Friday, 30 January 2015

Whitefly Damage

Eggs are laid and immature stages of whitefly develop on the undersides of leaves on most crops. Adults congregate on younger leaves in most crops and oviposition is heaviest on these leaves. The location on the plant of the various stages of the whitefly follows the development of the plant. Eggs and early instar nymphs are found on the young leaves and larger nymphs are usually more numerous on older leaves.
Adults congregate, feed, and mate on the under surfaces of the leaves of the host plant. This can occur in such numbers as to create "clouds" when disturbed. They appear to be more active during the sunny daylight periods, and do not fly as readily during early morning, late evening, or night hours.
The nymphal stages are sedentary, with the exception of the crawler, which after hatching moves a very short distance. Once a feeding site is selected the nymphs do not move. They suck juices from the plant with their piercing-sucking mouthparts. The nymphs are located on the undersides of the leaves and can become so numerous that they almost cover the entire undersurface area.
Direct crop damage occurs when whiteflies feed in plant phloem, remove plant sap and reduce plant vigor. With high populations plants may die. Whiteflies also excrete honeydew, which promotes sooty mold that interferes with photosynthesis and may lower harvest quality. In cotton, the sugars excreted during whitefly feeding make the cotton fibers sticky and can promote growth of sooty mold, both of which reduce quality. In some hosts, damage can result from whitefly feeding toxins that cause plant disorders such as silver leaf of squash and irregular ripening of tomato. Plant viruses also can be transmitted by whiteflies, such as the geminiviruses in tomatoes, peppers and cabbage, and certain clostroviruses like lettuce infectious yellows in lettuce and melons. Plant disorders and virus transmission are of particular concern because they can occur even when a whitefly population is small. In general, the older the plant when infected with virus or the later the onset of plant disorders, the less damage to the crop, so preventative action is critical. Prevention is also crucial in managing whiteflies in highly cosmetic crops such as ornamental plants, where even low numbers of whiteflies can affect marketability.
The sweetpotato whitefly currently is known to attack over 500 species of plants representing 74 plant families. They have been a particular problem on members of the squash family (squash, melons, cucumbers, pumpkins), tomato family (tomato, eggplant, potato), cotton family (cotton, okra, hibiscus), bean family (beans, soybean, peanuts), Gerber daisies, salvia, poinsettia, and many other ornamental plants. The poinsettia is a favored host and suffers color loss as well as leaf damage.
Whitefly management in a given crop will depend greatly on the severity of damage caused in that crop and the number of whiteflies required inflicting this damage. Very few whiteflies are required to transmit viruses, so where this is the major concern, the grower will want to avoid even small numbers of whiteflies. A combination of selected cultural practices, intensive chemical treatments or physical controls, and/or the development of host plant resistance, may be most effective. Where low levels of whiteflies are tolerable, other methods such as biological control can be more effective.

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