Black/White Fly Need a Killer!

So I can make a batch of garlic tea, give my plants a foliar feed with it

and at the same time get rid of the whitefly? 

Absolutely shokkyy. The odd reasonably powerful triggered hosing with plain old water in and around, up and down, before they gain a real foothold is the most effective remedy by far for the white and black fly. Gives the plants a watering at the same time and they seem to love the rush of the water, a bit like stroking cats' ears or brushing a dog.
I can't imagine why anyone with a tap nearby would do anything else ... it's the most natural remedy in the world.