[Bing]

I have one Rhubard plant growing very strong, I soak its leaves in rain water and use the stink rotten leaf water for pest control. But it seems the blackfly are gethering on the back of its leaves... the leaves are supose to be poisonous?! or nutritious? to blackfly?

[Yorkie]

I doubt it is that effective.

Also, you're mentioning a home-made pesticide.  Members do sometimes discuss making their own pesticides or uses for commercial products beyond those they are licensed for. Whilst not wishing to stifle free speech, this site does not suggest or condone the illegal use of pesticides and would suggest members consider this article:

http://www.allotment-garden.org/grow-your-own/home-made-pesticides