Whiteflies

Whiteflies (family Aleyrodidae) suck phloem sap and feed in groups on succulent shoots and the underside of leaves. Whiteflies develop through three life stages. Eggs hatch into mobile first instar (crawlers). After first instars (nymphs) settle to feed, whiteflies become immobile. Nymphs develop through four, increasingly larger instars. After maturing to the fourth instar (also called a pupa) a winged adult emerges.

Adult whiteflies have an orangish to yellow body and translucent wings that are thinly covered with powdery, white wax. At rest the adult resembles a tiny, whitish housefly or moth. Adults of many species at rest are about 1/16 inch long from head to wing tips, but giant whitefly adults are about 3/16 inch long.

Numerous whitefly species can infest ornamental crops in California. Whiteflies are identified to species by the appearance of older nymphs and especially fourth instars (pupae), including body coloration, spiracles (breathing tubes), and waxy projections. The adults of some species have dark wing markings that can also help determine the species. For side-by-side illustrations discriminating the adults and pupae of eight whitefly species that can infest flower and nursery crops see here. For a photo collage of whitefly adults and pupae and lists of their common ornamental host plants in California see Pest Notes: Whiteflies.

Bemisia spp. and greenhouse whitefly are the most common pest species. Greenhouse whitefly holds its wings relative flat (parallel to leaf surfaces). Bemisia adults generally hold wings more steeply pitched, rooflike over the body. This generally results in relatively more of the yellowish body of Bemisia being visible through a gap between wings unlike adult greenhouse whiteflies when viewed from above. Their distinctly different pupae best help to differentiate these species.

Whitefly eggs are oval, 1/100 inch (0.2 mm) long, and laid openly on plants. Eggs may occur singly on plant tissue or grouped in circles or crescents. When first laid eggs are pale green, yellowish, or white. Eggs darken to grayish before hatching. Whiteflies have multiple generations per year.

Greenhouse whitefly pupae are oval, pale whitish, and elevated in profile with edges perpendicular to the plant surface. The lower perimeter has a fringe of short filaments and the upper surface has relatively long, projecting filaments. Development from egg to adult takes about 21 to 26 days at 81°F.

Silverleaf and sweetpotato whiteflies is a group (complex) of species that cannot be separated by appearance, but they may differ in important ways. Sweetpotato whitely (Bemisia tabaci Q biotype) is highly resistant to numerous insecticides. Silverleaf whitefly (Bemisia tabaci B biotype, or Bemisia argentifolii) generally is less resistant to insecticides.

Bemisia pupae are oval, yellowish, slightly tapered at the rear end, and have few to no obvious marginal filaments. Tiny filaments may project from the top surface of nymphs, especially on hairy leaves; but these filaments are not obvious to the naked eye, especially in comparison with the long filaments of greenhouse whitefly. Development from egg to adult takes about 16 and 31 days when temperatures average 86° and 68°F, respectively.

Giant whitefly is larger than most whiteflies. Adults can be up to 3/16 inch long and have multiple grayish blotches on their whitish wings. Nymphs produce pendulous, long, hairlike filaments of wax up to 2 inches long that give infested foliage a bearded appearance and may be mistaken for a leaf fungus. The pupae are oval and raised in profile with sides perpendicular to the leaf surface. Adults and immatures can feed on over 60 plant species. Hosts most commonly infested include aralia, begonia, hibiscus, giant bird of paradise, mulberry, orchid tree, and xylosma. Introduced parasites have greatly reduced its abundance.

Ficus whitefly especially infests Ficus microcarpa, but also occurs on other fig species. Adults are about 1/16 inch long with grayish brown markings on whitish wings. The pale brown to yellow eggs occur mostly along the main vein on the underside of leaves. Nymphs resemble a scale insect. They are pale green to tan, flat, oval, and semi-transparent, blending with the color of the surrounding leaf surface.

Similar to aphids and psyllids, whitefly adults and nymphs suck phloem sap and excrete sticky honeydew. Honeydew attracts ants and supports the growth of blackish sooty mold (fungi). High populations of adults can annoy workers and customers. Whitefly feeding can stunt (slow) plant growth and cause heavily infested leaves to die and drop prematurely. Salivary secretions of Bemisia spp. and certain other whiteflies can cause physiological abnormalities in plants, such as white stem malady on poinsettia caused by Bemisia feeding.

