Answer: You have three options:
Handpicking. The large size of hornworms makes it easy to get hold of them. Once removed from the plant, they can be destroyed by snipping them in half with shears or dropping them into a bucket of water.
Rototilling. Turning up the soil after harvest will destroy any pupae that may be there.
Biological. Bacillus thuringensis, or BT (e.g., Dipel, Thuricide), is also considered very effective, especially on smaller larvae. Spray it as a precaution.
Natural enemies, such as the parasitic wasp that lays its eggs on the hornworm's back, are common. If found, such worms should be left in the garden so the emerging wasps can parasitize other hornworms.
Best wishes with your tomato plants!
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