfold, usually in early spring.
Fall Web Worm
The Fall web worm is hairy, pale green or yellow, and has black or reddish spots along its back and there is usually more than one generation each year. It makes its tent at the tips of branches and gradually extends down the branch toward the trunk, feeding on foliage inside the tent. As the leaves are devoured, the nest will be expanded to accommodate fresh foliage. Fall web worms make their tents in July and August and feed prominently on fruit, nut and Sweetgum trees. The adult moths will emerge in late spring, laying hundreds of eggs on the underside of leaves. The caterpillars mature in about 6 weeks and the nest will break and they will find places to overwinter in bark and debris. Fall web worms are usually not considered harmful to trees, except for the aesthetic quality of the tents. They are harmful when they feed on the regrowth of trees that were defoliated earlier in the season by other caterpillars.
Another fearsome caterpillar that is often seen and deserves mention is the Gypsy Moth.
Gypsy Moth
Lymantria dispar is one of North America’s most destructive forest pests, devastating populations of forest and shade trees. The gypsy moth caterpillar has five pairs of blue spots followed by six pairs of red spots along its back. And is most commonly seen on Oaks and Aspens, although they will feed on hundreds of different species of plants. The species originates from Europe and Asia, in the late 1800’s the Gypsy moth was accidentally introduced near Boston, MA. It was introduced by a hobbyist entomologist, Leopold Trouvelot, he brought gypsy moth egg masses home from a trip to France and was cultivating them on trees in his backyard when the larvae escaped. And about 10 years after this introduction, the gypsy moth population was widespread. The State and Federal Government began their attempts to eradicate the gypsy moth, unfortunately these attempts ultimately failed. The range of the gypsy moth has continued to spread throughout New England, eastern New York, and parts of New Jersey. When densities reach very high levels, trees may become completely defoliated. Evergreens are eaten when populations are very high, Evergreens do not regrow leaves as easily as deciduous trees and can die as a result of complete defoliation.
Pest Control
1. LET IT HAPPEN — Just let the pest be eradicated by natural predators. Although the trees can become defoliated and not look as pleasant, it usually does not kill the trees, most times the tree will leaf out again and be back to its natural health. Natural predators include birds, predaceous and parasitic insects (especially wasps), and disease organisms. Caterpillars with white eggs or cocoons attached to their back should not be destroyed because they are being hosts for native parasites and will bring them back to the nest and hopefully destroy the rest of the population.
2. HANDPICK EGG MASSES — Locate and remove egg masses during the Winter season since they are easier to see. Scrape off and dispose of masses so the future of the tree is not at stake. And the pest do not set up camp for next year.
3. MANUALLY REMOVE NESTS AND CATERPILLARS — for TENT CATERPILLARS start looking for nests as soon as the Amelanchier and the Sugar Maples start leafing out in early Spring. The tents can be pruned out, it’s most effective to do this at night or on cloudy, rainy days because the young caterpillars are gathered in the tent for protection and aren’t venturing out to feed. Or use a rake or long pole to pull down the webs and destroy the caterpillars inside and drop into soapy water. Or tear a hole in the sack and allow natural predators such as wasps, yellow jackets, and birds access. Planting predator attracting plants in close proximity to susceptible trees will help the natural destruction of the pests.
4. APPLY INSECTICIDE — If it becomes necessary, as a last resort apply insecticide. If you do decide to spray, always remember that the caterpillars are a member of our ecosystem and they will always be present. There will also be other species who will be feeding on them or in close proximity and chemical insecticides are contact poisons and can have a potential impact on a variety of beneficial insects (such as honeybees), Nesting birds, and other animals. Spraying is not effective against pupae or egg masses, and is less effective once caterpillars reach 1 inch long. The insecticide should be applied as soon as the tents are noticed. The tents are water resistant so spraying with a water based spray won’t do the trick, thoroughly cover the tents with the spray in order to kill and contain the population. Contacting a professional pesticide sprayer is always a good option when you’re dealing with spray chemicals and large quantities.
