Are leaves suddenly disappearing from your tomato plant? Take a closer look. Your plant is probably being attacked by hornworms. Despite their large size, these bright green caterpillars can easily hide among tomato leaves, staying out of sight until they have eaten most of the plant’s foliage. Inspect your plants for hornworms now before they strip it down to bare stems.
Tomato and tobacco hornworms are closely related large, bright green caterpillars that feed on the leaves of tomatoes, as well as tobacco, eggplant and occasionally peppers. They have a distinct single spine or “horn” on their rear end, but cannot sting. These caterpillars can reach up to four inches in length before they mature 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.
One of the simplest methods for controlling hornworms, as well as many types of insects, is to pick them off plants and drop them into a bucket of soapy water. 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.
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 local garden centers. When applying 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 from 0 to 21 days or more. If you already have ripening fruit on your tomato plants, be sure to choose a product with a short pre harvest interval.
Organic insecticides that control hornworms and other caterpillars include Bacillus thuringiensis, a bacterial disease that only effects caterpillars and is commonly referred to as B.t. B.t. is sold as Dipel, Thuricide, and several other brand names, and is most effective if applied when caterpillars are small. Spinosad is another option for controlling caterpillars organically. Derived from a soil dwelling bacterium, spinosad is the active ingredient in several insecticides, including Captain Jack’s Dead Bug Brew and Greenlight Spinosad Lawn and Garden Spray. Because they break down quickly, most organic pesticides need to be reapplied more often than synthetic sprays to maintain plant protection. Check product labeling for application timing recommendations and restrictions.
Synthetic insecticides that control caterpillars include products containing one of the following active ingredients: carbaryl (commonly sold as Sevin), cyfluthrin, permethrin, and bifenthrin. Active ingredients of all pesticides are listed on the product container and can usually be found on the front of the packaging. When treating tomatoes and other garden plants, be sure to only use products that are labeled for spraying vegetables. Spray late in the evening to minimize impacts on bees and other pollinators.
Gardeners occasionally find hornworms in their garden that have white cylindrical attachments all over their backs. These are the cocoons of a type of parasitic wasp, which feed on caterpillars 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 seek new caterpillars to prey upon.
Other common summer tomato problems include wilt diseases, leaf spot diseases, and fruit rots. If you suspect your tomatoes have a disease or insect problem, have the cause correctly diagnosed before taking any action. Visit Texas Cooperative Extension’s Tomato Problem Solver website, http://aggie-horticulture.tamu.edu/vegetable/tomato-problem-solver/, to diagnose your tomato problems or contact your local Extension office for assistance. Locate your local Extension office or post your questions to be answered online through the ‘Ask an Expert’ link at http://ces.ncsu.edu.
Charlotte D. Glen is the horticulture agent with the Pender County Cooperative Extension of NC State University, College of Agriculture & Life Sciences. She can be reached via e-mail at Charlotte_Glen@ncsu.edu.