Answer: The botanical Bt (bacillus thuringensis) is quite effective against caterpillars. It's a bacterium that acts as a stomach poison so you'll have to wait until it takes effect before you see any results. I'd avoid using strong pesticides in the garden at this time. When there's a devastating outbreak of tent caterpillars or winter moths, beneficial wasps generally arrive to do the bad guys in. You don't want to discourage the good guys. Bt only affects caterpillars so it's safe to use and won't harm bees or wasps.
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