Occasionally a pesky infestation of caterpillars appears on tomato plants, resulting in eaten leaves and stunted plant growth. While the lone caterpillar doesn't wreak enough damage to cause alarm, loss of too many leaves on a tomato plant stops development of new flowers and fruits. The plant must focus its energy on regrowing new stems with leaves. Hand-picking caterpillars and squishing them is an option, but when they hide and eat at night, it's a losing battle. Toxic pesticides kill all sorts of insect pests, as well as beneficial ones like honeybees and ladybirds. Luckily, one solution targets leaf-eating insects like caterpillars while sparing those that don't damage the plants.
Purchase spores or liquid bacteria culture of Bacillus thuringiensis (Bt). This organism, when applied to plant leaves, is consumed by leaf-eating insects like caterpillars, creates a hole in the stomach of the bug, causing it to die. Product trade names of Bt include Dipel (powder form) and Thuricide (liquid form).
Mix a solution of Bt with water in a spray bottle, according to product label directions.
Douse the tomato plant foliage with a spray of Bt solution, coating the leaf blade, stems and undersides--anywhere you see chewing damage from the caterpillars on the tomato plant.
Reapply the Bt spray once a week and after rain because the bacteria washes away. The caterpillars must consume foliage that has residue of Bt spray to be killed.
Bt does not immediately kill caterpillars on contact. The pests must first consume a bit of treated tomato leaf. Within about two days the caterpillars cease eating and die. Repeat application of Bt every week, as latent insect eggs may hatch and larvae resume eating tomato foliage. Spinosad, neem oil and pyrethrin are additional treatments to kill caterpillar pests on tomatoes, notes the Clemson University Extension.
Consult product label for advisories on harvest of tomatoes after the plant is sprayed with Bt. There may be waiting periods, such as a drying time, before you can touch plants, or harvest fruits or leaves.