Aphid onion attack

My growing onion sets have been attacked by little green bugs for the fourth time now - can anybody help? I didn't think aphids were supposed to like onions! I can get rid of them initially using soapy water but they keep coming back! They seem to live at the base of the green growth and eventually turn the leaves yellow and wilted. I am a complete novice with gardening so any help would be much appreciated.

Many Thanks