Sweetpotato Whitefly (Silverleaf Whitefly)

Sweetpotato whitefly is a major problem in California’s southern desert and in the San Joaquin Valley.

Several species of whiteflies may infest cotton. Proper identification of sweetpotato whitefly is important because other whitefly species do not usually cause economic damage in cotton. Use a hand lens to examine both immatures and adults. Sweetpotato 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 separating the two wings where they join. Bandedwinged whiteflies (Trialeurodes abutiloneus) 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 nymphal stage that has legs and antennae and is mobile. The 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 and yellowish with red eye spots. The edge of the pupae tapers down to the leaf surface and has few to no long waxy filaments around the edge. In contrast, greenhouse whitefly and banded-winged whitefly pupae have many long waxy filaments around the edge and the edge is somewhat vertical where it contacts the leaf surface.

Sweetpotato whitefly is a multihost pest. Problems in cotton develop from sweetpotato whiteflies that overwinter in cole crops, ornamentals, and weeds. Numbers often increase in spring melons. Once these alternative host crops are harvested or destroyed, whiteflies migrate into adjacent cotton fields. As temperatures warm up, numbers rapidly increase, with the highest numbers occurring in mid- to late summer.

Whiteflies are sucking insects and their feeding removes nutrients from the plant. Feeding by high populations may result in stunting, poor growth, defoliation, boll shed and reduced yields. As they feed, whiteflies produce large quantities of honeydew which, if deposited on fibers, will reduce cotton quality and may interfere with picking, ginning, and spinning. Honeydew also supports the growth of black sooty molds that stain lint, lowering its quality. The sweetpotato whitefly vectors the Cotton leaf crumple virus in southern California desert valleys.

Whiteflies are difficult to manage once their populations have reached high levels. Repeated exposure to insecticide treatments is very likely to lead to development of resistant strains. In general, the best approach is an integrated pest management strategy that relies first on cultural and biological control methods and uses chemical controls only when needed.

Several wasps, including species in the genera Encarsia and Eretmocerus, 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, such as the sevenspotted lady beetle, Coccinella septempunctata, do attack it but do not keep it below damaging numbers.

When possible, plant cotton at least one-half mile upwind from other key whitefly hosts such as melons, cole crops, and tomatoes. 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 and fallow fields, and harvest alfalfa on as short a schedule as possible. Before destroying weeds, however, check them for whitefly predators and parasites because they can be an important source of these natural enemies. In addition, allow the maximum time between whitefly host crops and produce vegetables and melons in the shortest season possible.

Where whitefly infestations are severe, plan for early crop termination and defoliation. After harvesting, promptly destroy stalks to prevent regrowth and limit additional whitefly buildup. Use glyphosate with the defoliation treatment to reduce regrowth.

Acala varieties, which require less time to mature than Pima varieties, may have fewer whitefly infestations. In general, all Pima varieties are more attractive to sweetpotato whitefly than upland cotton varieties. Of the upland varieties, hairy-leaf cottons are more susceptible than smooth-leaf varieties.

Cultural and biological controls and sprays of insecticidal soap, some oils, and azadirachtin are acceptable for use on organically grown cotton.

Some pyrethroid and organophosphate insecticides have lost their effectiveness for controlling whiteflies. Repeated applications of a product may build high resistance levels in whiteflies. To delay and manage resistance, do not treat successive generations of whiteflies with same product or with insecticides that have the same mode of action number; rotate insecticides with a different mode of action number during the season.

Unsprayed alternative host crops, from which sweetpotato whiteflies migrate into sprayed cotton fields, may be an important source of susceptible whitefly genes and therefore may act as resistance management agents.

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 cotton fields twice weekly. Whitefly adults and nymphs need to be monitored on undersides of leaves from early squaring to harvest. Check for whitefly adults on undersides of leaves—if 3 or more are found, rate the leaf as infested. For whitefly nymphs, place a quarter-sized ring between the central and left-side main veins and check for presence or absence of large nymphs. Score the leaf as infested if any large nymphs are present (3rd and 4th instars) within the quarter-sized ring.

To improve efficiency of your monitoring program, combine sampling of spider mites with other pests. From early squaring to boll development, combine sampling for spider mites, aphids, and whitefly as described in MONITORING SPIDER MITES, APHIDS, AND WHITEFLY. From open boll to harvest follow guidelines described under MONITORING FOR APHIDS AND WHITEFLY.

The treatment threshold is 40% of leaves infested with large nymphs or 40% of leaves infested with whitefly adults—but remember, a leaf is not called "infested" unless at least 3 whitefly adults are present. If using insect growth regulators (IGRs), nymphs must also be present to justify treatment. If high numbers of adults are at field edges, but no nymphs, an edge treatment with a non-IGR may be required.

Early-season treatments for sweetpotato whitefly nymphs should be limited to IGRs (buprofezin [Courier], pyriproxyfen [Knack], or spiromesifin [Oberon]), or nonpyrethroid insecticides. Pyrethroids should not be used until later in the season when the bolls are open, because they increase populations of spider mites and aphids by causing them to reproduce faster; they are more toxic to natural enemies of aphids, spider mites, and sweetpotato whiteflies than the other materials; and they are most effective against adult whiteflies, whereas nonpyrethroids are most effective against nymphs.

The IGRs (buprofezin and pyriproxyfen) may be applied only once per season; an application of either one may provide up to 6 weeks of whitefly control. Spiromesifen may be applied twice during the season. Sample carefully to be sure that an application is needed before applying IGRs, and use only full-field treatments. Use of insect growth regulators for whitefly control can reduce outbreaks of mites and aphids because of their selectivity.

In fields where whitefly populations are migrating in from overwintering sites or from other cotton fields and adults and eggs are present but nymphs are rare, a nonpyrethroid (acetamiprid [Assail], oxamyl [Vydate]) treatment can be used. In fields with young plants, an IGR may also be required after immigration from overwintering sites has subsided. Edge treatments of nonpyrethroids may also be useful under these conditions. Treat when leaf-turn samples average 10 or more adult whiteflies per leaf. If higher populations are present at the field margins than in the field centers, treat only field margins; this will help reduce cost as well as preserve beneficials. Treatment usually can be delayed until mid-July in the San Joaquin Valley. Holding off treatment until mid-July also reduces selection for pesticide resistance, which can develop rapidly in this pest.

Later in the season when bolls are open and lint is exposed, if there is a massive influx of sweetpotato whitefly from other cotton fields, use a pyrethroid such as bifenthrin (GCS Bifenthrin 2EC) or fenpropathrin (Danitol) in combination with an organophosphate to provide quick knockdown of adults.

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.

• Leaf ring sampling for whitefly nymphs