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Forest Tent Caterpillars Are Out Now

Jeffrey Hahn, Asst. Extension Entomologist

Did you have a problem with forest tent caterpillars (FTC) last year? If you did, expect to see them again soon as they have started to hatch during late April. You can recognize these caterpillars from their blue and black body and white footprint or keyhole shaped spots on their back. Despite their name, FTC do not construct conspicuous webs. If you find a large tent in a tree this spring, that is from eastern tent caterpillars.

Jeffrey Hahn

Photo 1: Forest tent caterpillar and damage

FTC are primarily a problem because they feed on the leaves of trees and shrubs, especially aspen, birch, oak, and linden/basswood. If they are abundant and their normal food is in short supply they will crawl down trees and also feed on fruits, vegetables, and flowers. They can also become nuisances when they wander around looking for sites to pupate (which has earned them the nickname 'armyworms'). This can lead them to crawl onto nearby buildings and other structures.

Populations of FTC are cyclical, with periods of few and increasing numbers of FTC lasting about 8 - 13 years. Eventually these increasing numbers hit outbreak numbers which lasts about three to four years. FTC populations in the Twin Cities, though, appear to be less cyclical.

Fortunately, healthy, mature trees can tolerate severe defoliation, even in several consecutive years. Young and unhealthy trees are more susceptible to injury and should be monitored closely for the potential need to treat. There are a variety of residual insecticides that you can use if you want to protect your plants. Consider using products that have a low impact on the environment, such as Bacillus thuringiensis, spinosad, and insecticidal soap. Bacillus thuringiensis is a particularly good product if the tree is flowering since it will not harm visiting honey bees.

There are also insecticides available to protect garden plants, including food crops. Be sure to check the label to be sure the particular product you want to use is cleared to treat the plants you wish to protect.
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