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The fungus Paecilomyces fumosoroseus consumes its host from the inside out.The powdery spores turn the infected insect white at first, then change to shades of pink.

Verticillium lecanii. This insect-parasitic fungus infects both aphids and white-flies.The fungal strain with large spores infects aphids; the one with smaller spores infects whiteflies. V.lecanii grows and multiplies in the greenhouse at temperatures of 59°-77°F. Fungal spore germination and infection occurs only when the relative humidity is greater than 90% for at least 10 hours per day. The spores germinate and bore through the whitefly, then grow inside it, killing it.The fungus appears as cottony white fluff on infected whiteflies which detracts from the appearance of some ornamentals.The fungus also causes some mortality of whitefly parasites inside the pest's body, but it seems to be compatible with other parasites. A commercial formulation of the fungus specific to whiteflies is available in Europe. V.lecanii is not yet registered for use in North America.

Possibilities for effective biological control

Although there are many different strategies for biological control of whiteflies in greenhouses, certain principles apply in most cases. Natural enemies are most effective if introduced when whitefly numbers are low. Biological control often fails if initiated after whitefly populations exceed one adult per 10 upper leaves on vegetable crops and fewer on ornamentals. Biological control usually does not eradicate whiteflies but maintains the pest population at a low density. On many crops, especially poinsettia, there may be a mixed infestation of greenhouse and silverleaf whiteflies. It is important, there-fore,to correctly identify the whiteflies in your crop so that the proper rates of the most effective natural enemies can be released. Residual insecticides should not be used within a month of parasite or predator release. If the whitefly population is high, reduce the number of whiteflies with a nonresidual spray, such as insecticidal soap, horticultural oil, natural pyrethrins (not synthetic pyrethroids), or insect growth regulators before releasing natural enemies. Natural enemy species introduced alone may provide effective whitefly control in certain situations. However, a combination of parasite releases, predator releases, applications of nonresidual insecticides, and leaf removal are often necessary to produce commercially acceptable crops.

The parasite Encarsia formosa is the most widely used insect for biological control of whiteflies. It has been used extensively for many years in commercial vegetable greenhouses throughout the world. It can control whiteflies on vegetable crops, including cucumber and tomatoes, and several ornamental crops, such as poinsettia. On vegetable crops that can tolerate a few whiteflies, such as tomato, a single inoculative release of Encarsia formosa may be sufficient for control. However, on most vegetable crops or short-term crops with a much lower tolerance for whiteflies, such as poinsettia cuttings, multiple inundative releases will be necessary. Many other promising natural enemies can be used in conjunction with Encarsia wasps.

The best time to introduce Encarsia is when the whitefly population is low. Wasp efficiency is seriously impaired when whitefly nymphs are too numerous (30-60 per in2).The wasps spend more time cleaning the excessive honeydew from themselves than searching for and parasitizing whiteflies. Consult the section on monitoring whiteflies for instructions on estimating whitefly populations.

Encarsia wasps are shipped in the pupal stage, glued to small cards that can be hung directly on the plants. Sometimes the adult Encarsia have already emerged, so open the parasite shipment in the greenhouse. Distribute the cards on the plants near whitefly infestations as soon as possible after the shipment arrives. Be sure to distribute the pupae evenly throughout the greenhouse because these very small wasps will disperse only about 6 feet from the release point. Place extra wasps in whitefly hot spots. Save several cards from each shipment to check for the percentage of wasps emerging from the pupal stage; quality can vary between shipments and suppliers.

Greenhouse growers typically introduce Encarsia wasps either by inundation or seasonal inoculation. Inundation, in which massive numbers of the parasite are released into the greenhouse at once, is the simplest but can be expen-sive.The more common method uses successive introductions of smaller numbers of parasites. Make the first release as soon as, or before, the first adult whiteflies are detected, either by visual inspection or on yellow sticky traps. Continue to make regular releases weekly or every other week until 60-80% of the whitefly pupae are parasitized. It is important to know how to recognize adult parasites and their parasitized whitefly pupae. Check the crop every 3-4 days to make sure adult Encarsia are present.The first parasitized whitefly pupae should be visible 2-3 weeks after the first release. As a general guide, about 25% of the whitefly pupae should be parasitized within 1 month of the first introduction, 50% after 2 months, and 80% after 3 months.To determine the percentage parasitized, count the number of black (parasitized) and white (nonparasitized) whitefly pupae on the underside of the leaves. Do not count pupae from which adult whiteflies or parasites have emerged.

