Certified GKH Gardening Expert
They are, certainly, from insects. Usually mites or midges cause these types of galls. This isn't as bad as it gets, and is a fairly mild case here.
It isn't going to harm your tree that much. It may be unsightly, but treating for mites or other insects to prevent future infestations will be the only method of control.
Here is an article to help you to combat leaf galls: