The miracle chemical to get rid of whiteflies is a systemic called imidacloprid, and I learned about it from the Tropical Hibiscus Mail List. You can buy it in Bayer Rose and Flower spray or Bayer Tree and Shrub drench. The spray is good for smaller plants, the drench for larger shrub sized plants. Both of these are only labelled for outdoor use, however, and I can't spray outdoors in the winter, obviously.
I overwinter mine indoors and find both of these impractical, not to mention MESSY, although the drench lasts for a year, so theoretically you could treat outdoors in the summer and be good for a year. So I bought a jug of marathon granules, which also contain the same chemical. Unfortunately, the granules are geared towards commercial growers and the smallest size I could find was a jug of 4 or 5 pounds, which costs around $130! I have a lot of plants, so I'll use it up eventually. We also use the granules in my job taking care of office plants - they work very well on fungus gnats, scale, mealybugs, thrips, pretty much any sucking insect EXCEPT spider mites, and we don't have to worry about spraying inside the offices.
My first winter with a lot of hibiscus I had spider mites and root rot. The second year I had horrid whiteflies. The third year I used the granues in March (that was March of 2006) and also used a horticultural oil for quick knockdown (the systemic takes a while to work its way into the plant, especially the woodier ones). I have not seen any whiteflies indoors since March of 2006 and I haven't re-treated yet. Some people claim that using imidacloprid can make plants more susceptible to spider mites, but I haven't found that in my experience.
