These aren't my problem, as they're on street trees, but we were interested to know what they are. We identified the trees as Tilia Americana (American Basswood), whose flowering structure looked so bizarre to us that I wasn't convinced that the red things didn't belong on the tree, but I don't see anything describing them, so I guess they're a pest. Ric sent me his photos so I could post them.
Thanks - looks just like them in this writeup: http://www.rhs.org.uk/Advice/profiles0704/lime_nail_gall_mite.asp It's amazing that they seem to be associated with just this one tree (which is also called Lime tree). Bladder gall mite growths don't seem to be as pretty as these ones, though they're the same shape.
