Whiteflies

Available natural enemies and their potential for control

There are a few available parasites of whiteflies. Some predators and pathogens can be used along with the parasites to improve control.The potential for successful biological control varies from moderate to high.

Order Homoptera: Aphids, leafhop-pers, planthoppers, mealybugs, scales, and whiteflies

Family Aleyrodidae: Whiteflies

Greenhouse whitefly, Trialeurodes vaporariorum

Sweetpotato whitefly,

Bemisia tabaci

Silverleaf whitefly,

Bemisia argentifolia

Bandedwing whitefly,

Trialeurodes abutilonea

Wh hiteflies infest a wide range of greenhouse crops.The greenhouse whitefly is a tropical species that has become established worldwide in greenhouses. It is a particularly destructive pest of cucumber, tomato, fuchsia, geranium, hibiscus, gerbera, and poinsettia. It attacks hundreds of other vegetable and ornamental crops.The greenhouse whitefly is the most common species found in northern greenhouses.The sweetpotato whitefly, from tropical and warm temperate regions, is also worldwide in distribution and attacks hundreds of ornamental plant species.The silverleaf whitefly, formerly known as the B strain of the sweetpotato whitefly, attacks poinsettia, hibiscus, rose, and other plants. It extracts much more sap than does the sweetpotato whitefly and also causes squash silverleaf, phytotoxemia in poinsettia, and irregular ripening disorders in tomato. Until 1993 the silverleaf whitefly was not distinguished from the sweetpotato whitefly, so much of the literature concerning sweetpotato whitefly may actually refer to silverleaf whitefly.The bandedwing whitefly is less common than the greenhouse, sweetpotato, or silverleaf whiteflies but occurs on many plant species. It commonly enters the greenhouse in late summer and early fall.

Damage

Whiteflies damage plants by removing plant sap and excreting large quantities of honeydew.The clear, sticky honeydew is often seen on the lower leaves. Honeydew serves as a medium for growth of black sooty mold fungus which interferes with photosynthesis and transpiration, and detracts from the beauty and marketability of the crops. Large numbers of adults and nymphs feeding on the leaves cause the leaves to wilt or turn yellow and drop, and may seriously reduce the yield of vegetables.

On ornamentals, whiteflies are pests primarily because their presence reduces the aesthetic value of a plant. Heavy infestations of silverleaf whitefly can cause stem and bract whitening on red poinsettia cultivars.

Whiteflies also act as efficient vectors of several viral diseases.The greenhouse whitefly can transmit the beet pseudo yellow virus to cucumbers, while the sweetpotato whitefly can transmit several viruses such as those causing tomato yellow leaf curl and cucumber vein yellowing.

Description and life cycle

Adult whiteflies are about 1/16 inch (1-3 mm) long and covered with a white, waxy powder. Most species appear very similar, but they can be distinguished from each other. Greenhouse whiteflies hold their wings rather flat over the abdomen. Sweetpotato and silverleaf whiteflies hold their wings roof-like and close to the sides of the abdomen.The bandedwing whitefly has light gray bars across the wings.The adults of all these species congregate on the undersides of leaves, usually at the top of the plant.

Female greenhouse whiteflies lay 6-20 eggs daily, in a perfect circle or a portion of a circle, on the underside of the leaf. The cigar-shaped eggs are deposited as the female moves around in a circle with her mouthparts inserted in the plant as a pivot. Sweetpotato whitefly eggs are laid randomly, in small groups or singly, which makes them more difficult to detect.The eggs are white when first laid but later turn dark brown.They hatch in 7-10 days into mobile first-instar nymphs —"crawlers"that resemble scale crawlers—which search for a suitable place to settle, seldom moving more than 3/4 inch from the eggshell.The next two instars are sessile, translucent green or yellow, rather flat, scale-like, and inconspicuous.The fourth and final instar, often referred to as the pupa (although this species undergoes simple metamorphosis), becomes opaque yellow and mounded.

The nymphs and pupae of many species have distinctive white, waxy spines, but the sweetpotato whitefly has no spines. Greenhouse whitefly pupae are cake-shaped with erect side walls, whereas the sweetpotato and silverleaf whitefly pupae are mounded. In the greenhouse the generations usually overlap completely. Greenhouse whiteflies develop best at moderate temperatures around 75°F, while sweetpotato and silverleaf whiteflies prefer temperatures over 81°F.

