Well first of all do not drench the soil, that is a waste of money and time.
Next you can use yellow sticky cards but that will only trap part of the population and will also trap your beneficials. Yellow attracts good and bad insects. It also only traps the adults, not the immature stage, which is the white on the underside of the leaves.
About using insecticides on whitefly. People have so over used insecticides on whiteflies (WF) that there is a strain WF have developed resistance to most of the common chemical insecticides. So if you spray, it might be like spraying water on them but with still killing the beneficials that would attack the WF. This strain is called the Q biotype whitefly. You can find info about it all over the net. There has been a lot of money poured into the research on this pest, hoping to find solutions.
Now in telling you that, things like soaps and oils kill by physical means and not chemical means, so the soaps and oils will work well on the IMMATURE stage of the whitefly. That is were you need to plan your attack, on the immatures. Also you need to be targeting the undersides of your leaves when you spray (that's where the immatures are). I know you say that is "impractical" but that's just the way it is. We, as a society, have gotten spoiled over the last 50 years with all these very toxic pesticides that work fast and easy but now we are discovering the long term impact of these quick cure. We are now having to go back and learn how to grow a better
plant to prevent having these problems because the hard pesticides are not available anymore.
Don't forget with soaps and oils you to have to watch for phytotoxicity, burning from soaps or oils when you use them. Homeowners tend not to read the label and if they do they mix concentrates to strong, thinking it will work better, and end up burning the
plant. SO FOLLOW THE DIRECTIONS closely. Dont spray in the middle of they day, make sure the
plant was watered the day before and follow all those other warning they give you about soaps and oils.
If it were me? I would toss the fushia and not ever grow it. Switch to something different. It is worth all this work and insecticide for it? This is back to right
plant right place. These
plants are not happy there and the WF are moving right in. Why not find something that will be problem free.
PS don't forget your pesticides kill the beneficials too
whiteflies on buglady consulting