Staff Writer Gardeners have a new insect pest to worry about this spring. The ash whitefly (Siphonium phillyreae) from Europe arrived in Phoenix and Tucson last November by way of California. We should expect to see its appearance in Cochise County in late April.
The gnat-sized ash whitefly infests and seriously damages a variety of landscape trees and ornamentals. In California it has attacked apples, pears, pomegranates, apricots, peaches, citrus, olive, ash, and other, shade trees. The whitefly has apparently not affected any native desert plants, but infestations appear to be spreading to more host plants than are normally attacked in its native European range from Ireland to Egypt.
Ash whiteflies can kill full grown trees by repeatedly destroying all of the tree's leaves. Leaves affected by the whitefly turn yellow, curl, and drop prematurely. In addition, whiteflies excrete a sugary honeydew that attracts sooty mold fungi, further damaging the tree. Infestations of flying adult ash whitefly have been compared to a light snow flurry in appearance. They are multivolting, producing more than one generation per year, and do best at temperatures around 77 degrees.
Experiments in California have shown that common pesticides are completely ineffective against the ash whitefly. However, laboratory experiments with two biological controls indicate that the whitefly's natural European predators may be used successfully in the U.S. to control the insect. Field studies are currently under way using a small parasitic wasp, Encarsia Formosa, and a ladybird beetle, Clitostethus arcuatus, both imported from Europe. The ladybird beetle has not cleared quarantine yet, but the wasp has recently been released. Serious infestations of ash whitefly generally do not occur in Europe because of the presence of these natural predators. It will take some time to develop a suitable biological control program in the U.S.
Concerned growers can contact Dr. David Byrne, University of Arizona entomologist, for more information.