I posted a question some time ago about getting rid of wooly aphids from a pyra cantha bush ,the answer is listed someway back, but I will say it is very hard work I think I will wait till the winter months and give it a good going over with jeyes fluid as a winter wash it stinks but hopefully it will do the trick.As far as the canker goes it is caused when the aphid burrows into the bark and is much the same as the scars that occur when you cut a branch off.It does have some effect though as it slows down the rate of growth of the tree as I have learnt ,I havent pruned the bush this year and I usually prune it twice this is all I have learnt so far good luck