How to Manage Pests
Pests in Gardens and Landscapes
The giant whitefly, Aleurodicus dugesii, an insect native to Mexico, was first discovered in the United States in Texas in 1991. It is now established in multiple states including California, Arizona, Florida, Louisiana, and Hawaii. Giant whitefly was first discovered in San Diego County in 1992, after which it spread rapidly northward along the coast and into interior valleys to San Luis Obispo County and into northern California. This insect may now be widespread in southern California, the Central Coast, the San Francisco Bay Area, and the southern Sacramento Valley.
Giant whitefly is a serious pest of many ornamental plant species commonly found in nurseries, landscapes, and home gardens. The plant species most affected by the giant whitefly include begonia, hibiscus, bird of paradise, orchid tree, banana, mulberry, xylosma, aralia, and various vegetables. Certain varieties of citrus and avocado are also affected. As giant whitefly populations become established in new areas and adapt to vegetation, the list of known host plants (Table 1) is likely to grow.
IDENTIFICATION AND BIOLOGY
Giant whitefly gets its name from its large size (adults can be up to 3/16 inch long) relative to many other whitefly species in North America. This species can also be distinguished from most other whitefly species by the wax they deposit. During the early stages of infestation, the adults produce spirals of wax on the undersides of the leaves and deposit their eggs in these wax bands. As the populations grow, the wax becomes heavier and hangs down from the lower leaf surfaces. When populations of giant whitefly reach high levels, the whiteflies and their waxy deposits occur on both upper and lower leaf surfaces in extraordinary amounts.
Giant whitefly has three developmental stages: an egg, a progression of four immature stages called nymphs, and an adult stage. Eggs, which are approximately the size of a pin head, are deposited in succession in a spiral pattern on stalks imbedded into the undersides of leaves. Females typically cover the eggs in wax when laid.
Giant whitefly nymphs are small and have oval bodies lacking wings, with no apparent legs or antennae. After hatching, the mobile first stage nymphs, called crawlers, disperse over the leaf surface to locate a suitable place to feed. They then settle and insert their mouthparts into the plant phloem to feed, remaining immobile until they reach adulthood. During the progression of each nymphal stage, individuals more than double in size. The nymphs produce long hairlike filaments of wax that give a bearded appearance to affected leaves; this wax can be mistaken for a fungus. Wax production is especially noticeable during the third and fourth nymphal stages.
Adult giant whiteflies emerge from the last nymphal stage as a winged insect. Giant whiteflies exhibit a strong tendency to feed in groups, and after emergence the majority will remain on the same plant to feed and lay eggs. All stages of development generally occur on the undersides of leaves.
Giant whitefly can damage plants directly by its feeding activity. Both nymphs and adult giant whiteflies feed by inserting their needlelike mouthparts into the leaf’s vascular tissue or phloem and suck out the plant’s sap. Feeding by giant whitefly deprives the host plant of water and nutrients and at high infestation levels can lead to severe leaf senescence and abscission, followed by plant dieback. However, giant whitefly feeding rarely causes plant death.
During feeding, giant whiteflies excrete a sticky, sugary solution called honeydew that accumulates on leaves. This sugary solution provides an excellent medium for the growth of black sooty mold fungus. Sooty mold is not only unattractive, but it reduces the photosynthetic (food-producing) abilities of the leaves. Learn more about sooty mold in the Pest Notes: Sooty Mold.
Giant whitefly is somewhat unique among whiteflies, because it produces such large amounts of visible wax. This wax detracts from the aesthetic value of the plants. The wax filaments generally attain lengths of up to 2 inches, but in areas shielded from the wind these filaments can reach astounding lengths of more than 10 inches. The wax filaments easily break off and can stick to surrounding objects, further reducing the aesthetics of the area.
Manage giant whiteflies in your landscape with an integrated approach that includes removal of infested leaves and, if necessary, washing whiteflies off leaves with water. When choosing plants, consider species less susceptible to giant whitefly. When planting susceptible species, put them in full sun, if possible, since giant whitefly prefers shaded areas. Biological control by parasitic wasps or generalist predators also can reduce giant whitefly populations.
Leaf or Plant Removal
The tendency of giant whitefly adults to remain on the plant where they developed leads to a strongly clustered distribution. This clustering behavior means that removing relatively few leaves can destroy large numbers of whiteflies. Leaf removal is most effective when populations are restricted to a few plants or leaves. Thus, monitoring to detect early infestations is extremely important for management of giant whitefly. Place infested plant material in plastic bags, seal, and remove them from the property. If the infested leaves are left in the open after removal, adult whiteflies may continue to grow from mature nymphs and may infest new plants.
