Whitefly Information from the University of California
Whiteflies are tiny, sap-sucking insects that are frequently abundant in vegetable and ornamental plantings. They excrete sticky honeydew and cause yellowing or death of leaves. Outbreaks often occur when the natural biological control is disrupted. Management is difficult.
IDENTIFICATION AND LIFE CYCLE
Whiteflies usually occur in groups on the undersides of leaves. They derive their name from the mealy white wax covering the adult’s wings and body. Adults are tiny insects with yellowish bodies and whitish wings. Although adults of some species have distinctive wing markings, many species are most readily distinguished in the last nymphal (immature) stage, which is wingless .
Whiteflies develop rapidly in warm weather, and populations can build up quickly in situations where natural enemies are destroyed and weather is favorable. Most whiteflies, especially the most common pest species—greenhouse whitefly (Trialeurodes vaporariorum) and silverleaf or sweetpotato whiteflies (Bemisia species)—have a wide host range that includes many weeds and crops. In many parts of California, they breed all year, moving from one host to another as plants are harvested or dry up.
Whiteflies normally lay their tiny, oblong eggs on the undersides of leaves. The eggs hatch, and the young whiteflies gradually increase in size through four nymphal stages called instars. The first nymphal stage (crawler) is barely visible even with a hand lens. The crawlers move around for several hours, then settle and remain immobile. Later nymphal stages are oval and flattened like small scale insects. The legs and antennae are greatly reduced, and older nymphs do not move. The winged adult emerges from the last nymphal stage (for convenience sometimes incorrectly called a pupa). All stages feed by sucking plant juices from leaves and excreting excess liquid as drops of honeydew as they feed.
Whiteflies suck phloem sap. Large populations can cause leaves to turn yellow, appear dry, or fall off plants. Like aphids, whiteflies excrete honeydew, so leaves may be sticky or covered with black sooty mold. The honeydew attracts ants, which interfere with the activities of natural enemies that may control whiteflies and other pests.
Feeding by the immature silverleaf whitefly, Bemisia argentifolii, can cause plant distortion, discoloration, or silvering of leaves and may cause serious losses in some vegetable crops. Some whiteflies transmit viruses to certain vegetable crops. With the notable exception of the citrus whitefly, whiteflies are not normally a problem in fruit trees, but several whiteflies can be problems on ornamental trees. Low levels of whiteflies are not usually damaging. Adults by themselves will not cause significant damage unless they are transmitting a plant pathogen. Generally, plant losses do not occur unless there is a significant population of whitefly nymphs.