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 in control of giant whitefly. Place infested material in plastic bags, seal, and remove the bags from the property. If the infested leaves are left in the open after removal, a portion of the adult whitefly population may migrate to new plants.
Leaf removal will work better on some plants than others. Giant whitefly is currently found on many different plant species, and it reproduces more successfully on some than on others. 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 syringing with water.
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.
Syringing
The use of a strong stream of water directed to the undersides of infested leaves (syringing) can be very effective in managing giant whitefly. Comparison studies with several pesticides indicate that syringing performed as well or better than chemical treatments. With high whitefly populations, syringing is recommended at least once a week. As populations decrease, intervals can be lengthened to once every 2 or 3 weeks. An additional advantage of syringing is improved plant appearance. Moreover, unlike insecticide sprays, water syringing will not have the negative impact on biological control programs.
Biological Control
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 Asian lady beetle, Harmonia axyridis, also feeds on this pest.
Parasitic wasps often found parasitizing giant whitefly include Entedononecremnus krauteri and Encarsia hispida. Entedononecremnus krauteri is a dark parasitic wasp with red eyes and is often observed on the upper surface of leaves where it lays eggs through the leaf surface into whiteflies on the undersides of leaves. Two other parasites of giant whitefly, Encarsiella noyesii and Idioporus affinis, have more recently been introduced into California from Mexico to help control populations of the giant whitefly. These tiny, stingless, parasitic wasps lay their eggs inside the whitefly larvae. When the wasp eggs hatch, their larvae feed on the giant whitefly larvae. The wasp then pupates and emerges, leaving behind either a hollow, yellowish shell (Idioporus affinis) or a clear shell in which the black skin that the parasite shed is visible (Encarsiella noyesii). In both shells, with the aid of a magnifying glass, you can see the round exit holes through which the parasites emerged. While these parasites don't completely eradicate all of the whiteflies, they can dramatically reduce their numbers to all but a small number at the base of plants. For most situations, this level of control is considered satisfactory.
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 parasites mentioned above are being released by University of California and California state scientists. It is hoped that these parasites will permanently establish and distribute themselves throughout infested areas; release by home gardeners should not be necessary.
Chemical Control
Although some insecticides are registered to control giant whitefly either by foliar or soil application, their use is not recommended if parasites are present in the area. To determine if parasitic wasps are present, use a hand lens to examine several leaves containing whitefly nymphs. Look for a dark discoloration of the nymph or the tiny holes that parasites make when they emerge from the dead larva. If there is evidence of parasites, insecticide treatments should not be necessary.
If you do choose to use insecticides, select least-toxic products such as insecticidal soaps or oils. Remember to follow label directions and spray undersides of leaves where whitefly colonies are found.