How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

Several species of whiteflies may infest cole crops. Proper identification of silverleaf whitefly is important because other whitefly species do not cause economic damage in cole crops. Use a hand lens to examine both immatures and adults. Silverleaf whitefly adults are tiny (0.06 inch or 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 is most similar in appearance, hold their wings flatter over the back, and there is no space between them where the wings meet in the center of the back.

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

DAMAGE

Whiteflies damage cole crops by sucking enormous quantities of sap and covering plants with sticky honeydew. Black sooty mold grows over the honeydew, lowering the photosynthetic capacity of the plant. Feeding by silverleaf whitefly stunts plant growth and development; as a result harvest may be delayed. Silverleaf whitefly feeding on broccoli causes a bleaching or whitening in stems and leaf petioles.

MANAGEMENT

Silverleaf whitefly is a major problem in California's southern desert and an increasing problem in the southern San Joaquin Valley. Biological control can be helpful in controlling light populations of this pest, and cultural practices are important in helping to prevent severe infestation. Monitor for silverleaf whitefly and apply insecticides when necessary. If whiteflies are migrating into the field, treatments to field borders may be adequate.

Biological Control
Silverleaf whitefly is an introduced pest that has escaped its natural enemies. Some native parasites and predators do attack it, but do not keep it below damaging numbers. These include several wasps in the Encarsia and Eretmocerus genera and predatory bigeyed bugs, lacewing larvae, and lady beetles that feed on nymphs. Several exotic species of parasites in the Encarsia and Eretmocerus genera also have been introduced into southern California to assist in biological control.

Cultural Control
Populations peak in late summer and begin to decrease by November. Delaying planting or using host-free periods may decrease severity of attack. When possible, plant cole crops at least one-half mile upwind from other key whitefly hosts, such as melons and cotton. Maintain good sanitation in winter and spring host plants. Attempt to produce the crop in the shortest season possible; proper management of irrigation and nitrogen will assist in this. Remove/destroy all crop residue as soon as possible.

Organically Acceptable Methods
Biological and cultural control, and sprays of insecticidal soap and narrow range oils are organically acceptable management tools.

Monitoring and Treatment Decisions
Routinely check field margins for whiteflies; these areas are usually infested first. Be especially alert for rapid population buildup when nearby host crops are in decline. During these critical periods, check cole crop 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 cole crops.

Effective treatments consist of combining a pyrethroid (bifenthrin-Capture) with a cyclodiene (endosulfan-Thiodan), carbamate (methomyl-Lannate), or organophosphate (acephate-Orthene, chlorpyrifos-Lorsban). Bifenthrin and endosulfan provide acceptable control of light populations when used alone, but use combinations of materials on moderate to heavy populations. An alternative to combining insecticides is to use one of the neonicotinoids (imidacloprid-Admire, or acetamiprid-Assail). Insecticidal soaps and oils are not as effective as other materials and require frequent applications and excellent coverage.

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: Cole Crops
UC ANR Publication 3442
Insects
W. E. Chaney, UC Cooperative Extension, Monterey Co.
E. T. Natwick, UC Cooperative Extension, Imperial Co.
Acknowledgments for contributions to the insects section:
W. J. Bentley, UC IPM Program, Kearney Agricultural Center, Parlier
N. C. Toscano, Entomology, UC Riverside

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