I didn't have any problems until I ordered some infested citrus trees from California. Then I got sooty mold (which grows on white fly poop).

My solution was to get a concentrated jug of neem oil and go bonkers spraying. I think the jug was enough to fill 85 spray bottles (after diluting), and I think I probably went through a half dozen bottles, twice a week for a couple weeks. But after that the problem was over and they haven't been back.

It helps that I'm in Kansas, since there aren't wild white flies outside to keep re-colonizing my greenhouses (I think it gets too cold, like you suggested). So once they're gone, they're gone until I accidentally re-introduce them. But it's a different story in California.

If I were in California, I'd probably have to re-spray with neem oil every month. I'd probably look into biological controls, too. Maybe there's a good creature you could stock that would eat all the white flies? That's how it is with spider mites: you can just stock with good predatory mites, and they keep the bad spider mites under control.