Answer: What you've heard is true. Once the caterpillars are nestled inside their bags, sprays will be unaffective. Late June or early July, when the caterpillars are actively crawling and feeding is the time to spray. Insecticides recommended for controlling bagworms include Bacillus thuringiensis kurstaki (sold as Dipel or Thuricide) and spinosad (Conserve). The bacterium Bacillus thuringiensis is effective on young caterpillars, but the material must be ingested--so thorough plant coverage is essential. Spinosad very effectively works by contact and ingestion.
Best wishes with your trees!
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