Ithaca has seen a large population of leaf-eating caterpillars crawling everywhere around town this summer, and an outbreak of spongy moths continues in the state for a second year. Many worry for their trees’ safety after seeing the impact of insects on North American forests. What are these caterpillars, and are they a danger to our forests or city trees?
There are three main types of leaf-eating caterpillar in New York State (NYS): forest tent caterpillars, eastern tent caterpillars, and spongy moth caterpillars. While tent caterpillars are native to New York, spongy moths have only been around for 150 years or so; they were introduced to the New England states by accident when some escaped from a failed experiment. Spongy moths spread all across the East Coast and have since been “naturalized” in our forests, meaning that they do not need a human’s help to survive and reproduce, but that doesn’t mean that they are harmless to have around–they damage trees and compete with native tent caterpillars for food and space. Leaf-eaters aren’t a constant threat, though, they come in cycles. Usually an outbreak lasts a few years, followed by a few years of silence. NYS leaf-eaters feast in the spring, leaving trees little time to photosynthesize after growing their leaves. However, the caterpillars disappear around late July, allowing deciduous trees– such as maples or oaks–to grow a secondary set of smaller leaves. Deciduous trees can survive 2 to 3 years of this before running out of steam and dying. Evergreen trees don’t fare quite as well. They’re not built to regrow leaves very often, nor live without them for long, and will die if they lose more than around half of their leaves.
Climate change can also hurt a tree’s ability to survive defoliation: a tree living in a drought may be unable to grow a second set of leaves, and excessive heat (often exacerbated by city sidewalks) or poor site conditions can make it hard for a tree to survive repeated leaf-eating. This can create a destructive cycle: a tree struggling with defoliation will have a harder time fighting disease, and a tree fighting disease will have a harder time surviving defoliation.
Trees in a forest are often in healthier living conditions, making them likely to survive caterpillar feeding for even several years in a row. If a forest tree is struggling, it may even be less of a target in the future, as it’s surrounded by full, healthy trees. In places similar to Ithaca, trees may lack these protections, but caterpillar danger is unlikely–few caterpillars any trees are spread farther apart and there’s more anti-caterpillar action taken outside of forests, making such areas undesirable for the leaf-eaters.
With all this in mind, the forest tent caterpillar, eastern tent caterpillar, and spongy moth caterpillar species are not much to worry about. Defoliated trees in the summer may ruin the appearance of a place, but rarely die off. The greatest worry an Ithacan should have is getting a rash by touching a spongy moth caterpillar, which can be treated with an antihistamine. Trees co- evolved to survive leaf-eating caterpillars, disease, birds, and everything in between, so as long as we don’t hurt their conditions too much, they will be just fine.