Monitoring

Biological control of whiteflies will be difficult unless the infestation is detected early.The greenhouse should be thoroughly inspected for problems before the crop is introduced. Check hanging baskets and weeds under the benches. Begin monitoring susceptible

greenhouse crops at planting.Whiteflies are generally not found uniformly throughout a crop at first.They will be concentrated in a few areas and many areas will be without insects. As the population grows, the infested areas expand and multiply.

You should monitor whiteflies in two ways. Use yellow sticky traps to detect the first adult whiteflies and to monitor adult whitefly populations. In addition, visually examine plants to determine the number of immatures present. Yellow sticky traps placed throughout the crop will detect adults at very low levels—one adult for every 10 plants. Traps are commercially available or can be made by the grower. Hang the traps just above the crop canopy to detect greenhouse whitefly.You can place the traps within the crop canopy, or even on the ground or on a greenhouse bench for detecting sweetpotato whitefly, especially in tall crops. Also place traps near doors, air-intake vents, and among newly arriving plants.The number of traps needed depends on the crop, but on average space them 45-60 ft apart (one per 1000 ft2). Check the traps once or twice weekly. Counting the whiteflies on a 1-inch wide vertical band of either side of the sticky traps gives a good approximation of total trap densities and saves time.

Sticky traps are useful for detecting the first invasion of whiteflies, and they provide information on the relative abundance of whitefly adults. But many variables, such as trap size and shape, amount of sunlight,and air movement affect trap catches. Do not rely on traps alone to monitor whitefly populations. If adults are detected, eggs and nymphs will be present, and visual plant inspection should be performed.

You can see whiteflies most often on relatively new or young tissue.Turn the leaves over to examine the undersides for whiteflies. Adult whiteflies can easily be seen, but a 10X-15X magnifier or hand lens is helpful for observing immature stages.Whitefly populations will vary considerably on different plants and even on different cultivars of the same type of plant. Maintain scouting records of observed pest levels for all plant species and cultivars. Review these records to determine which plants should be monitored more carefully.

On many crops there can be mixed infestations of different species of whiteflies. Determine which species are present before implementing control, as natural enemy efficacy can vary depending on the whitefly species present.

If you are using Encarsia formosa wasps for biological control, monitor both the whitefly and the wasp populations. Some adult parasites will also be caught on yellow sticky traps. Learn to recognize the tiny adult wasps and black parasitized nymphs. Record parasitism rates weekly until the whitefly populations are under control, and periodically thereafter. Remove yellow sticky traps before releasing wasps and then put them back up 3-4 days after releases are made.

Natural enemies

Natural enemies of whiteflies include many parasitic wasps, predators, and several species of fungus. Exploration for and evaluation of new natural enemies, including many not mentioned below, continues.

Parasites

Several tiny parasitic wasps that attack whiteflies have been used successfully for biological control. None of these wasp families or species have common names.Those discussed here are in the families Aphelinidae and Platygastridae. The taxonomy of these wasps is very confused, and the true identify of many species or strains is questionable. The encyrtid Encarsia formosa is widely used to control the greenhouse whitefly in greenhouses. Most of the other wasps listed below are not yet commercially available.

Amitus fuscipennis. This Central American platygastrid wasp is less than V25 inch (1 mm) long.The adults are black with reddish brown legs and antennae, and brown wings.Their larvae parasitize whitefly nymphs, and the adult wasps emerge from the whitefly pupae.The females are very short-lived, but they can parasitize many hosts if they are available during this time. A. fuscipennis' effectiveness against greenhouse and sweetpotato whiteflies on poinsettia is being studied.This wasp is produced by a grower cooperative in Colombia but is not commercially available in the United States.

