Soapy water kills white flies. Yellow sticky traps are almost irresistible to them, they are strongly attracted to the yellow. I put them out every 5 feet, haven't had whitefly trouble since.
Caution, please only buy the Seabright labs brand, as other brands are so sticky that birds and small animals will stick to them, very cruel.
They last a whole season, here's a link.
https://www.amazon.com/Yellow-Sticky...w+sticky+traps