Order Hemiptera: Family Aleyrodidae
Pest description and damage Whiteflies are tiny, pure white, moth-like sucking insects that prefer to feed and lay eggs on the undersides of leaves. The nymphs are yellowish and before becoming adults the third instar molts to the fourth and last instar, an immobile stage that is thickened like a small, round, layered birthday cake. Male and female whitefly adults emerge to mate and lay eggs. Tiny black eggs are laid in small circles, half circles or as scattered singles. Small circles that appear dusted with a white powder may be noted. Damage varies by species but the whitefly sucks plant sap from the phloem or plant cells. Leaves may turn yellow or dry and fall, or leaves may be covered by honeydew followed by the sooty mold fungus. Some can transmit viruses. Hosts are various and include rhododendron and azalea, Indian plum, ceanothus, fuchsia, and many other plants. The presence of whiteflies on the undersides of evergreen broadleaf plants in fall and winter may not be predictive of summer problems. Fuchsias can be severely damaged by high populations of whiteflies.
Biology and life cycle This insect has four instars: nearly invisible 1st instars, yellowish flat 2nd and 3rd instars and the 4th thickened instar before molting to the adult stage. Tiny, black, oval eggs are laid in little circles or scattered singly across the leaf surface. In warm weather populations can build quickly to cause considerable damage. It is often the adults that overwinter.
Pest monitoring Ruffle plants to get whiteflies flying. If the adult whiteflies are present, check the undersides of leaves to verify young, feeding stages are present or check for honeydew and sooty mold. The presence of small dusty white circular areas may not mean whiteflies are a problem. Note whether the level of damage is worth control efforts.
Keep plants healthy and watered according to their needs. Whitefly populations increase with high nitrogen levels: do not over-fertilize plants. Wash plants with a gentle stream of water directed to the underside of plants, or disturb foliage and vacuum adults. Prune back and encourage host plant refoliation. A combination of disturbing the foliage, vacuuming flying adults and using sticky yellow cards attractive to whiteflies provides some measure of control, but stay vigilant.
The most noted biological control is Encarsia formosa, but there are several Encarsia species that can be purchased for release. These consist of small cards which can be clipped on plants. Make sure to put one card in a jar to observe hatch date and to determine if parasitoids are alive. Other predators and parasitoids are also active and together they often bring high populations back into check. See Flint reference for extensive biological and cultural discussion.
For more information
Flint, M.L. 2002. Whiteflies. University of California (http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7401.html#TABLE1)
Rosetta, R. 2016. Whiteflies. PNW Nursery IPM.( http://oregonstate.edu/dept/nurspest/whiteflies.htm)