Summer Veggie Pest: Tomato Hornworm

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Tomato hornworms are large, bright green caterpillars that feed on the leaves of tomatoes, as well as tobacco plants. They have a distinct single spine or “horn” on their abdomen but cannot sting. These caterpillars will reach up to four inches in length before maturing into large grey moths. Despite their large size, hornworms are not usually noticed until they strip a considerable amount of foliage from a tomato plant, which can happen in a matter of a few days if several caterpillars are feeding on the same plant. If left unchecked, hornworms can devour all of a tomato plant’s foliage, severely weakening the plant. Hornworms will continue to multiply through the rest of the growing season, making control now and throughout the summer essential to keep your tomatoes productive.

An image of a tomato hornwormOne of the simplest methods for controlling hornworms is to pick them off the plants and dispose of them. Controlling hornworms by hand picking requires almost daily inspection to stay on top of the population. Adult hornworms, known as sphinx moths, lay new eggs every night, and each female can lay up to 2000 eggs during her short life span.

An image of parasitic wasps attacking a tomato hornworm

Parasitic wasps attack a tomato hornworm.

A less labor-intensive way of controlling hornworms is to spray tomato plants with an appropriate insect control product. Several insecticides are effective, with products containing organic and synthetic active ingredients available at your local garden center. Examples include products that contain spinosad, B.t., neem oil or azadirachtin as the active ingredient. When applying

Image of adult hornworm, known as Spinx moth.

Adult hornworm, known as Spinx moth.

any insecticide be sure to read and follow all label directions and pay careful attention to the pre-harvest interval. This is the number of days you must wait between the time you spray the plant and the time you harvest. Pre-harvest intervals vary among insecticides. If you already have ripening fruit on your tomato plants, be sure to choose a product with a short pre-harvest interval. Check product labeling for application timing recommendations and restrictions. Active ingredients of all pesticides are listed on the product container and can usually be found on the front of the packaging.

Be sure to spray late in the evening to minimize impacts on bees and other pollinators. You’ll occasionally find hornworms in the garden that have white cylindrical attachments all over their backs. These are the cocoons of a type of parasitic wasp, which actually feed on hornworms, as they develop, eventually causing them to die. Parasitic wasps are very small and do not sting people. If you find a caterpillar in your garden that has been attacked by parasitic wasps, leave it there so the wasps may continue to develop. When mature they will hatch out of the cocoons and feed upon the hornworm they are attached to.