|Eriophyid mite on aspen leaves|
The good news is that the gall does not affect plant health, and usually the same tree is not hit two years in a row. So, it is just an aesthetic issue. The affected leaves can be cut out if they are really bothersome, but it is not necessary.
The distortion is caused by chemical secretions produced during feeding or egg laying. The chemicals act like natural plant growth hormones. The leaves only become distorted when the leaf is attacked early in the growing season when it is rapidly expanding. Fully expanded leaves do not have the same reaction.
For more information, please see this fact sheet: