The first time I bumped into a tomato hornworm in my garden I nearly surrendered my will to live. I had never seen such a huge caterpillar and two thoughts ran through my head: 1. The end is nigh! 2. Maybe it will transform into a stunning exotic endangered moth. Neither happened.
Hornworms are garden pests common throughout all regions. At least anywhere they can find their favorite crops: tomatoes, tobacco, potatoes, peppers, and eggplant. After a summer of devouring your garden, the caterpillars burrow into the soil, pupate, and emerge in the spring as the five-spotted hawk moth (tomato hornworms) or Carolina sphinx moth (tobacco hornworms).
To prevent summer caterpillar infestion, till your soil well in the spring to expose the caterpillars' cocoons. Also incorporate companion plants such as marigold, borage, basil, and parsley. Upon infestation, the simplest method of hornworm removal is to pick them off the plants in the evening when caterpillars are active. If worm picking isn't your bag, apply the tried-and-true, all-natural, Bt caterpillar powder or spray, such as Safer Brand's Caterpillar II Killer. It is a biological control that does not harm beneficial insects, animals, or humans. Although diatomaceous earth may have an effect on the moths/caterpillars as the enter and emerge from the soil, it's not a practical solution once the caterpillars have climbed up to the top of the plants where they feast.
Despite the destructive nature of their hornworm larvae, sphinx & hawk moths are impressive sights. Almost as impressive as the fact that They Might Be Giants wrote a song about the particularly fascinating hummingbird hawk moth:
P.S. Although diatomaceous earth (DE), mentioned above, may not be the best post-infestation option for hornworm control, it is a safe and indispensable home & garden staple. It has gained popularity in recent years as a pet and kid safe option for treating indoor pests like ants, cockroaches, fleas, and bedbugs. In the garden it's effective in the treatment of many terrestrial pests, yet doesn't appear to be harm bees. A 10-lb bag is inexpensive and will last years if kept dry. Many people believe in incorporating DE into their diet. In fact, food-grade DE is likely already present in our food in the form of an insect control additive in grain products.