Gall wasps are so named because they inject chemicals into trees that create abnormal growth patterns (galls) that shelter the wasps' own developing larvae. In this cool recent study, paleontologists looked at the fossil record (over 2,900 fossil leaves from the western U.S., with the oldest about 45 million years old) in order to see how stable the relationship is between gall wasps and their preferred host trees. This is possible because the galls are sometimes preserved in the fossil record. It turns out that galls associated with Cynipini wasps only occur on the leaves of just two types of oak trees, and that the association remains stable over about 30 million years: An interesting case of stability in an ecological relationship! This raises some interesting evolutionary questions about why parasites do (or in this case, don't) shift their hosts.
E.H. Leckey and D. H. Smith, "Host fidelity over geologic time: restricted use of oaks by oak gall wasps," Journal of Paleontology, vol. 89, no. 2. (March 2015), 236-244.