Efficacy of leaf removal will vary among plant species. While giant whitefly may currently be found on many different plant species (Table 1), suitability of these plants as reproductive hosts varies. On less preferred hosts, such as yellow hibiscus, removal of leaves should be sufficient to control populations. On more preferred host plants, such as red hibiscus, giant bird of paradise, and xylosma, control will require early detection, rigorous leaf removal, and spraying the plant with water (see below).
In high traffic or visible areas that are adjacent to walkways, doors, or windows, removing or replacing plants infested by giant whitefly may be an option.
The use of a strong stream of water directed to the undersides of infested leaves can be very effective in managing giant whitefly. Comparison studies indicate that spraying plants with water alone controlled giant whitefly as well or better than several different chemical treatments. With high whitefly populations, water sprays are recommended at least once a week. As populations decrease, intervals can be lengthened to once every 2 or 3 weeks.
An additional advantage of hosing off plants is improved plant appearance. Moreover, unlike insecticide sprays, water sprays will have limited negative impact on natural enemies.
Native insect predators such as green lacewings (Chrysopa and Chrysoperla spp.), larvae of syrphid flies (also called flower flies and hover flies), and lady beetle adults and larvae (Cycloneda polita and Delphastus catalinae) attack giant whitefly in California but do not provide adequate biological control. The introduced multicolored Asian lady beetle, Harmonia axyridis, also feeds on this pest.
Parasitic wasps (also called parasitoids) are the most important natural enemies of giant whitefly. These tiny, stingless wasps lay their eggs inside whitefly nymphs. When the wasp eggs hatch, their larvae feed on the nymphs, eventually killing them. The wasps then pupate and emerge, leaving behind the exoskeletons of the whitefly nymphs with tell-tale emergence holes.
While native parasitoid wasps such as Encarsia hispida are present, they are rare and provide little control. To address the lack of adequate control by native natural enemies, three non-native parasitoids were released into California during the mid-1990s. The wasp Entedononecremnus krauteri was introduced to San Diego from Texas in 1995. Entedononecremnus krauteri has a dark body color with red eyes and is often observed on the upper surface of leaves, where it lays eggs through the leaf surface into the nymphs on the undersides of leaves. While present in high numbers for a few years after release, Entedononecremnus krauteri populations have declined, and it seems to be the least impactful of the three introduced parasitoids. This is likely due to competition with the other two introduced species.
The parasitoids Encarsia noyesi and Idioporus affinis were introduced into California from Mexico in 1996 to help control populations of the giant whitefly. Unlike Entedononecremnus krauteri, these species forage amongst the nymphs on the underside of leaves. Both species are present in high numbers in California and appear to contribute significantly to the biological control of giant whitefly. While these parasitoids don't completely eradicate all of the whiteflies, they can dramatically reduce their numbers to a small population at the base of plants. For most situations, this level of control is considered satisfactory. Parasitism by Entedononecremnus krauteri and Encarsia noyesi can be identified by their black pupae within the whitefly nymphs, while Idioporus pupae are black and yellow.
Some natural enemies are commercially available for release against giant whitefly, but there is no evidence that purchasing and releasing natural enemies is effective. The whitefly parasitoids mentioned above have become permanently established and distributed themselves throughout infested areas; therefore release by home gardeners is not necessary.
Insecticides are not generally recommended because they kill biological control agents. As mentioned previously, a forceful stream of water directed at colonies can be effective and will maintain control from natural enemies.
Although some insecticides are available to control giant whitefly either by foliar or soil application, their use is not recommended if parasitic wasps are present in the area. To determine if parasitic wasps are present, use a magnifying or hand lens to examine several leaves containing whitefly nymphs. Look for adult parasitoids, nymphs that have a dark color, or the tiny holes that parasitoids make when they emerge from the dead nymphs. If there is evidence of parasitoids, insecticides should not be used.
If you choose to use insecticides, select least-toxic products such as insecticidal soaps or oils. Always read and follow label directions.
Dreistadt SH, Clark JK, Flint ML. 2001. Integrated Pest Management for Floriculture and Nurseries. UCANR Publication 3402. Oakland, CA.
Windbiel-Rojas K, Messenger-Sikes B. 2020. Pest Notes: Sooty Mold. UC ANR Publication 74108. Oakland, CA.
UC IPM Natural Enemies Gallery. http://ipm.ucanr.edu/PMG/NE/index.html
TECHNICAL EDITOR: K Windbiel-Rojas
ANR ASSOCIATE EDITOR: AM Sutherland
EDITOR: B Messenger-Sikes
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