Why do sycamore leaves have mites and fleas, the red things?

Sometimes it’s tough being a tree…

This leaf is heavily infested with gall mites. But wait until you see how bad things are underneath...

The red galls on the top of the leaf are caused by the leaf’s reaction to damage inflicted by gall mites feeding. In response to the grazing, the leaf makes a little dome (the gall) and later on the mite lays its eggs inside that gall, where the larvae will hatch out and continue feeding and growing, perpetuating the infestation.

It seems tough for this tree, but it should be fine in the long run:)