Encarsia formosa. This wasp originated in the semitropical areas of the New World and was one of the first recorded parasites in greenhouses. It has been used since the 1920s to control the greenhouse whitefly in warm greenhouses. E. formosa will also parasitize sweetpotato whitefly in greenhouses, but this whitefly is not a good host for this wasp, so control is not as effective. The tiny females, about 1/40 inch (0.6 mm) long, are black with a yellow abdomen and opalescent wings. Males are somewhat larger than females, completely black, and extremely rare.The parthenogenic—or asexually reproductive—females deposit 50-100 eggs individually inside the bodies of third-instar nymphs or pupae.The wasp larvae develop through four instars in about 2 weeks at optimal temperatures. As the larvae grow they kill the whiteflies. Parasitized pupae of greenhouse whitefly turn black in about 10 days, while the pupae of parasitized sweetpotato whiteflies turn amber brown. Both are easily distinguished from unparasitized pupae.Wasp larvae pupate within the whitefly body. Adults emerge about 10 days later.The adult wasps also kill whitefly nymphs by feeding on them through holes made with the ovipositor.This species is widely available commercially.

Encarsia formosa larvae have pupated and emerged as adults from these whitefly nymphs.

Encarsia luteola (=E. deserti). This species resembles E.formosa but is slightly smaller, lighter brown, and males are produced regularly. In the field it is usually found on upper leaves. It will parasitize greenhouse whitefly in the lab, but parasitized nymphs do not turn black. Researchers in southern California and Arizona are evaluating E.luteola for control of sweetpotato and silverleaf whitefly.This species is commercially available.

Encarsia pergandiella (=E. bemisiae=E. tabacivora=E. versicolor). This aphe-linid wasp, native to North America, parasitizes greenhouse, sweetpotato, and bandedwing whiteflies.The 1/40-inch (0.5-mm) adults are yellow and brown. Over a 2-week period, females lay an average of 50 eggs singly inside whitefly nymphs, preferring second or third instars. Parasitized whitefly nymphs do not turn black.The adults also feed on body fluids of third-instar nymphs and pupae.This wasp develops faster than E. formosa under cool conditions. However, its use in greenhouses may be limited because some strains hyperparasitize their own larvae or those of other Encarsia species to produce males.This wasp is not available commercially in the United States.

Other Encarsia species. Several other species of Encarsia have been mentioned as possible biological control agents for whiteflies in greenhouses.The development of these species varies only slightly from that of E.formosa. These potentially useful species include

■ E.inaron (=E.aleyrodis=E.parteno-pea),a Mediterranean species that parasitizes the pupae of greenhouse and sweetpotato whitefly;

■ E.lutea parasitizes sweetpotato whitefly pupae;

■ E.meritoria,from California, will parasitize bandedwing,greenhouse, and sweetpotato whiteflies but prefers other species, such as the iris whitefly (Aleyrodes spiraeoides). Females are golden yellow, and males are brown with yellow heads and legs;

■ E. transvena (=E.sublutea),a tropical species.The lemon-yellow adult females parasitize third-instar nymphs of greenhouse or sweetpotato whiteflies and host feed on second-instar nymphs and pupae, as well as on whitefly honeydew; and

■ E. tricolor,a European parasite of the greenhouse whitefly that oviposits in pupae, but whose efficiency in greenhouses is lower than that of E. formosa.It does not survive above 93°F.

None of these species is commercially available yet.

Eretmocerus eremicus (=californicus).

This aphelinid wasp, indigenous to desert areas of California and Arizona, is a parasite of greenhouse, bandedwing, silverleaf, and sweetpotato whiteflies. The V32-inch (0.75-mm) adult females are lemon-yellow with green eyes.The males are yellow with light-brown markings. Both sexes have very long antennae. Females lay eggs beneath whitefly nymphs of any stage but prefer the second and third instars.They will not lay eggs near whitefly pupae.The first instar wasp larvae feed externally until the host pupates.Then the wasp larvae chew inside the host and complete their development.The adults do not host feed on all species of whitefly, but do host feed on sweetpotato and silverleaf whiteflies on poin-settia.This wasp does not reproduce well on sweetpotato whitefly on poinsettia, but it reproduces very well on the same whitefly on other plants, such as hibiscus. It does best under hot conditions as would be found in late summer in desert valleys (80°-110°F),but its longevity is considerably reduced. At 70°-75°F its development is too slow to control whitefly outbreaks.This species is commercially available.

The female Eretmocerus eremicus lays an egg beneath a whitefly nymph.