There is a third, do-it-yourself method for introducing Encarsia to the crop— using banker plants. In an area that is separated from the production area, infest young tomato, cucumber, or tobacco plants with whiteflies when the plants have about six leaves.Three weeks later introduce enough Encarsia adults to parasitize at least 85% of the whitefly nymphs. Check in 2-3 weeks to be sure that the wasps have parasitized large numbers of pupae. A week or two before introducing the banker plants into the crop, treat the plants with a nonpersistent insecticide, such as pyrethrum,to kill any adult whiteflies.

Place the banker plants uniformly throughout the greenhouse no more than 20 ft apart.This provides a continuous source of parasites emerging over 8-10 weeks that will disperse to whitefly-infested crop plants.When only a few whiteflies are present in the crop, the emerging parasites survive on the whiteflies and honeydew present on the bankers.The banker plants must be established early in the growing season so that a large wasp population, up to 10,000 per plant, has developed by the time whiteflies infest the crop.This may not be practical in large commercial greenhouses.

Whatever your introduction method, be aware that excessive leaf removal can eliminate many parasites when they are about to emerge. Check the leaves for black parasitized pupae. If there are many black pupae that lack exit holes, leave the prunings in the greenhouse for a week or two to allow time for the Encarsia to emerge. If there are more white than black whitefly nymphs, discard the prunings.Otherwise,you should remove leaves regularly and frequently so that large numbers of leaves with parasites are not removed all at once.

Recommended release rates vary considerably depending on the whitefly density, the time of year, the crop and its growth stage, and the type of whitefly. Most suppliers provide detailed instructions for the release and use of Encarsia and can make recommendations about the number of wasps to release based on specific situations. However, the number of Encarsia that actually emerge may be considerably different from the number promised by the insectary. Use quality parasites from reliable insec-taries. Large differences from the expected numbers can drastically affect control. Check each parasite shipment for the number of emerging wasps, and adjust release rates as necessary.To check wasp emergence, count exit holes on several cards at initial release and 1 week later. Or, place several cards within airtight containers with transparent lids, place the containers out of direct sunlight,and check the number of adults that emerge after 1 week.

In general, for very low initial infestations of less than one whitefly per plant, release rates of 1.5 parasite per 10 ft2 are recommended. For higher initial infestations (still less than one whitefly per upper leaf), releases of 3-9 parasites per 10 ft2 should be made at 10-14 day intervals. For greenhouse tomatoes and peppers, weekly releases of one wasp per every four plants is suggested. For cucumber the rate is one wasp per every two plants,and on poinsettias,two wasps per plant, released weekly. Release rates are better established for vegetable crops than for ornamentals, which have to be virtually pest-free to be marketable.Encarsia can provide control comparable to conventional insecticides on many ornamentals, although a final "clean up" treatment with an insecticide may be necessary prior to sale. Initial whitefly infestation on poinsettia usually occurs during plant propagation. For greenhouse whitefly, release one parasite per plant per week on the stock plants to achieve long-term control of a light infestation. Weekly introductions of 50 Encarsia per 150 newly potted poinsettia cuttings provide good control of a low infestation of greenhouse whitefly.This rate should also be suitable for fuchsia, gerbera, and other ornamentals. In one trial,four Encarsia per ft2 controlled greenhouse whitefly on gerbera so the plants were only marginally infested at the end of the experiment.

Most of the information on the use of Encarsia was obtained against greenhouse whitefly, so modifications may be necessary for control of other whiteflies. Encarsia parasitizes sweetpotato and sil-verleaf whitefly, for example, but not as effectively as it does greenhouse whitefly. On sweetpotato whitefly, Encarsia reproduces less, takes longer to mature, and produces lower-quality off-spring.The fact that the wasp is produced commercially almost exclusively on greenhouse whitefly may explain these problems.The rearing history of Encarsia can affect its acceptance on subsequent hosts. Also, there are different strains of Encarsia that vary in their effectiveness against different whitefly species. Double or triple release rates and regular introductions throughout the cropping season may be necessary to compensate for these problems.

