Tent Caterpillar

By Elaine Farragher


Tent caterpillars have been a particularly noticeable pest this year as well as last year. My former passive curiosity has been replaced with a rather bitter antipathy as I have watched some of my favourite bushes denuded by these hungry and very efficient insects. I spent some very enjoyable hours gleefully zapping their tent cities with a propane torch in an attempt to contain their destructive populations. However, for the ones too high to reach, I could only watch helplessly as the hordes of munchers stripped the leaves off one of my best apple trees.

Tent caterpillars are a favourite food of black billed cuckoos whose populations increase in years of high infestation. Unfortunately, not too many other birds will eat them so the well-stuffed cuckoos have a poor chance of keeping up. The one cuckoo I saw around my apple trees was having a negligible effect but I cheered him anyway for his efforts.

By this time of year the worst is over, the damage has been done and the caterpillars have become moths. Now they are looking for mates, laying eggs and dying off after their brief adult existence. Good riddance – until next year, that is – which should be the last of a cycle of infestation which generally lasts three years.

Late summer marks the beginning of the life cycle of the tent caterpillar. Now is when you can begin to discover their egg cases which are shiny and dark, about an inch long, encircling the twigs of their favourite trees – usually black cherry, apple and choke cherry. The two to three hundred eggs are protected by a hardened frothy substance which in turn is covered by the dark water-proof coating. There may be as many as twenty egg cases on a single tree.

Immediately after hatching in the spring, they start to feed on the leaves of the host tree. After a week or so of building up their strength, these communal insects together start building their tent city in a fork of branches in the tree. When you try to rip apart one of these tents with a stick, it is a little surprising to find that it is a tent-within-a-tent- within-a-tent, and so on, forming a rather tough, many-layered mass of silk. As the caterpillars grow, they shed their skin and deposit this and other waste materials in the inner layers of the tent, while they spin new silken webbing to house themselves in a new and cleaner outer layer. Rather like a bad housekeeper who must keep moving to avoid the mess created in the old dwelling.

In feeding habits these caterpillars are somewhat like us in that they have three meals a day – morning, mid-day and evening between which times they either go home to the nest, or hang around in a tight squirming cluster in the sun outside. The evening feeding is their longest, and it is after dark that they do the greatest damage.

When the caterpillars shed their skin, for the fifth and last time, they drop out of the tree and go off on their own to look for a place to spin the cocoon from which the adult moth will emerge. Their search for a secure spot often takes them across roads on which they are squished by passing cars. This year they were so numerous that they were considered a hazard in some areas as the roads became slippery with their carcasses.

If the caterpillar manages to find a handy crevice, it spins a tough woven cocoon and folds itself in half to fit into it. This enclosure in turn is covered with a yellow material which stiffens the cocoon. When dry this layer becomes very powdery and floats off at the slightest touch – an easy way to identify a tent caterpillar cocoon.

The adult moth which emerges is light brown, thick-bodied and between an inch and an inch and a half long with a faint white stripe on its wings. Their entire life cycle is a lead-up to their brief few weeks when they are finally moths and capable of mating and laying eggs. Once their eggs are laid they die off quickly.

If you have favourite cherry or apple trees which you would like to see healthy and fruitful next year, be on the lookout now for the black shiny egg cases which usually entirely encircle the twigs. The trees and bushes where they are found will be breakfast, lunch and dinner for the emerging caterpillars in the spring. Fortunately, these insects usually do not destroy the tree that they dine on. A tree can lose its leaves one season and still bounce back the next. In any case the defoliation is usually not complete, so even if the tree is victim two years in a row it will usually survive.


Elaine Farragher, August 1988



Subject headings

Insects – Nature