Eretmocerus haldemani. This is primarily a parasite of sweetpotato and band-edwing whiteflies, although sometimes it will parasitize greenhouse whiteflies. Females are lemon yellow and males are dark yellow with brown markings. Females lay eggs beneath whitefly nymphs. First-instar wasp larvae feed externally for 3-4 days, then enter the whitefly nymph and molt to second instars.The second instars continue development only after the host pupates. Its development is similar to that of E. eremicus.It is also most effective under hot conditions.E.haldemani is not commercially available.

Eretmocerus mundus. This Mediterranean parasite of sweetpotato whitefly lays eggs under any nymphal whitefly stage, although it prefers second and third instars.The eggs hatch only after the host pupates.The wasp larvae then enter the whitefly bodies and develop. In the field they are usually found on the lower leaves of the plants. E. mundus is not commercially available.

Predators

A handful of specialists and many general predators feed on various stages of whiteflies. Only a few have been investigated for their ability to control whiteflies in greenhouses.

Chrysoperla (=Chrysopa) carnea. The larvae of green lacewings feed primarily on aphids but will also feed on immature whiteflies and other insects in the absence of aphids.They develop more slowly on whiteflies than on aphids and rarely reproduce in greenhouses. Green lacewings are available from many commercial suppliers. (See "Aphids"for a description and life cycle information.)

Chrysoperla comanche—Comanche lacewing. This is another green lacewing species that feeds on aphids and a variety of other insect pests. In the absence of aphids, the larvae feed voraciously on whitefly eggs and nymphs, and will occasionally consume adults. This species is closely related to C.rufi-labris, but is more adapted to dry conditions. It is commercially available.

Chrysoperla rufilabris. This species is similar to C. carnea in biology and development, but it is better adapted for development under humid conditions than C. carnea.This generalist predator is commercially available.

Delphastus pusillus. This small, black lady beetle feeds on bandedwing, greenhouse, silverleaf, and sweetpotato whiteflies. Females are all black, while males have a brown head.The adult females and larvae feed primarily on whitefly eggs but will also eat whitefly nymphs and spider mites. However, they do not reproduce well without whitefly eggs. Females, which can live up to 2 months, consume more than 150 whitefly eggs or nymphs per day. They lay their eggs at the tops of plants near whitefly eggs.The beetle larvae eat nearly 1000 eggs each during their 2-week developmental period.They discriminate between parasitized whitefly nymphs (those in which the parasite has developed for at least a week) and non-parasitized ones, and avoid feeding on the parasitized nymphs.They pupate on the lower leaves, in leaf litter, or in other protected locations. If they pupate in saucers at the bottom of potted plants or other places that hold water, the beetles will drown when the plants are watered.This beetle is commercially available.

Other lady beetles. Larvae and adults of Coleomegilla maculata and the convergent lady beetle (Hippodamia conver-gens) have been reported as predators of bandedwing whitefly, feeding on eggs, nymphs, and pupae. C. maculata, Coccinella septempunctata, Cycloneda sanguinea, and several others have been reported as predators of sweetpotato whitefly. None of these is as efficient as the other described predators, and they do not reproduce in greenhouses.

The tiny black lady beetle Delphastus pusillus adult consumes more than 150 whitefly eggs and nymphs per day; larvae eat nearly 1000 eggs during development.

Dicyphus tamaninii. This Mediterranean mirid bug eats a wide range of prey and is an important predator of whiteflies and other insect pests on tomatoes in southern Europe.The bugs prefer whitefly nymphs to other insects, tending to feed on prey on the underside of leaves.When prey is scarce they will feed on tomato fruit but not cucumber fruit.They require approximately 19 days to complete development at 77°F.This bug is not commercially available.

Geocorispunctipes. This 1/16-3/16 inch (2-4 mm) bigeyed bug, endemic to the southwestern United States, is usually dark brown or black. During nymphal development and as adults, they consume numerous aphids,small caterpillars, nymphal and pupal stages of whiteflies, other small insects, and spider mites. Supplementary green plant material and seeds in their diet improves development, reproduction, and survival. Sunflower seeds scattered on plants have enhanced Geocoris populations in experimental vegetable crop fields.This bug has not been investigated for use in greenhouses, but is commercially available.