The host plant also influences the ability of Encarsia to control whiteflies. Large, dense hairs (trichomes) can interfere with parasites searching for prey and reduce its efficiency. Biological control with Encarsia may be improved by selecting plant varieties with fewer hairs or none. In one experiment, wasps were able to parasitize 20% more whiteflies on a hairless cucumber than on a normal, hairy cucumber. By contrast, certain host plants are particularly good hosts for whiteflies, making it harder for the wasp to keep up with whitefly populations. On plants such as cucumber and eggplant, whiteflies produce more eggs, develop faster, and live longer than they do on plants such as tomato. Large populations on favorable host plants produce excessive honeydew excretion, which interferes with normal parasite searching behavior.To compensate for the better pest development on highly susceptible plants, introduce more parasites. For example, release rates of 14 wasps per plant are suggested for eggplant, but only five per plant are needed for tomato.

Encarsia wasps fly and lay eggs best in warm (61°-82°F), bright greenhouses. For maximum whitefly control, keep relative humidity at 50-70% and daytime air temperature around 75°F. Low temperatures allow the whitefly to multiply faster than the parasite, resulting in poor control. Cool, cloudy conditions and the low light levels of winter also reduce Encarsia's ability to control whiteflies. Dutch researchers are developing a strain that is more effective at lower temperatures. Under cool conditions, you may need to increase release rates or complement control using other natural enemies or a fungal pathogen such as Beauveria bassiana.If you release more wasps, you may also need to decrease the distance between release sites to compensate for slower reproduction and remove adult whiteflies in the tops of plants by vacuuming the upper foliage or by hanging numerous yellow sticky traps or ribbons.Whiteflies are strongly attracted to yellow sticky traps, while the parasites are less attracted—as long as sufficient whitefly nymphs are present. If you find more than 15 whiteflies caught on yellow traps in vegetable crops, consider spraying with a nonresidual insecticide.

Many wasps other than Encarsia are also good parasites of whiteflies. Eretmocerus eremicus is better than Encarsia against silverleaf whitefly because of its greater mobility and ability to attack more nymphs.This species is also less susceptible to pesticides than Encarsia.It is often shipped as pupae mixed in a carrier, such as bran or vermiculite. Sprinkle the mixture into leaf axils or around the base of plants. Be sure that ants, which can carry off the pupae, are not present.

Recommended release rates are similar to those for Encarsia;release two wasps per 15 ft2 every 1-2 weeks as a preventative measure.When whitefly infestations are established, a minimum of three weekly introductions at rates of 3-9 wasps per 10 ft2 (depending on whitefly density) is suggested.

Predators of whiteflies can also be effective in biological control programs. Most of them do not feed exclusively on whiteflies, but these predators will provide control if whiteflies are the predominant pest.The lady beetle Delphastus pusillus is the most whitefly-specific predator available, although it will feed on spider mites if no whiteflies are available. It prefers high whitefly numbers, so it is particularly useful for release in whitefly hot spots. It may eradicate whiteflies if the population is not very dispersed. A release rate of seven per 10 ft2 is suggested.

The mirid Macrolophus caliginosus is another good predator of whiteflies, particularly when used in combination with other natural enemies. It remains active at relatively low temperatures, and therefore can successfully supplement control by other species that are active only at higher temperatures.The bugs must be released early in the season because of a slow population build-up. They are supplied as adults and nymphs mixed with vermiculite.The mixture can be shaken directly from the container

Whiteflies

Enemies

Encarsia formosa nymph -

pupa adult

Encarsia pergandiella

Encarsia inaron

Encarsia transvena

Eretmocerus spp.

Chrysoperla spp.

Delphastus pusillus

Lady beetles

Dicyphus tamaninii

Macrolophus caliginosus

Orius spp.

Predatory mites

Aschersonia aleyrodis

Beauveria bassiana

Metarhizium anisopliae

Paecilomyces fumosoroseus

Verticillium lecanii

Natural enemy attacks the host • • • Natural enemy host feeds as an adult - Width of bar indicates degree of effectiveness egg nymph -

pupa adult onto leaves. Release 5-10 bugs per 10 ft2 into the first whitefly hot spots on a weekly basis. Continue releasing until populations of M.caliginosus reach 0.5-2 per 10 ft2 throughout the greenhouse (at least 2 months). Do not use this mirid on gerbera because the bug can cause damage to the flowers.

