How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

The most common species of whitefly infesting potato is the silverleaf whitefly. The adults are tiny (0.06 inch, 1.5 mm long), yellowish insects with white wings. Silverleaf whiteflies hold their wings somewhat vertically tilted, or rooflike, over the body and the wings do not meet over the back but have a small space separating them. Other species of whitefly have been observed in potatoes but their populations tend to be localized within the field and do not cause damage. Populations of silverleaf whitefly can be found relatively uniformly throughout the field in fall plantings.

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. Mature nymphs of silverleaf whitefly are oval, whitish, soft, and have few to no long waxy filaments. In contrast, greenhouse whitefly, Trialeurodes vaporariorum, has many long waxy filaments and the edge of the body is somewhat vertical where it contacts the leaf surface.

DAMAGE

Silverleaf whitefly damages leaves by feeding, which causes leaves to yellow and curl, and by the production of honeydew, which causes leaves to appear shiny or blackened (from sooty mold growing on the honeydew). Damage is similar to that caused by aphid feeding: they debilitate the plants. Whiteflies cause the most damage to winter-harvested potatoes in the southern San Joaquin Valley. Fields near defoliated cotton can be severely infested.

MANAGEMENT

Whitefly populations are often held in check by beneficial insects. If populations do reach high levels, it may be necessary to treat in fall.

Biological Control
Several wasps, including species in the Encarsia and Eretmocerus spp., 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
When possible, plant potatoes at least one-half mile upwind from key silverleaf whitefly hosts such as melons, cole crops, and cotton. Maintain good sanitation in areas of winter/spring host crops and weeds by destroying and removing all crop residues as soon as possible. Control weeds in noncrop areas including hedge rows and fallow fields and harvest alfalfa on as short a schedule as possible. In addition, allow the maximum time between whitefly host crops and produce vegetables and melons in the shortest season possible.

Organically Acceptable Methods
Biological and cultural controls are acceptable to use on an organically certified crop.

Monitoring and Treatment Decisions
Routinely check field margins for whiteflies; these areas are usually infested first. Record your results (example form—108 KB, PDF). Be especially alert for rapid population build up when nearby host crops are in decline. During these critical periods, check fields twice weekly. If beneficials are present, allow them 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. Treatment thresholds have not been determined for silverleaf whitefly in potato, but potatoes can take large populations of whitefly before treatment is necessary. If populations reach high levels in fall, a treatment may be warranted.

• Summary of Important Links

IMPORTANT LINKS

• Surveying foliage pests form (108KB, PDF)

PUBLICATION

UC IPM Pest Management Guidelines: Potato
UC ANR Publication 3463
Insects
L. D. Godfrey, Entomology, UC Davis
D. R. Haviland, UC Cooperative Extension, Kern Co.

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