USING THE FOSSIL RECORD TO EXAMINE LONG-TERM PATTERNS IN THE EVOLUTION OF INSECTS AND THEIR HOST PLANTS. DO GALL WASPS MAINTAIN HOST-FIDELITY OVER GEOLOGIC TIME?

Oak leaf galls are a rare but persistent feature in fossil floras from western North America. Made by wasps (Hymenoptera: Cynipini) that specialize on oaks, both the galls and gall-attachment scars have been recognized from numerous Miocene and Pliocene floras in California, Idaho, Nevada, Oregon and Washington. Modern cynipid wasps are very host specific and studies based on molecular analyses have indicated that host plant shifts should have been rare throughout the group’s evolutionary history. If fidelity to a specific host plant is maintained through time, then cynipid galls should only occur on the same host species through time and across floras. To examine the host fidelity of cynipid wasps, we examined 20 Neogene floras from western North America to test the hypothesis that host use is highly conserved in this group. Over 2,500 oak leaves from at least 6 species were examined, yielding approximately 60 galled leaves. Galls and gall scars were restricted almost exclusively to Q. simulata (a red oak) and Q. hannibali (an intermediate oak). The unique and consistent shape of gall scars on leaves, including three-dimensional preservation of galls on some leaves, has allowed them to be identified as those made by cynipids including the modern genus Antron, a specialist on white oaks. Only a few cynipid gall scars were found on other oak species, including one potential gall on a white oak in one of the oldest floras in which galls were observed.

Our results provide evidence for a long-lived association, approximately 10 million years, between Antron and their red and intermediate oak hosts. However, the modern association between Antron and the white oaks may represent a relatively recent host shift, or at least the abandonment of a long-standing host relationship, as opposed to a relationship that has been maintained through the evolutionary history of these groups. Although there is evidence of specialization and long-lived associations between cynipid wasps and their oak hosts, they are not as one would have predicted based on modern associations, nor on modern phylogenetic data.