The most I could find out about Kwas is that it is a soup made by fermenting bread. I find nothing about using it to control Whitefly.
Whitefly can be difficult to control because they have developed immunities to most of the commonly used insecticides. The best method of control is to encourage the predators of Whitefly and place Yellow Sticky Traps that can trap some of the adults.

Here is a link that might be useful: About Whiteflies

Thanks. I should have mentioned that I learned about kwas from 2 older threads on this forum. The first was called " How do I get rid of white fly?" from 7/30/2012. That led me to "One of the best recipes in Organic gardening" from 10/16/2009.

I'm wondering if I can use vaseline instead of motor oil on yellow boards to trap the whiteflies?

Also, I'm on my 2nd try to make kwas. Neither time did it turn white as it mentioned it should in the 2nd thread.