Whiteflies

Whiteflies are small flying insects that are about 0.04 inch (1 mm) long. The body and wings of adults are covered with a fine whitish powdery wax that is opaque in appearance. Whiteflies colonize the underside of leaves; adults and eggs are commonly found on the lower surface of younger leaves and the scalelike nymphal stages on somewhat older leaves.

Distinguishing whitefly species is difficult; use a hand lens to examine both immatures and adults. Adult sweetpotato whiteflies, sometimes called silverleaf whiteflies, hold their wings somewhat vertically tilted like the peaked roof of a house, instead of flat over their bodies like the greenhouse whitefly. During the last part of the fourth larval stage, often called the pupa, whiteflies develop red eyes and stop feeding. The pupal stage is the stage that is easiest to identify sweetpotato whitefly; pupae have no waxy filaments around their edges as do most other species of whiteflies.

Extremely dense populations of sweetpotato whitefly may immigrate into late-August or September plantings of carrots in the low deserts of southern California and damage seedlings. In light to moderate infestations, leaves show no distinctive symptoms as a result of whitefly feeding; however, copious quantities of honeydew are deposited on leaves, resulting in a shiny, sticky appearance.

Whiteflies can be very difficult to control with insecticides. Natural or introduced biological methods provide the best long-term solution to keeping whitefly numbers at a minimum along with reducing host plants in areas of heavy infestation. If you treat for sweetpotato whitefly, make applications before pests build up and contaminate carrot tops with honeydew.

Several parasitic wasps, including species in the Encarsia and Eretmocerus genera, control whiteflies. Whitefly nymphs are also preyed upon by bigeyed bugs, green lacewing larvae, brown lacewing larvae and lady beetles. Encourage buildup of beneficial insects by avoiding the use of nonselective pesticides and by protecting their habitat.

Remove field bindweed and other weeds in and adjacent to the crop field as well as crop residues. Host-free periods are valuable for controlling several of the whitefly species.

Use biological controls, cultural controls, and soap sprays in a certified organic crop.

Carrots are not a preferred host of whiteflies and in some areas treatment with insecticides is rarely justified. Where sweetpotato whitefly is a chronic problem, a treatment with imidacloprid at planting will effectively control whiteflies during the critical period of stand establishment. If treatment is not applied at planting, occasionally dense colonies of sweetpotato whitefly nymphs can necessitate treatment of bunching carrots to prevent honeydew and sooty molds from severely contaminating the carrot tops.

For foliar treatment, combinations of a pyrethroid insecticide (mode-of-action group 3A) with methomyl provide superior control over applications of a single insecticide. Rotate pyrethroid treatments with a treatment of insecticidal soap to help slow development of insecticide resistance. Insecticidal soaps control all immature stages of whiteflies, including eggs, whereas the other insecticides control first instar nymphs and adults only; thus, treatment timing is critical.