I passed a European alder tree (Alnus glutinosa) on Willow Path at the Arnold Arboretum a couple days ago (accession 1399-73*B), and noticed that many of its branches were covered in a white fungus/mold-like substance. Upon closer inspection, the mold moved!
This was no mold. It was an infestation of woolly alder aphids (Prociphilus tessellatus).
Woolly alder aphids feed on the sap of alder bushes and trees in the summer. They secrete a white waxy thread-like substance that covers their abdomens and the branches they feed on.
Initially, the aphids that are born have no wings. They simply remain in place feeding. On this particular alder tree, I found aphids using the trunk as a highway, scurrying up and down. They lacked large amounts of the white wax that aphids attached to the branches produced.
As fall approaches, these aphids produce winged forms. They fly off to red or silver maple trees where they lay eggs. The nymphs feed on maple sap and eventually produce a winged generation that flies off in search of an alder. More on their life cycle at The Adirondack Almanack.
The wingless forms start off small in comparison to the winged forms (see photo below) but increase in size as they feed.
As with other aphids, these ones secrete sweet honeydew, which attracts ants and wasps. I observed a wasp, possibly a yellow jacket, land near an aphid colony.
As extensive as these colonies appear, most sources I checked say that they do not cause lasting harm to the host tree.