Answer: I think tomato hornworms are the problem with your tomatoes. They are voracious feeders and can level a tomato plant in just hours. Bt (sold as Dipel) will take care of them in a hurry. It will also work on other worms/caterpillars. I'd reserve the Sevin for some other time. Sevin kills all insects, including bees and wasps, and you need those for pollination of the flowers. Bt won't harm other insects.
Best wishes with your garden!
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