Many systemic and contact insecticides have been tested for control of whiteflies, but few give effective control. Currently registered systemic insecticides, such as oxamyl, have been only partially effective. Certain contact insecticide combinations, especially pyrethroids such as fenpropathrin or bifenthrin plus organo-phosphates such as acephate or metamidophos, have provided excellent control in greenhouse and field studies as long as there was thorough coverage of the foliage. However, by exposing pest populations to two types of chemicals at once, combinations may accelerate selection for resistance to both materials. Therefore, tank mixes should be resorted to only when single applications are not effective. Other products with contact activity, such as oils, soaps and K-salts of fatty acids, can be very effective with thorough coverage, but in field tests they are often less effective because of poor coverage.
Good coverage of the foliage with contact insecticides is essential for best results. Most whiteflies are located on the undersides of leaves where they are protected from overtop applications, and the immature stages (except for the crawler) are immobile and do not increase their exposure to insecticides by moving around the plant. Use drop nozzles where appropriate, adequate pressure, and calibrate and maintain equipment carefully.
Specific insecticides should be selected according to the stage(s) of whitefly to be controlled. For example, growth regulators often control immature stages by affecting nymphal development, but do not provide good adult control. On the other hand, short residual contact insecticides may control adults, but not affect egg hatch.
Whiteflies have become resistant to insecticides throughout the U.S., threatening the success of traditional chemical control techniques in other areas. The effectiveness of the few currently registered insecticides could be lost if they are excessively and repeatedly applied. There are techniques for monitoring resistance to determine which insecticides are still active against whiteflies. Generally, if an insecticide treatment is properly made with sufficient coverage and yet is ineffective, then that whitefly population should be tested for resistance to the product.
There is a possibility that treating a resistant whitefly population with certain insecticides could actually accelerate population growth. This could be because more eggs are laid when the insect is under biochemical stress, or because beneficial arthropods are eliminated. To minimize this potential problem, insecticide applications should be used judiciously and combined with non-chemical control tactics. Furthermore, distinct classes of chemical compounds should be rotated at least every other spray. Distinct classes of insecticide include the pyrethroids (Ambush, Asana, Danitol, Karate, etc.), organo-phosphates (Orthene, Monitor, Lorsban), carbamates (Vydate), chlorinated hydrocarbons (Thiodan), insect growth regulators (Applaud, fenoxicarb), oils, and soaps and detergents. Resistance to soaps and oils is unlikely to ever develop, so these materials should be used as much as possible.
The insecticides mentioned in this section may not be reccommended on labels for use in your area or for your crop. Check with your State Cooperative Extension office and read the insecticide label before applying any insecticide.