Tent Caterpillar Saga Comes To A Close

Last week I wrote about the epidemic of Western Tent Caterpillars. As the saga comes to an end, I’m exploring a little more in-depth about these fascinating creatures. Because Tent Caterpillars devour tree foliage, many people believe their trees will die. Alarm spreads through the neighborhood as fast as the caterpillars do. A neighbor had me in a panic saying the tree will die if more than a certain percent of the leaves are eaten. In actuality, the trees rarely ever die. What usually happens is the tree leafs out again within a few weeks. In some cases, heavy defoliation can kill trees. This usually happens after several years of severe insect damage followed by severe drought.

In a moment of panic, many tree lovers often do things that are more harmful to their trees than the actual caterpillars do. If you can keep your mind when all around you are losing theirs, sit back and enjoy the show. Watch one of the most fascinating life cycles you will ever see. It began early spring, just when the buds on the trees started to open, the eggs of the Western Tent Caterpillar hatched and began devouring the leaves of your Alders, Willows, Aspen, Birch, Fruit Trees, and shrubs. The caterpillars molted several times during their five to six week growing period, increasing in size each time. You can see the old skins within the tents. After their last molt, the caterpillars revolt from their communal lifestyle and become gypsies wondering over large areas searching for food. Their food choices become more random and they may even feed on many garden plants. At this stage we see a larger caterpillars spreading out, appearing to take over. You will find them all over far from their tents as they cross your driveways and perch on your house. After this short-lived stage, they go back into a more secluded lifestyle. They may choose to go back to their old tent or find a new spot to call home. They spin their cocoon just about anywhere.

As a native insect the Tent Caterpillars are food for many other insects. Tachinid flies choose to lays eggs on the caterpillars back. Once the egg hatches, a tiny maggot burrows into the caterpillar to feed on its body. This parasitic fly eventually kills the caterpillar. Tachinids, not available commercially as beneficial insect control, can be attracted to your yard. The mature flies feed on nectar that attract Tachinid Flies. You can plant a garden containing herbs such as parsley, cilantro, thyme, lemon balm and dill to attract them into your yard. You can see the tiny white egg on the head of the caterpillar below.

Parasitized Caterpillar
Tachinid Fly egg on head

Tent caterpillars are also subject to a viral disease called wilt, which spreads through heavily infested trees. The virus quickly destroys the caterpillar communes. You can pick out the dead caterpillars because they droop or smear when killed by a virus. The nest below appears to have been devastated by the a virus.

Tent Caterpillar Virus
Western Tent Caterpillar Virus

Such natural controls eventually reduce population levels gradually. Because an outbreak can persist up to 4 years before nature effectively controls the populations, many trees can be damaged. As conditions become more favorable for the caterpillars, the  population gradually rebuilds. Wide fluctuations in caterpillar populations show a delicate balance of ecological forces. When we choose to interfere with the natural cycle, we can often make the problem worse. By spraying your trees with broad spectrum insecticides, the parasitic flies and wasps may also die, leaving no predator for remaining caterpillars and other insects the beneficial insects hunt. Human intervention is not necessary to end the devastation.


Western Tent Caterpillars – What Should You Do About Them?

Everyone seems to be noticing the Western Tent Caterpillars this season. They are not new, but they are causing extensive damage to trees this year. Soon they will be flying around as moths.

Western Tent Caterpillars

If you see the tents in your trees or shrubs this spring they established their home there long ago. Last years adult moths laid eggs on your tree branches. The larvae quietly overwintered there. This spring their tents appeared on the branches. They will partly defoliate some of the branches. The weeping birch pictured below shows fairly extreme defoliation with a large nest. The tree owner watched the defoliation happen in just a day or two. Sometimes tent caterpillars will even defoliate the entire tree. The caterpillars usually don’t kill your tree, especially if it is mature.


The tree below seems to have succumbed to the caterpillars. It shows no signs of life and tents cover the branches. It is also possible that the tree died from some other cause over the winter.


Western Tent Caterpillars are cyclical. Epidemics tend to run in two to three-year cycles. A virus tends to spread through the caterpillars when populations become epidemic,  which knocks the populations back to normal levels. The foliage on your trees will most likely grow back. The best course of action is to cut the nest out of the tree in the morning or evening when they are inside their tents, place it in a bag and throw it away. If you cannot reach the nest, you can attempt to use a high pressure hose to spray them down. It’s best not to spray a broad spectrum insecticide because it can kill beneficial insects, which helps control the populations.