A bittersweet vine (Celastrus sp.) twists around a tree trunk in the Arboretum with tiny black creatures crawling on it. I believe these are black bean aphids (Aphis fabae). These aphids are sucking the sap out of the vine. While they prefer eating from the bounty of the bean family, they are known to attack bittersweet.
Like other aphids, they produce a sweet waste product called honeydew. Ants are attracted to the honeydew and collect it, tending aphids as we do cows. In the photo below, a black carpenter ant (Camponotus pennsylvanicus) watches over a black bean aphid, waiting for its sugary treat.
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!