in 2004 i had a real problem with aphids causing leaf curl on my plum tree, it looked awful as all the leaves curled up and went brown, and i feared for the tree's survival. After doing lots of reading the best advice i could find was to use a systemic and spray just before the flower buds break open. i did it last year in late jan and it worked very well as only one stem on one branch had the problem. this year i sprayed last monday, amazing how much the cold winter has delayed everything.

anyways as someone above has said they overwinter in the bark, but apparently as the buds start to swell on the tree they start their seasonal life cycle.

as yours is an apple which come into spring later than plums your leaf buds probably haven't started to swell. however when they do just use any systemic that kills wooly aphids as this should stop them before they start. thereafter just keep checking, and spray again if you need to.

that said due to the amount of sprays i've given my tree i wouldn't eat the plums, but as its the only mature tree in my back garden it gives real structure so i wouldn't contemplate removing it