Manage whiteflies with biological and cultural controls and the application of effective insecticides. Pesticide resistance is especially a problem with Bemisia tabaci Q biotype; certain scientific laboratories can perform genetic tests to identify which biotype(s) of Bemisia tabaci are present to help in selecting controls. When employing insecticides, rotate to a different mode of action every 21 days or sooner. Include insecticides such as insecticidal soap, horticultural oil, and microbials to minimize selection for pesticide resistance. For a detailed discussion of effective insecticide use, see Chemical Class Rotations for Control of Bemisia tabaci (Hemiptera: Aleyrodidae) on Poinsettia and Their Effect on Cryptic Species Population Composition (PDF).

Numerous species of parasitic wasps (e.g., Encarsia and Eretmocerus spp.) feed on whiteflies and generally cause the populations of most whitely species to remain low in the field. Predators of whiteflies include adults and larvae of small lady beetles, bigeyed bugs, brown lacewings, dustywings, green lacewings, minute pirate bugs, and larvae of syrphid flies. Former pests now controlled by introduced natural enemies include ash whitefly (Siphoninus phillyreae) and bayberry whitefly (Parabemisia myricae). In many situations giant whitefly and woolly whitefly (Aleurothrixus floccosus) are also satisfactorily controlled by parasitic wasps.

Where broad-spectrum, residual (persistent) insecticides are avoided for all pests, resident natural enemies will be more effective. Certain natural enemies can be purchased and released to control particular whiteflies. Encarsia formosa, a black and yellowish parasitic wasp about 1/16 inch (1.5 mm) long, is highly effective for controlling greenhouse whitefly. Begin any releases when greenhouse whiteflies are first observed or even before and in any case before populations are high. When greenhouse whiteflies are first observed and early during the crop production cycle, a general recommendation is to release 2 to 5 adult parasites per plant per week for 8 to 10 weeks. Greenhouse whitefly pupae normally are pale yellowish to white, but turn black when parasitized by Encarsia formosa. Unlike the irregular slit left when an adult whitefly emergences, emerging parasitic wasps leave a circular exit hole in whitefly pupae they have killed.

Adults and larvae of the Delphastus pusillus lady beetle and certain other natural enemies may also be available for release to help control particular whiteflies. Consult The Biology and Management of the Silverleaf Whitefly, Bemisia argentifolii Bellows and Perring (Homoptera: Aleyrodidae) on Greenhouse Grown Ornamentals from UC Riverside for details on using natural enemies to manage Bemisia. For more information, see Biological Control, and Releasing Natural Enemies for Biological Control of Agricultural Pests.

Carefully inspect new plants and propagation stock to ensure they are pest free. Rogue or effectively treat infested plants. Practice excellent sanitation. Keep production areas free of weeds, which can serve as hosts for whitefly populations. Promptly remove and dispose of crop debris and weeds in covered containers.

Exclude adult whiteflies by growing field crops beneath floating row covers or exclusion netting held above the plant canopy or in screened hoop houses. Cover greenhouse vents and other openings with screens with a pore width of 405 microns (0.405 mm, about 4/250 inch) or smaller. Install double doors with positive-pressure ventilation (air constantly flowing outwards) to reduce pest movement into growing enclosures.

Reflective mulch

For field-grown crops, applying reflective mulch in row middles or entirely covering the soil surface and planting through holes in the mulch can greatly reduce the extent of whitefly infestation while plants are small. Reflective mulch also reduces the extent of infection from insect-vectored viruses when crops are young and most susceptible to these plant pathogens. Reflective mulch can increase crop growth and cut-flower yield and reduce need for irrigation by conserving soil moisture. See REFLECTIVE MULCHES for more information.

Biological and cultural controls are organically acceptable management methods. Certain formulations of the botanically derived azadirachtin (Azatin), neem oil, and pyrethrins without piperonyl butoxide (PyGanic), the microbials Beauveria bassiana and Isaria fumosorosea, certain narrow-range oils (Organic JMS Stylet-Oil), and potassium salts of fatty acids (insecticidal soap) are acceptable for organic production.

Place yellow sticky traps throughout growing areas to capture and monitor the relative abundance of adult whiteflies. In crops most susceptible to whiteflies, deploy 1 trap per 1,000 sq. ft. of growing area. In crops less susceptible, 1 trap per 10,000 sq. ft. of growing area can be sufficient. Also visually inspect crop foliage at least weekly for whitefly adults, eggs, nymphs, and pupae. Using a 10X hand lens can increase the chance of seeing the early instars most susceptible to insecticides.Examine both the underside of leaves and the tips of green shoots for whiteflies. For more information, see Monitoring with Sticky Traps and Sticky Trap Monitoring of Insect Pests.

Action thresholds vary with the crop. For example, cut flowers such as gerbera can tolerate more whiteflies than poinsettias since only the gerbera flowers are harvested and marketed. For more information, see Establishing Action Thresholds.

Selected Products Registered for Greenhouse or Nursery Ornamentals