Macrolophus caliginosus. This predatory mirid bug attacks all stages of whitefly, but prefers eggs and nymphs. It will also feed on aphids,and to a lesser extent spider mites, moth eggs, leafminer larvae, and thrips, but populations develop most rapidly on whiteflies. The slender adult bugs are bright green, about 1/4 inch (6 mm) long with long legs and antennae. Females lay 100-250 eggs in leaves and stems, depending on temperature and food.The eggs take about 2 weeks to hatch.The yellowish-green nymphs are found mainly on the underside of leaves where their prey is. Both nymphs and adults actively search for their prey. Adults eat 40-50 whitefly eggs per day.They insert their mouth-parts into the whitefly and suck out the contents, leaving only the skin behind. This bug will feed on plant sap, but does not cause damage on most crops. It can cause damage to gerbera flowers, and may reduce tomato fruit set on some cultivars when prey is limited.This predator is commercially available in Europe where it is widely used to control whitefly, thrips, mites, and caterpillars. It is not currently permitted to be imported into North America, although some Canadian suppliers advertise it.

Orius spp. Several species of minute pirate bugs are generalist predators that feed on whiteflies, thrips, spider mites, aphids,and caterpillar eggs,as well as pollen.They are also cannibalistic under crowded conditions.The black, 1/16-3/16 inch (2-5 mm) long adults are ovoid and somewhat flattened, with distinctively patterned black and white wings.Their eggs are laid in leaf tissues with one end sticking out.The tiny Orius nymphs are pinkish-yellow to light brown. Both nymphs and adults are very active and feed on all stages of whiteflies. Most species enter diapause under short days and lower temperatures. Several species of Orius are available commercially.

Predatory mites. Several phytoseiid mites are known predators of sweet-potato whitefly on cotton in Sudan. Amblyseius aleyrodis feeds on whitefly eggs and nymphs, as well as spider mite eggs, pollen, and plant juices. Immature mites require 15-20 whitefly eggs or nymphs for their development and the adults consume up to three eggs or two nymphs daily.Euseius delhiensis (=rubini), E.scutalis, and Amblyseius swirskii have also been reported as predators of sweetpotato whitefly eggs. None of these predatory mites has been investigated as whitefly predators in greenhouses, but E. delhiensis may be commercially available.

Pathogens

Several fungi kill whiteflies.Most require high humidity for infection and development, so they will be most useful in high-humidity greenhouses. Because they are considered pesticides, they must be approved by the Environmental Protection Agency before they can be sold commercially.

Aschersonia aleyrodis. This fungus, first described on citrus whitefly nymphs in Florida, is highly specific to whitefly nymphs.The older the whitefly nymph, the less likely it is to be infected by this fungus. Infected whitefly nymphs typically have a "fried egg"look,with a yellowish discoloration in the center of the body. Under humid conditions bright orange, slimy spore masses form on the outside of infected nymphs.The first signs of infection, a change in color from normal whitish to yellow, occurs in 24-48 hours.White external mycelial growth begins in 4-6 days, and the orange color develops under the appropriate conditions in 7-9 days. Infection is limited by relative humidity, just like for other fungi. It tolerates a wide range of temperatures (59°-86°F), but does best at 73°-77°F. It does not appear to infect parasitized nymphs and is harmless to adult parasites. It is not commercially available.

Beauveria bassiana. This fungus infects numerous species of insects, including whiteflies,thrips,and aphids (see "Aphids"for more information). All stages of the whitefly may be infected. Several brands and formulations of Beauveria are commercially available for use in greenhouses.

Metarhizium anisopliae. This soil-borne fungus infects over 200 species of insects, including whiteflies and aphids (see "Aphids"for more information). It is not registered for use in the United States on any greenhouse crops.

Paecilomyces fumosoroseus. This fungus infects all stages of whiteflies, as well as thrips and aphids, but it is most effective against whitefly nymphs (see "Aphids"for more information). Subsequent reinfections are not common in greenhouses, so reapplication may be necessary.This fungus is limited by the fact that it requires a relative humidity of around 90% for infection. It has been investigated in several states for use on greenhouse ornamentals and is commercially available in Europe, but is not available in North America.

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