If your plants are becoming weak and the leaves a yellow and dying, give them a good shake. If they release hordes of what appears to be flying dandruff, you have a whitefly problem (Note: in western states, leafhoppers are also called whiteflies.) Even without a good shaking out, whiteflies are clearly visible on plants. The adult is approximately the size of a pinhead, mothlike, dusty and white winged. The nymph is yellowish, legless, flat and oval; it may resemble scale at certain stages. The yellow, cone shaped eggs are laid on the underside of leaves. Both the nymphs and adult whiteflies weaken plants by sucking juices from leaves, buds and stems. In addition to being general weakened with yellowed, dying foliage, affected plants and fruit may be undersized and poorly colored. Another indication of a whitefly infestation is shiny, sticky honeydew covering fruit and leaves, an excretion which furthermore encourages growth of black fungus. Most fruits and vegetables are vulnerable to whitefly damage, and they are also common greenhouse pests.
Since whitefly infestations are difficult to control once they have infested your garden plants, prevention is paramount. Inspect purchased bedding or vegetable plants carefully to avoid bringing home infested specimens. If whiteflies have previously plagued your vegetable and fruit crops, you may want to use a preventive spray of insecticidal soap early in the growing season. Otherwise, as soon as you spot a light infestation of whiteflies on any of your plants, spray with insecticidal soap every 2 to 3 days for 2 weeks, making sure to get undersides of leaves. For serious infestations, you may have to resort to pyrethrum, but do this only after you have tried soap spray. Destroy heavily infested annuals as soon as you detect the problem, because otherwise the whiteflies will spread quickly throughout the garden.