Q: What is wrong with my aspen leaf?
A: That is an aspen leaf miner. It’s a small bug that lays eggs inside of an aspen leaf. As the larvae begins to grow, it starts to feed off of the chlorophyll. Although the leaf might have some discoloration and damage, it will not harm the entire tree - only that leaf.
It isn’t something to worry about since they are almost never found in large numbers. No treatment is necessary.