The larvae of the hawk or sphinx moths, also known as hornworms or tomato hornworms, are particularly destructive to delicate tomato plants and their tantalizing soft-fleshed fruit. Even just a couple of hornworms can wreak havoc on tomato plants, chewing their way through the leaves and stems and munching through the tomatoes. Hornworms emerge from over-winter hibernation in midsummer, just as you start eagerly anticipating your first tomato harvest of the season.
You can plant your tomato plants close to trees and shrubs where hungry birds may be nest or seek shelter. A propped up flowerpot provides ideal shelter for toads and frogs who will happily enjoy a hornworm meal while leaving your tomatoes alone. If you notice only a few early hornworms, appearing most frequently at dusk, you can pick them off by hand. About 3 inches long, with a flexible horn -- which is really the spine -- at the end of their bodies, hornworms may be difficult to see at first because of their green color.
Homemade spray may deter hornworms from eating your tomato plants. In some cases, a strong burst of water will be sufficient to knock the worms to the ground. Other options include crushing four to six cloves of garlic and mixing them together with one tablespoon of dish soap and one gallon of water. Strain with a piece of cheesecloth to remove the garlic pieces. Letting the mixture steep for 24 hours before using increases its potency. Transfer to a handheld spray bottle and spray the tops and bottoms of the leaves. Stored in the refrigerator, the spray will keep for up to one month.
Most commonly referred to as Bt, bacillus thuringiensis is a naturally occurring bacteria, safe for people and animals and will not harm helpful garden insects such as worms or lady bugs. Bt occurs naturally in soil and is a widely used, highly effective microbial treatment. When applied to the tomato foliage on top and underneath leaves, Bt will last at most a week, although often less and may need to be reapplied every few days.
Planting natural deterrents to hornworms near your tomatoes can prevent an infestation from taking place. Companion plants such as borage and marigolds are natural deterrents to hornworms. In addition to the bright splash of color provided by marigolds, borage also adds calcium and potassium to your soil, enriching it even further. Maintaining healthy tomato plants will also act as a deterrent to hornworms, as they will first feast on weaker plants.