If tomato hornworms are the worst of your tomato problems, then you are doing great. I much prefer to deal with problems I can actually see and identify, rather than the more insidious invisible ones.
There are a couple of easy ways to know you have hornworms, even if you don’t see them at first. When you notice that your tomato plants are rapidly being eaten by something, with damage concentrated on the upper branches, hornworms are most likely at work. The easiest way to spot them is to look for their droppings on the leaves below the damage. These look like little brown segments of corncob. The larger the droppings, the larger the worm, so be prepared. If you slowly raise your eyes from where you see the freshest droppings, scanning left and right, you are likely to spot the culprit(s) chowing down.
Once spotted, hand pick the worm, throw it on the ground and stomp on it. If you have chickens, they will love this tasty fluorescent green snack. If you are having trouble spotting the hornworms, you can sometimes get them to move by spraying your plants with water. They don’t like being disturbed and will thrash when you get them wet. Another way to see them, although I’ve never actually done it, is to shine a black light on your plants at night. They will glow green against purple foliage.
If hand-picking isn’t your thing, there is a much easier solution called Bt (Bacillus thuringiensis), which is a bacterium that only affects caterpillars. It’s registered organic and will not hurt you, your children, grandchildren, pets or any other creature other than the caterpillars. You can purchase Bt at most local nurseries. Ask for the Bt liquid concentrate. Common brand names include Thuricide and Monterey Bt.
Follow the instructions and mix the Bt with water in a spray bottle. Spray when the caterpillars are actively feeding. Wet both the tops and undersides of the leaves and the buds with the solution. Once the caterpillars ingest the Bt, they will stop feeding and die within a few days. Continue monitoring your plants for new damage and droppings, and spray again if needed. Then keep the Bt on hand in a cool dry place inside, because you will probably need it again towards the fall when there is likely to be another hornworm outbreak.
A longer term strategy is to increase your population of native parasitic wasps, because they lay their eggs on the backs of hornworms. The developing wasp larva gorge themselves on the hornworm’s insides. I can assure you, a parasitized hornworm is not going to damage your tomatoes anymore. The best way to attract predatory insects that will assist you in dealing with hornworms and many other insect pests, is to diversify your backyard ecosystem with flowers.
Add members of the Apiaceae family of plants around your garden. These include the “umbellifers” such as parsley, carrot, dill, coriander, and fennel. These plants produce umbels, clusters of tiny flowers on terminal stalks, that are the perfect size for the wasps’ small mouth parts. The nectar from these small flowers sustains the wasps and other predatory insects.
Other great nectar flowers to use include buckwheat, clover, chamomile, feverfew, yarrow, sweet alyssum, plains coreopsis, cosmos, California bluebell and the culinary herbs (especially thyme). Some say that interplanting your tomatoes with dill, borage, basil and/or marigolds will repel hornworms, although I have not adequately tested this approach to comment on whether it works or not.