Answer: Without knowing which specific plants are being eaten and which specific kind of worm it is, it is a bit difficult to give advice. Bt or Bacillis thuringiensis (in products such as Dipel) will kill caterpillars. You could try applying that per the label directions.
However, it would be better to get a definite identification of the pest and then knowing that, decide how to proceed. If for instance it is a tomato horn worm on a tomato plant, handpicking might be the best/easiest control. Or if it is webworm caterpillars in a tree, it might be better/easier to remove their webbing at dusk when they are inside it rather than try to treat the entire tree.
Your local Cornell cooperative extension should be able to help you identify the worms and then recommend appropriate treatment. They will also be able to tell you how/when to apply it to obtainmaximum results. I'm sorry I can't be more specific long distance. Good luck with your plants.
Q&A Library Searching Tips