I thought i would share this with you & some of you may already do this.
I tried this last year on my cabbages & brussels etc. After picking some rubarb i cut off the leaves & put them in one of my black water buts with the lid let it brew for about a week gave it a stir & wow what a smell, After picking myself off the floor i waterd some of my cabbages with it as they had a touch of the whitefly, continued with this on a regular basis, every night!! & after a week i noticed this :P no white fly,green fly,black fly, no butterflies were around the plants no catterpillers,no flea beetle :D. but on the plants i left they were infested with white fly etc :(.
So after a while i started with the other plants and there was a massive reduction in the white flys etc, natures pesticide ;D.
I think its down to the toxins in the leaf that does the trick & the plants looked really well, in fact one comment i had last year they were the best cabbage & brussels on the whole site ;D ;D, ouch my head its getting bigger!!
Just one thing i found a water butt with a tap is better & try and filter the water :D a pair of tights do well catches all the bits from the water.