Greenhouse Whitefly
Trialeurodes vaporariorum (Westwood), Aleyrodidae, HEMIPTERA


DESCRIPTION

Adult - About 1.5 mm long, the adult is a white insect which resembles a tiny moth.

Egg - The oblong eggs, pale green to purple, are inserted into the lower leaf surface, often in a circle or a crescent. Each egg is about 0.2 mm long.

Nymph - The tiny, first instar nymph (or crawler) is yellow with red eyes and has functional legs and antennae. Second and third instar nymphs are flattened, immobile scale-like insects.

Pupa - About 0.75 mm long, the oval, flattened pupa is a pale green (normal) or black (parasitized) insect with long hairs on its back and around its body margin. After the adult has emerged, a white, almost transparent pupal skin is left behind.


BIOLOGY

Distribution - Greenhouse whiteflies are worldwide pests of greenhouse-grown ornamentals and vegetables. First discovered in England in 1856, the were found in the northeastern U.S. in 1870. Tropical Central or South America are suggested origins of the greenhouse whitefly.

Host Plants - Greenhouse whiteflies infest a wide variety of ornamental and vegetable crops, and they can survive outdoors during the growing season, particularly in sheltered locations. Some of the more important hosts include bean, melon, lettuce, cucumber, tomato, squash, potato, eggplant, strawberry, grape, tobacco, and rose. Even trees such as avocado may be infested.

Damage - Nymphs and adults extract plant sap through their needle-like mouthparts, the adults preferring to feed on tender new growth. As a result, leaves turn yellow and drop from infested plants. In addition to having an unthrifty appearance, plants may be stunted and unreproductive. Sooty mold often grows on leaves covered with honeydew secretions of whiteflies and interfere further with the growth of plants.

Life History - Greenhouse whiteflies reproduce relatively slowly (one generation every 30 to 45 days), but each female lays about 250 eggs and lives as long as two months. Adults usually are found on the lower surface of new leaves. There they insert their eggs which hatch 5 to 7 days later. New crawlers move about the plant for a day or two, often from leaf to leaf, before inserting their mouthparts to feed. Once this occurs, they probably do not move again until mature. The crawlers molt into later nymphal instars and then into pupae. Finally, a new generation of whitish-yellow adults emerges. They soon are covered with a white waxy bloom.


CONTROL

Parasitic wasps, Encarsia formosa, sometimes help keep greenhouse whitefly populations under control. However, at temperatures below 24 degrees C (75 F), the reproduction of these parasites is inhibited. As a result, whitefly infestations are encouraged by lower greenhouse temperatures.

Control of whiteflies is difficult because the eggs and immature forms are resistant to many aerosol and insecticide sprays. One must make regular applications of pesticides to control emerging adults until the last of a whole generation of immature whiteflies has emerged. Methomyl and some of the synthetic pyrethroid and synthetic insect-growth-regulator pesticides, however, are extremely effective and need not be applied as often. For specific chemical control recommendations, consult the current North Carolina Agricultural Chemicals Manual.

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