Greenhouses provide ideal growing conditions that lets plants and food crops grow year-round. The controlled environment of the greenhouse allows farmers to grow crops in areas where soil or climate make it difficult to grow food. Greenhouses protect plants from excessive heat and cold, dust storms, blizzards and in most cases, insects. There are times when insects invade greenhouses and they must be effectively controlled to ensure the well-being of the plants.
The rapid-producing aphid will quickly populate a greenhouse if left untreated. Female aphids give birth just seven days after they are born. Aphids, often called plant lice, are 1/16 to 1/8 of an inch long, are tan in color and have a soft body. They suck the sap from the stems and leaves of plants. Signs of aphid damage are the curling and wrinkling of leaves and stems. Aphids leave behind a waste called honeydew that covers leaves of the plant with a black, sooty mold.
Treat aphids with insecticides that have the active ingredients acephate, kinoprine, endosulfan or malathion which are specifically used to fight aphids. The University of Kentucky recommends applying insecticides two to three times over a seven-day period. Always follow manufacturers' labels when using chemicals.
Tiny, light tan or brown thrips find it easy to infiltrate the greenhouse environment. They are 1/8 inch long and have two or four veinless wings. Thrips cut leaves and then suck the sap from the wound. Injured leaves or flowers become misshapen and leaves develop silver or gray spots. Thrips also spreads viruses from one plant to another.
Thrips are difficult to control but there are ways to stop them. The University of California suggests introducing a parasitic wasp called Thripobius semiluteus into the greenhouse. The wasp attacks only greenhouse thrips and has been helpful in eliminating thrips. Chemical treatment of thrip isn't always successful, but neem oil, azadirachtin and insecticidal soaps are most effective Apply treatments early when thrips damage is first noticed and repeat every five to 10 days to kill newly hatched thrips. Always follow the manufacturer's label when using chemicals.
The most common greenhouse pest is the whitefly. Two varieties of this pest, the greenhouse whitefly and the sliverleaf whitefly, attack both flower and vegetable plants in the greenhouse. They are powdery-white and are small in size--1/12 inch long. Whiteflies attack the underside of plant leaves, sucking sap from the leaf. They also leave behind a honeydew that turns black and covers plant leaves. Plants attacked by whiteflies become weak and begin to wilt or droop.
A parasitic wasp, the Encarsia formosa, attacks the larvae of the whitefly and helps control infestations. Insecticides only work on the adult white fly. Neem oil, imidacloprid, pyrethrins and malathion are all effective in controlling whiteflies. The University of Missouri says to apply insecticides four to five times at five- to seven-day intervals until whiteflies are under control.