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.
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.