Field surveys of cynipid gall-inducer occurrences on Quercus species were conducted in Florida, North Carolina, and Pennsylvania. Thirty-four oak species were surveyed and 247 species of cynipid gall-inducing insects were identified. All cynipids demonstrated strong host-species and organ fidelity, and several gall inducers showed preferences based on habitat or host-plant size. Cynipids that initiated galls on members of the white-oak Section (Quercus) never infested oaks in the red-oak Section (Lobatae). This specialization creates effective niche partitioning among cynipids. The host-association patterns of such specialists should reflect similarities among oaks. Consequently we used principal components and hierarchical cluster analyses to group oak species according to their cynipid distributions. Cynipids distinguished small differences among their hosts including between several problematical oak species: Q. laurifolia-hemisphaerica, Q. prinoides-muehlenbergii, and Q. virginiana-geminata. A dendrogram of oak species based on cynipid distributions was reasonably congruent with botanical arrangements. However, our classification disagrees with botanical arrangements in several ways: Q. arkansana is distinct and not clustered with Q. marilandica and Q. laevis, and Q. marilandica is clustered near Q. imbricaria and Q. ilicifolia. Collaborative efforts between taxonomic botanists and entomologists will be useful in resolving a variety of plant and insect systematic problems.

Key words: classification, Cynipinae, Fagaceae, gall, host-plant association, Quercus