Whiteflies

Several species of whiteflies may infest tomato. Proper identification of sweetpotato whiteflies and greenhouse whiteflies is important because other whitefly species do not cause economic damage in tomato. Use a hand lens to examine both immatures and adults. Whitefly adults are tiny (0.06 inch, 1.5 mm long), yellowish insects with white wings. Sweetpotato whiteflies hold their wings somewhat vertically tilted, or rooflike, over the body; the wings do not meet over the back but have a small space separating them. Greenhouse whitefly adults are very similar in appearance to the sweetpotato whitefly but hold their wings flatter over the back and there is no space between the wings where they meet in the center of the back. Bandedwinged whiteflies, Trialeurodes abutilonia, have grayish or brownish bands across their wings.

Whiteflies are found mostly on the undersides of leaves. They fly readily when plants are disturbed. The tiny, elongated eggs hatch into a first instar nymphal stage that has legs and antennae and is mobile; first instar nymphs are commonly referred to as "crawlers". Both legs and antennae are lost after the first molt and subsequent nymphal instars remain fixed to the leaf surface and have a scalelike appearance. The last nymphal instar, often called the pupa or the red-eyed nymph, is the 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.

Currently sweetpotato whitefly is only a problem of tomatoes grown in southern California and areas of the southern and central San Joaquin Valley. Greenhouse whiteflies are found in all but the lower desert growing areas.

Both species of whitefly cause damage to 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). Feeding by sweetpotato whitefly is especially damaging because it also causes fruit to ripen unevenly.

In recent years, the greenhouse whitefly has been found to be the vector of tomato infectious chlorosis virus, a virus capable of causing heavy losses in the production of fresh-market and greenhouse tomatoes.

Bemisia species of whiteflies transmit gemini viruses such as that cause TOMATO YELLOW LEAF CURL, which has recently been found in the Imperial Valley and Coachella Valley. The spread of this virus in the state is a major threat to tomato production. To prevent the spread of Tomato yellow leaf curl virus into other areas of California, do not bring transplants into California from out-of-state or move transplants or other Bemisia-infested hosts from an area that is known to be infested with the disease to uninfested areas. Use only virus-free tomato transplants grown and shipped from the Imperial Valley, which are monitored by CDFA and the county agricultural commissioner's office.

An integrated pest management program for whiteflies includes following good cultural practices, such as host-free periods, conserving natural enemies, routinely monitoring fields for trouble spots, and using pesticides only when necessary.

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 beetle larvae. 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 parasitic wasp, Encarsia formosa, has been used successfully to control greenhouse whitefly in greenhouses or protected crop situations elsewhere in the world where tomatoes are more commonly grown in this manner.

The best control for whiteflies is to maximize the distance and time interval between host crops.

• When possible, plant tomatoes at least one-half mile upwind from key sweetpotato whitefly hosts such as melons, cole crops, and cotton.
• Maintain good sanitation in areas of winter and spring host crops and weeds by destroying and removing all crop residues as soon as possible. Control weeds in noncrop areas including head rows (headland areas) and fallow fields.
• Harvest alfalfa on as short a schedule as possible.
• Allow the maximum time between harvest and subsequent planting of sweetpotato whitefly host crops.
• Grow vegetables and melons in the shortest time-span possible.

Adult sweetpotato whiteflies are repelled by silver- or aluminum-colored mulches. Place reflective polyethylene mulches on planting beds before seeding or transplanting to significantly reduce rate of colonization by whiteflies and delay the buildup of damaging numbers of whiteflies by 4 to 6 weeks. This delay in infestation can be especially important if virus transmission is a major concern. The mulches lose their effectiveness when more than 60% of the surface is covered by foliage. Therefore, they are effective only for the first few weeks after seedling emergence or transplanting of either spring or fall tomatoes.

Greenhouse whiteflies are often induced by applications of broad-spectrum pesticides. Avoid such materials early in the season.

Cultural and biological control as well as sprays of insecticidal soaps and oil plus azadirachtin are acceptable for use on organically certified produce.

Routinely check field margins for whiteflies; these areas are usually infested first. Be especially alert for the rapid migration of whiteflies when nearby host crops are in decline. During these critical periods, check fields twice weekly. Yellow 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. The treatment threshold for sweetpotato whitefly is about 4 adults per leaf in a random 30-leaf sample of healthy leaves. Thresholds have not yet been established for greenhouse whitefly.

• Photos of whiteflies