have you tried Mosquito Dunks? they utilize a micro-organism that attacks the larvae and are quite effective against mosquitos and fungus gnats, ...maybe they would be equally effective against white flies.But it is a help if you have any pests. I have been battling white flies forever and if i keep my plants neat they are easier to control.
They look like donuts made of sawdust and each donut is made to be tossed into a pond so, unless you have a LARGE reservoir, lol, i'd suggest crumbling up one of the donuts and lightly top dressing the girls.
Hope this helps, SOG