Sweetpotato Whitefly

Several species of whiteflies may infest lettuce. Only sweetpotato whitefly can cause economically important damage. Use a hand lens to distinguish sweetpotato whitefly from other whiteflies by examining both immature and adult stages.

Whiteflies are found mostly on the undersides of leaves. They fly readily when plants are disturbed. The tiny, oval eggs hatch into a first larval stage that has legs and antennae and is mobile. Both legs and antennae are lost after the first molt and subsequent stages remain fixed to the leaf surface. The last nymphal stage, often called the pupa or the "red-eye" nymph, is the stage that is easiest to identify. Sweetpotato whitefly pupae are oval, whitish, and soft. The edge of the pupa tapers down to the leaf surface and has few to no long waxy filaments around the edge. In contrast, greenhouse whitefly pupae have many long waxy filament around the edge and the edge is somewhat vertical where it contacts the leaf surface.

Sweetpotato whitefly is a major pest in the low desert lettuce production areas both due to direct feeding damage and virus transmission. It is not a major pest in the Salinas and San Joaquin Valleys. It does not occur on spring lettuce and generally only very low numbers occur on fall lettuce. Feeding by whiteflies produces sticky honeydew on the leaves. A black, sooty mold can often develop on the excreted honeydew. Sweetpotato whitefly feeding may cause a stunting and yellowing of head lettuce.

Several wasps, including species in the Encarsia and Eretmocerus genera, parasitize whiteflies. Whitefly nymphs are also preyed upon by bigeyed bugs, lacewing larvae, and lady beetles. Sweetpotato whitefly is an introduced pest that has escaped its natural enemies. Some indigenous native parasites and predators do attack it, but do not keep it below damaging numbers. The lady beetle Delphastus pusillus is being introduced into southern California to enhance biological control in the area.

Plant your earliest lettuce at least one-half mile upwind from cotton, melon, or cole crop fields. Maintain proper field sanitation by destroying crop residues that may harbor whiteflies after harvest. Remove weed that host the whitefly and virus. Whitefly numbers in desert areas decrease in mid-October and November, so whenever possible, delay planting to avoid infestation. Present research indicates sprinklers may reduce whitefly numbers and virus incidence.

Use biological and cultural controls and sprays of azadirachtin or entomopathogenic fungus in an organically certified crop.

To prevent sweetpotato whitefly infestation:

• Routinely check field margins for whiteflies; these areas are usually infested first.
• Be especially alert for rapid number increase when nearby host crops senesce (dry out) or are harvested. During these critical periods, check lettuce fields twice weekly.
• Use sticky traps to detect initial whitefly migrations into fields.
• Allow natural enemies an opportunity to manage light whitefly infestations.
• When higher numbers are present at the field margins and not the field centers, use targeted insecticide applications only to the field margins which will slow movement of whiteflies into the field for a short period. This approach will reduce treatment costs and help preserve natural enemies in the field.

Thresholds are not available for sweetpotato whitefly in lettuce.

The most effective treatment is to apply imidacloprid (Admire) at planting. This application will provide control in fields where sweetpotato whitefly problems are anticipated.

Whitefly control with insecticides is maximized by thorough spray coverage. Ground application may give more complete coverage than air.