Pruning Prunus

I have a little plum tree/bushes it’s not fruited yet, it’s about 5ft high and I’ve pruned it most years to make it a tidier shape. I’m guessing that’s the reason. It’s got some fruit on it this year but also some curled new leaf which the ants seem very keen on. I’m wondering if there’s some bug in there.

 
I understand that you need to cover peach trees over winter or they get peach leaf curl. The leaves fall off and that means the fruit can end up small.