Pest description and damage Whiteflies are small white insects that hold their wings rooflike over their abdomens rather than flat like true flies. Infested leaves, usually the tender young leaves at the branch tips, are mottled yellow on the upper surface, and the margins often curl. Rhododendron whiteflies often leave white, sometimes circular, powdery residues on the leaf surface. Whiteflies can produce large amounts of sticky honeydew, which can develop a heavy coating of black sooty mold. The rhododendron whitefly is mainly an aesthetic pest, although heavy infestations can damage susceptible plants.
Biology and life history The insect overwinters as a nymph. There are several species of whiteflies. Some overwinter as adults on the undersides of evergreen leaves (salal, Oregon grape, rhododendron) but there is no apparent damage. They disappear in spring and no young are found on those leaves. Adults occur from mid-May to early August. Tiny black eggs are laid in circles, half-circles, or they are scattered about singly on the undersides of leaves. There is one generation per year.
Rhododendron species with smooth surfaces underneath the leaf are most affected. Species with a thick and leathery epidermis are often unaffected. Do not over fertilize. Whitefly numbers increase with increasing foliar nitrogen levels. If necessary, switch to a slow-release fertilizer and/or one with less nitrogen. Lightly vacuum plants in the early morning to remove adult whiteflies when they are cold and less likely to fly away. Whiteflies are also known to spend winter under rhododendron leaves without establishing on those plants. Check for eggs, or larvae to be sure that there is a live population.
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For more information
Flint, M.L. 2002. Whiteflies. UC-IPM On-Line (http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7401.html)