Answer: Tent caterpillars are large enough to be easily hand-picked and squished, or drowned in a soapy water solution. If you must use a pesticide, the safest, most effective botanical on the market is Bacillus thuringiensis (Bt). It's a stomach poison, so the caterpillars need to consume some treated leaf material before the poison takes effect. There are some parasitic wasps that use the caterpillars as hosts for their eggs, so if you kill the caterpillars, you may very well be killing some beneficial insects, as well. Try hand-picking the caterpillars to see if you can get the population down to an acceptable number first. If not, then Bt is your next line of defense.
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