Tomatoes are pretty “user friendly”, but that doesn’t mean you won’t have to check your plants. A hungry tomato hornworm can decimate a tomato patch if left to itself, and a horde of grasshoppers can, well, plague you!
So what’s a tomato-loving gardener to do? You can fight back! The most common insects and their suggested means of control is listed below. Note: I am listing more organic methods of control — you can also use commercial pesticides, but read the labels carefully if you go that route.
Tomato Insects and Controlling Them
Aphids: These are very small insects, coming in various sizes (1/16 to 1/8 inch in length) and colors (brown, yellow, pink, or black). They harm tomato plants by piercing the stems and sucking the juices out of the plant. Aphids also secrete a sugary fluid known as honeydew, which gums up the plant and serves as a medium for the growth of sooty mold; a fungus. Preferred method of control is to spray the plant with warm, slightly soapy water (make sure to get the underside of the leaves, too!) Backup control is to dust the plants with diatomaceous earth. Repeat the treatment in a week.
Thrips: Thrips are small (about 1/16 inch long) and slender insects. They live in flowers, on tender leaves and leaflets, feeding on the sap. One part of the thrips’ mouth acts like a rasp and tears the surface of the plant, exposing the contents. The fluid is then sucked up through another part of the mouth. The adults range in color from yellow to black, while the immature wingless nymphs are light in color. Their feeding usually causes leaf-curl (leaves curl in an upward fashion) or some type of distortion. Preferred method of control is spray them with warm, slight soapy water. Backup control is to dust the plants with diatomaceous earth.
Spider Mites: Spider mites are about 1/60 of an inch long (in other words, really tiny), and they may be whitish, green or red, depending upon the species. They are eight-legged (the insects have six legs) and are close relatives to spiders rather than insects. Spider mites live on the sap of the plant which is drawn by piercing the leaf with two sharp, slender lances attached to the mouth. Mites spin webs as do spiders, and are able to be blown from field to field by floating or parachuting in the wind. In periods of hot dry weather the leaves of host plants become blotched with pale yellow, reddish-brown spots ranging from small to large areas on both upper and lower leaf surfaces. The leaves become pale and sickly in appearance, gradually die, and drop off the plant (the leaves look as though they have been dusted with some sort of powder, caused by molted skins of the mites). The mites are also pests of plants grown inside or in greenhouses. The preferred method of control is hosing off with cold water, followed by a thorough dusting of diatomaceous earth. A second treatment should be applied from five to seven days following the first, in order to control spider mites. This is because the first application kills the nymphs and adults, but the eggs might still hatch, giving birth to a new generation.
Tomato Hornworm: These fierce-looking critters are able to munch a tomato plant in no time, if left unchecked. You can recognize these caterpillars by the distinctive “horn” (which is actually located on its rear). Preferred method of control is to pluck them off and drown them in soapy water.
Cutworm: These worms are able to “mow down” young seedlings, by munching clear through the stem! The preferred method of control is to place a “collar” around the tomato plant. This can be as simple as taking the tissue roll (or a paper towel roll, cut in half) and carefully placing it over the tomato plant and pushing it an inch or so into the spoil. Tin foil also works.
Leafhoppers: Leafhoppers vary in size from 1/20 to 1/4 inch but a few will reach 1/2 inch in length. Most are wedge-shaped, broad at the head and pointed behind. They are often brilliantly colored, and may have solid, striped, spotted or banded color patterns. However, some are dull colored (brown, tan, gray). When leafhoppers are abundant on any crop, the plants show a lack of vigor, growth is retarded and, in most cases, the leaves have a mottled appearance, or turn yellow, red or brown. This is due to the leafhoppers sucking out the sap. Several species of leafhoppers transmit plant viruses. The adults lay eggs in the plant stem, buds or leaves, which hatch into wingless nymphs (which feed the same way as the adults). Control is usually accomplished by proper spraying of the crop with a recommended insecticide (Safer brand preferred).
Whiteflies: This insect is about 1/32″ long, and is light in color. It inhabits and feeds on the undersurfaces of leaves by penetrating the tissue and removing plant sap with its piercing-sucking mouthparts. Adults congregate, feed, and mate on the undersurfaces of the leaves, and this can occur in such numbers that “clouds” of insects appear, when disturbed. Other forms of damage include the removal of plant sap, vine, leaf, and plant breakdown, yellowing, leaf shedding and abnormalities of fruiting structure. Control is difficult, but try warm soapy water.
Colorado potato beetle: The adult is about 3/8 of an inch long and has alternate black and yellow stripes running lengthwise down the back of its body (five of each color on each wing cover). It lays patches of about one dozen yellowish-orange colored eggs on the underside of the leaves. The smooth skinned larvae are pink to red in color and have two rows of small black spots on their sides. The larvae reach 1/2 inch in length when fully mature and possess well-developed legs. Both the adults and larvae feed by chewing the leaves and terminal growth of the host plant. Preferred method of control is Bt.
Nematodes: The nemesis of Florida gardeners (but found all over) is the nematode. These tiny worms infiltrate the root system, crippling and even killing the plant. The preferred method of control is to plant marigolds the season before planting the tomato plants (marigolds give off a chemical that wards off the nematodes) or at least plant the marigolds along withe the tomatoes. Backup method is to “solarize” the soil for at least a week before planting the tomato plants. (To solarize the soil, till the ground, then cover it with a thick, clear plastic. The sun will “bake” the soil, killing the nematodes. Since you also kill all the “good” bacteria by this method, so be sure to spread some compost over the solarized soil a week or two before planting.)