To organically protect tomatoes from the dreaded tomato hornworm, employ herbs three ways in the garden. Establish hornworm-repelling herbs near tomatoes to keep the predators from climbing their vines, plant herbs that attract insects that feed on hornworms and consider growing other herbs that hornworms love to eat. This could prevent them from ever getting around to chowing down on your tomatoes.
The classic pairing of tomatoes and basil in the kitchen garden didn't come about because tomatoes and basil taste heavenly together. Basil boasts the additional reputation of scaring away tomato hornworms as well as mosquitoes, aphids and flies.Whichever basil variety you choose, be sure to plant it in front of the tomatoes as the sun-loving herbs hate being in the shadow of taller plants.
Borage is one of the more well-known companion plants for tomatoes because of its ability to repel hornworms. The herbs, which grow up to 2-½ feet tall, add beauty to a kitchen garden with blue flowers and graceful, arching foliage. Plant them in front of, or interspersed among, tomato plants. Use borage flowers to garnish a salad, or, when crystallized, to dress up cupcakes. The leaves add a cucumber-flavored freshness to drinks and summer soups, as well as Middle Eastern dishes such as cucumbers with yogurt.
Dill, like many other members of the Umbelliferae botanical family, attracts the parasitic wasp, which lays its eggs on the backs of hornworms. As the wasp eggs begin to hatch, the larvae literally paralyze the hormworm. The tiny wasps seem to love the "umbrella" shape of dill and other Umbelliferae because they provide a flat surface on which to land and feature small nectar-filled flowers. Dill has the additional advantage, ironically, of being a favorite snack of the tomato hornworm. The herb acts as a "trap crop"---a plant which may distract the hornworm from turning to the tomato as a food source---while also hosting the tiny wasps that feed on the hornworms. Use dill's yellow flower heads to flavor homemade pickles, and its chopped leaves for potato dishes, egg salad and to flavor fish.
Another member of the Umbelliferae family that attracts the parasitic wasp, parsley makes a good choice to plant near tomatoes. It can grow in full sun or light shade, so position it to the side of your tomatoes if you need the space in front for other herbs or vegetables. Grow curly or flat-leafed parsley. The former looks a bit prettier in the garden and to garnish a plate. Cooks traditionally favor the latter for its slightly stronger flavor.
Certain all-purpose, pest-controlling plants may also be useful in combating the hornworm. Some gardeners place marigolds and mints near tomatoes and other vulnerable crops. It's best to keep the mints, which grow quite quickly, in containers to prevent them from overtaking the tomatoes.