How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

Silverleaf 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 silverleaf 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. 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 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. Silverleaf 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 bandedwinged whitefly pupae have many long waxy filaments around the edge and the edge is somewhat vertical where it contacts the leaf surface.

Silverleaf whitefly problems in cotton develop from populations that overwinter in cole crops, ornamentals, and weeds. Populations often build in spring melons and migrate to cotton. Once temperatures warm up in summer, populations can build rapidly with the highest populations occurring in mid- to late summer.

DAMAGE

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 silverleaf whitefly vectors the cotton leaf crumple virus in southern California desert valleys.

MANAGEMENT

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.

Biological Control
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. Silverleaf 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.

Cultural Control
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/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 silverleaf whitefly than upland cotton varieties. Of the upland varieties, hairy leaf cottons are more susceptible than smooth-leaf varieties.

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

Resistance
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.

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 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 (3^rd and 4^th 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. Monitoring forms are available on the online version of this guideline.

The treatment threshold is 40% leaves infested with large nymphs or 40% 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 silverleaf 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 silverleaf 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/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], chlorpyrifos [Lorsban], oxamyl [Vydate], endosulfan [Thionex]) 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/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 silverleaf whitefly from other cotton fields, use a pyrethroid such as bifenthrin (Capture]) or fenpropathrin (Danitol) in combination with DEF, endosulfan, or 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.

PUBLICATION

UC IPM Pest Management Guidelines: Cotton
UC ANR Publication 3444
Insects and Mites
L. D. Godfrey, Entomology, UC Davis
P. B. Goodell, UC IPM Program, Kearney Agricultural Center, Parlier
E. T. Natwick, UC Cooperative Extension, Imperial Co.
D. R. Haviland, UC IPM Program, Kern Co.
Acknowledgment for contributions to the insects and mites section:
E. E. Grafton-Cardwell, Kearney Agricultural Center, Parlier
N. C. Toscano, Entomology, UC Davis

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