I think so. Mind you I haven't had probs with whitefly, so I haven't used this method. Sylvestris is the lovely big hhp one? I'd perhaps have thought one of the smaller bedding types would be easier to handle, tho I don't like them as flowers. I suppose you have to sneak up on it when the whitefly aren't looking, pop a big bag over the top, and whisk it out of the g.h. sharpish........