If you mean the grey mealy aphid that collects in clusters in the growing tips of cabbage, kale sprout plants etc.,I find that a strong blast from a hose is all that is needed -do this once or twice thoroughly and the problem generally goes away. Also very effective on young shoots of roses etc. at this time of year. Doing this means that blue tits and other small birds can then pick them up from the ground without coming to any harm. I don’t like using any sprays at all as they will damage other life. Putting bird feeders near your veg patch will attract many birds who will hang around and pick off pests too. Get nature working with you – it will, if you let it.