Q: For the past three summers I have had a problem with whiteflies in my garden. I first notice them on the underside of leaves on my tomatoes. Before the summer is over, they have attacked any other plant that is still green. What do you recommend?
A: If you hit them early, whiteflies can be controlled with organic insecticides like neem oil or spinosad (click for sources).
You can also use synthetic contact insecticides like permethrin (Eight Vegetable, Fruit and Flower) or cyfluthrin (Bayer Vegetable and Garden).
If you want systemic action, imidacloprid (Bayer Fruit, Citrus and Vegetable Insect Control) will do a fine job on whiteflies.