How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

Several species of whiteflies may infest lettuce. Use a hand lens to distinguish silverleaf whitefly from other whiteflies by examining both immatures and adults. Silverleaf whitefly adults are tiny (0.06 inch, 1.5 mm long), yellowish insects with white wings. Their wings are held somewhat vertically tilted, or rooflike, over the body and generally do not meet over the back but have a small space separating them. Greenhouse whitefly (Trialeurodes vaporariorum) adults, the species that are most similar in appearance, hold their wings flatter over the back and there is no space where the two wings meet in the center of the back. Banded whiteflies (Trialeurodes abutilonia) have brownish bands across their wings.

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. Silverleaf 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.

DAMAGE

Lettuce is not a major host of silverleaf whitefly in the San Joaquin Valley. It does not occur on spring lettuce and generally only very low populations occur on fall lettuce. Feeding by whiteflies produces sticky honeydew on the leaves. A black, sooty mold often grows on the excreted honeydew. Silverleaf whitefly feeding can cause a stunting and yellowing of head lettuce.

MANAGEMENT

Biological Control
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. Silverleaf 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 assist in biological control.

Cultural Control
Plant your earliest lettuce at least one-half mile upwind from cotton, melon, or cole crop fields. Destroy crop residues from these crops because they may harbor whiteflies after harvest. Remove weeds that host the whitefly and the virus. Whitefly populations in desert areas decrease in mid-October and November, so whenever possible, delay planting to avoid infestation. Present research indicates sprinklers may reduce whitefly populations and virus incidence.

Organically Acceptable Methods
Biological and cultural controls are organically acceptable.

Monitoring and Treatment Decisions
Routinely check field margins for whiteflies; these areas are usually infested first. Be especially alert for rapid population build up when nearby host crops are in decline. During these critical periods, check lettuce fields twice weekly. Sticky traps may be useful in detecting initial whitefly migrations into fields. Allow beneficials an opportunity to control light whitefly infestations. If higher populations are present at the field margins than the field centers, then treat only the field margins. This approach will reduce treatment costs and help preserve beneficials in the field. Thresholds are not available for silverleaf whitefly in lettuce.

The most effective treatment is to apply imidacloprid (Admire) at planting. This application will provide control in fields where silverleaf whitefly problems are anticipated. Otherwise, an application of bifenthrin (Capture) can provide acceptable control of light populations. For moderate-to-heavy populations, use imidacloprid at planting or methomyl (Lannate) during the crop season. Rotate classes of insecticides to manage resistance. This includes all insecticides used in the field, including those used for other insect pests during the current season.

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

PUBLICATION

UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450
Insects and Other Arthropods
E. T. Natwick, UC Cooperative Extension, Imperial Co.
W. E. Chaney, UC Cooperative Extension, Monterey Co.
Acknowledgment for contributions to the insects and mites section:
N. C. Toscano, Entomology, UC Riverside

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