Jeffrey Hahn, Asst. Extension Entomologist
If you are thinking about treating forest tent caterpillars, check their size because the odds are it is too late to treat them any more this year. If they are between 1 3/4 - 2 inches long, they are either finishing or are done with their feeding. Forest tent caterpillars, sometimes called (incorrectly) armyworms have been active since mid to late April and typically feed 5 - 6 weeks. The best time to treat them is when they are half full grown size, or about one inch long.
Older larvae can cause problems by coming down from trees and wandering around looking for food. Sometimes they can severely damage nearby plants, including those in gardens. Other times, they are annoying when they crawl onto homes, sidewalks, decks, patios, outdoor furniture, and other objects. They may even pupate on homes and other things, further being nuisances.
They are difficult to deal with, especially when they are abundant. You can try to create a barrier with an insecticide, such as permethrin (be sure any insecticide you use is labeled for the area or plants you intend to treat). Otherwise use physical removal (avoid crushing the caterpillars if possible when they are homes). It is likely you will need to be persistent as effective physical removal is often time consuming.
Click here for more information on forest tent caterpillars.