By Whitney Cranshaw, Colorado State University Cooperative Extension Entomologist
Not much stands between a vegetable gardener and a crop of succulent, vine-ripened tomatoes, unless it's a horde of insects. The following information will help you identify and eliminate major insect pests.
Tomato hornworms are the most familiar insect pests. These caterpillars grow to an awesome size, three inches or longer, with a fearsome looking "horn" on their back end, the purpose of which is unknown other than to scare gardeners. The rapid loss of tomato leaves as well as big green droppings at the plant's base, signals a hornworm at work. While the caterpillar stage usually takes three to four weeks to complete, hornworms grow so much in the last few days, they seem to appear almost overnight.
Despite their size, hornworms are quite easy to control. They are very susceptible to Bacillus thuringiensis, the biological insecticide sold under trade names such as Dipel, Thuricide, and Caterpillar Attack. Most other garden insecticides (Sevin, Malathion) also are effective.
I hunt hornworms as a form of recreational pest control, and I recently met a man who trained his dog to find them! Because they are large, hornworms are easy to hand-pick or to cut with a knife or scissors. They are, however, difficult to find, as they blend into their background. Hornworms are most visible early in the morning, when they likely are on the plant's exterior. See Hornworms
Though hornworns are large, the most damaging tomato insect often is the over-looked potato/tomato psyllid. Developing psyllids are small (aphid-sized), pale green or yellow, and somewhat scale-like in appearance. Adults are banded black and white and jump readily when disturbed.
Psyllids feed on the underside of leaves, sucking plant sap and introducing a saliva that disrupts plant growth. New growth may appear twisted and discolored. Tomatoes from psyllid-infested plants are small and mealy textured. Psyllid-injured Big Boy and other tomato varieties may grow no larger than a small marble.
Tomato psyllids do not occur regularly, as they overwinter in southern areas and are carried by winds into Colorado. Heavy flights were detected in several locations earlier this year. Check plants for evidence of psyllids, and look for their characteristic droppings that consist of small waxy beads. These droppings also resemble granulated sugar or salt.
Controls are warranted when psyllids are present. Sulfur dusts, permethrin, esfenvalerate or insecticidal soaps are effective treatments. Apply thoroughly, including on leaf undersides, where psyllids nymphs occur. See Psyllids
Greenhouse whitefly is another common tomato pest. The flying adult stage is familiar to most gardeners, but the pale, motionless nymphs that feed on leaf undersides are not. Nymphs and adults remove sap and reduce plant vigor. Whiteflies also may excrete sticky honeydew, which is more of a nuisance than a serious problem.
Once established, whiteflies are difficult to control. They resist most insecticides. Sprays must cover the leaf undersides to kill the nymphs. Pyrethrins and malathion may provide some control, but insecticidal soaps probably are the best. (NOTE: Most whitefly sprays sold in nurseries contain resmethrin, and are not legal to use on edible crops such as tomatoes.)
Greenhouse whitefly is not native to Colorado, and it cannot survive our outdoor temperatures. Problems arise annually, however, from plants grown in whitefly-infested greenhouses or other indoor areas. Therefore the best means of managing whitefly is to just say no: Don't grow or purchase transplants that may have come from whitefly-infested places.
Flea Beetles (damage seen above) are small, black chewing insects that create pits in the leaves and fruits of many plants including tomatoes. They are first seen in the spring as they favor the early growth of the plants they affect. See Flea Beetles.
Photo: Judy Sedbrook
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010