The green lacewings Chrysoperla carnea, C. rufilabris, and C. comanche are most efficient when leaves of adjacent plants touch because the larvae are better able to spread among the plants. Releases of 8-25 C. rufilabris larvae per plant controlled sweetpotato whitefly on hibiscus in Texas greenhouses—well enough that all of the plants were marketable. However, lacewing development was not normal, indicating that the white-flies alone were not an adequate food source. Orius spp. also provide some control of whiteflies and other pests.

Many of these predators can be released in conjunction with E.formosa. Supplementary releases of green lacewings or the lady beetle Delphastus pusillus will control whiteflies on many crops, especially vegetables.These predators may feed on the same instars parasitized by the wasp, but the beetles avoid feeding on whiteflies in the late stages of parasitism. If the beetles are released 2 weeks after the parasite, the wasps should be developed enough that beetles avoid them. Otherwise, you may wish to release these predators only in areas of high whitefly numbers.

The fungus Beauveria bassiana has been effective in controlling whiteflies under a wide range of commercial production conditions.The fungus has controlled whitefly on hibiscus, mandevilla, poin-settia, and tropical foliage plants in the greenhouse. It worked as well as or better than the best conventional insecticide programs, even where infestations had reached critical levels. Beneficial insects are not affected unless sprayed directly, so this pathogen could be used in conjunction with Encarsia.Unlike parasites,fungi are able to control high whitefly populations. Fungal applications may be useful in reducing whitefly populations before introducing Encarsia or in conjunction with the parasite. Under the appropriate conditions, fungi may eliminate the need for insecticides to control whiteflies. However, B. bassiana must be used within an IPM framework because all fungi may be killed by certain fungicides used to control plant diseases.

The fungi Aschersonia aleyrodis, Metarhizium anisopliae, Paecilomyces fumosoroseus, and Verticillium lecanii are promising biological control agents but are not currently available for use in the United States.Whiteflies can be suppressed for several months following a single application of V.lecanii if the temperature remains between 59° and 77°F and the humidity is greater than 90% for at least 10 hours per day.This fungus can kill 80-97% of the nymphs, and subsequent infection kills many adults emerging from surviving nymphs. V. lecanii is pathogenic to Encarsia when applied directly, but whiteflies are even more susceptible, so the fungus and wasp can coexist.

Experimental sprays of Paecilomyces fumosoroseus significantly reduced sweet potato whitefly populations on ornamentals (hibiscus, mandevilla, poin-settia, and crossandra) when applied weekly.P. fumosoroseus kills sweetpotato whitefly more quickly and with higher mortality than other fungi. Under normal greenhouse conditions infection is detectable 7-10 days after application. However, many of the whiteflies killed by the fungus do not show the typical coloration unless placed in humid conditions to allow sporulation. P.fumosoroseus also provides better control than conventional insecticides because it kills all whitefly stages, including adults and eggs, whereas most con ventional chemicals do not. It is compatible with Encarsia, Eretmocerus, and the predator Delphastus pusillus and is tolerant of some fungicides.

Aschersonia aleyrodis requires several applications in order to control white-flies.This fungus is effective mainly against the younger nymphal stages of the whitefly and does not affect Encarsia formosa, which oviposits in old nymphs or pupae, so the two are very compatible for use together in greenhouses.

Fungi are applied as spores suspended for spraying in water that contains a wetting agent and other adjuvants.The first diseased whiteflies appear 3-7 days after the spore application, but whitefly numbers do not decline until a week or two after spraying. High humidity is necessary for fungal sporulation and infec-tion.When humidity is low, performance is unpredictable.You can increase humidity by dampening the plants with water sprays (be cautious; this may encourage plant disease). Late afternoon applications reduce spore injury by ultraviolet light and desiccation, since the greenhouse is relatively more humid at night. Humid areas with moderate temperatures, such as rooting benches and shade-cloth covered areas used to induce inflorescence in chrysanthemums, may be the only practical areas for application. Multiple sprays are necessary for A. aleyrodis, B. bassiana, and P. fumosoroseus, which will not spread in the greenhouse, and for V.lecanii if humidity is not high enough to allow continuous infection.These fungi are killed by many fungicides.

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