Answer: Whiteflies can be a terrible nuisance, but wherever there are populations of bad bugs (whiteflies), there's a population of good bugs (such as lace wings) waiting in the wings - providing you don't kill the good guys off with insecticides! When the natural balance of predators is exterminated, whitefly populations can soar.
Insecticidal soap is a good defense, but the application must be thorough (undersides of leaves, especially lower leaves, and all the stems), and repeat applications are necessary.
There are a number of biological controls that you can try to get rid of white flies. A predacious wasp, Encarsia formosa, can be released to control white flies. The wasps are available through: American Insectaries, Escondido, (619) 751-1436, Applied Bio-Control, (209) 874-1862, and Beneficial Insectary, (800) 477-3715.
Since you're dealing with an outdoor white fly problem, you might want to try releasing lacewings or minute pirate bugs, both available through the above suppliers.
Other control methods include yellow sticky traps - basically cardboard painted bright yellow and coated with mineral oil or petroleum jelly, and suspended near the tops of plants. As mentioned before, Insecticidal soaps work well against white flies, too. Just be sure to spray both top and undersides of leaves. Insecticidal soaps can cause burning of leaves if applied during hot weather, so use caution.
Good luck in getting rid of those pesky pests!
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