There is something growing on the leaves of my maple tree

[fontanals]

It starts with spring, as the leaves grow little red bumps are visible on
each leaf. As summer rolls around, these bumps multiply and turn green
until the whole leaf is covered and drops off the tree. Does anyone know
what they are and are they harming the tree?

 

[Treeman14]

Those are galls, most of which are caused by wasps (there are hundreds of different gall makers). Most likely maple bladder galls. Check out these websites for descriptions and more info:

www.ag.ohio-state.edu/~ohioline/hyg-fact/2000/2004.html

www.bostocktree.on.ca/handouts/maplebgm.htm

www.oldhouseweb.com/gardening/garden/01701256.shtml