Whitefly and any other fly

[Ger-annie-um]

I've seen references to using potato water for spraying whitefly. My question is.. is it just water you'd boil your dinnertime spuds in, does it work for green and blackfly and how often do you need to spray? Sorry, should have said questions plural

[ameno]

It's just the water you boiled potatoes in, yes. Although there are specific potato-water ratios out there to get the mix just right.
The idea is that the starch from the potatoes comes out into the water when you cook them, and this starch water then kills small insects by clogging their breathing pores (invertebrates don't breath through their mouths, but rather have a series on tiny holes down their sides).
Because it kills only by direct contact (and you need to hit them with a decent amount, too), you should spray whenever you see living pests.

[Penellype]

I've had good success with this on whitefly in the tunnel at my allotment. Because the starch will clog a fine sprayer I use a veterinary dosing syringe and squirt it on. The adults tend to fly away but I've had leaves that were covered in nymphs on the underside and after potato water they are now covered in dead nymphs, so it does work. There are also far fewer adults (probably less than 10% remaining). I'm spraying when I have potato water, about once a week, and I don't fuss about ratios.

I haven't tried it on aphids - I have hoverflies which are doing a decent job with those and I don't want to kill the hoverfly larvae.