Malocasoma, the Caterpillar Tent Maker

Malacosoma disstria, Forest Tent Caterpillar

Dr. Bob Gilbert – Columnist

Dr. Bob Gilbert

Spring is here and a few tent caterpillars have been spotted. These tents are almost always located in the inconspicuous native Wild Cherry trees. Oaks and Black-gums are also occasional hosts. At first glance it appears the tents have appeared even before the trees have leafed out. Closer inspection reveals the early leaves have already been eaten. In a heavy infestation year the entire tree looks like it has been defoliated. 

tent caterpillars and frass

There are two species of Tent Caterpillars – the Eastern Tent (Malocasoma americana) and the Forest Tent (Malocasoma disstria).  Malocasoma is a family of moths.  Maloca is a South American term for a communal structure.  Soma means body. The moth’s larvae form is a caterpillar that exists as a communal inhabitant in a woven tent.  Fortunately, the Forest Tent species is uncommon here; it is found mostly in hardwood cypress forests. It has white spots like footprints on the top of the body. It does not form tents but sheets or pads of silk on the very tips of branches. They do not colonize living a somewhat nomadic existence. 

The Eastern Tent species has a white line down its back. The tents are formed in forks of branches. These caterpillars are communal. Both species have long hairs covering their bodies. Blue jays and the Yellow-billed and Black-billed Cuckoos appear to be the only birds that can digest these insects. When there is a bumper crop of tent caterpillars the Cuckoo population increases. The population of these caterpillars fluctuates in cycles.  Last year there are fewer tents.  Silk gathered from the tents line many bird nests.

Above, closeup of egg mass with silk trails. At right, a one-inch long tent caterpillar.

What is interesting about the tents is that they are constructed to capture heat from the early morning sun. Studies have shown that the caterpillar has to have a body temperature of at least 55 degrees in order to digest leaves. The caterpillars move to different rooms in the tent as the sun changes positions.  They start to forage early in the morning leaving a hormone (pheromone) trail and/or a silk trail that enables them to get back to home base where they rest and digest. Buried in the layers of the silk tent are droppings called frass. They are only able to partially digest an ingested leaf so there is always lots of frass. 

Tent caterpillars are one of the earliest insects to hatch in the spring, likely because young leaves are easier to digest? These caterpillars have a short life span. They hatch in March and after 5 to 6 larval molts called instars, they spin a cocoon and pupate. Two months later the adult moths hatch, breed and then the female lays eggs in a mass and dies. The adult female moth may only live a day or two. Like other insects with this type of life cycle the adults do not feed. The egg mass is laid around a small branch and is covered with a shiny material called spumaline. It is able to absorb some moisture necessary for the larvae but stays hard enough to prevent other insects like wasps from laying their eggs directly on larvae. In a heavy year the Forest Tent species produces so much frass that people have to stay inside. It rains frass.

Moths of the tent caterpillar are short lived.  Most of their life cycle is spent in a larva or caterpillar form. This is very similar to our cicada.  It spends most of its life underground only briefly emerging above ground to reproduce and then dies.  

 Almost all healthy deciduous trees can tolerate being defoliated two to even three times a year. The second and third growth produces smaller leaves. It does use up a lot of energy for a tree to defoliate but this does not kill the trees. But if this happens year after year the tree will become weaker.

A third insect, the gypsy moth, also produces webs and defoliated trees. These insects were mistakenly introduced in an attempt to develop silk. Fortunately, gypsy moths have not reached us yet. 

Sprays can control both tent caterpillars, Sevin being the most popular insecticide. But you can in early spring look for eggs masses on branches that could be cut off and destroyed. But do remember that these caterpillars are the food source of our native, elusive and very unique cuckoos.

 Dr. Bob Gilbert is the cofounder of Smith Gilbert Gardens in Kennesaw, Ga.

Karen Lawrence is a professional wildlife and horticultural photographer from Franklin.

Note: This article now abridged first appeared in The Franklin Press in April 2011.