The insect order Hymenoptera originated during the Permian nearly 300 million years ago. Ancestrally herbivorous hymenopteran lineages today make up the paraphyletic suborder 'Symphyta,' which encompasses circa 8200 species with very diverse host-plant associations. We used phylogeny-based statistical analyses to explore drivers of diversity dynamics within the 'Symphyta,' with a particular focus on the hypothesis that diversification of herbivorous insects has been driven by the explosive radiation of angiosperms during and after the Cretaceous. Our ancestral-state estimates reveal that the first symphytans fed on gymnosperms, and that shifts onto angiosperms and pteridophytes—and back—have occurred at different time intervals in different groups. Trait-dependent analyses indicate that average net diversification rates do not differ between symphytan lineages feeding on angiosperms, gymnosperms, or pteridophytes, but trait-independent models show that the highest diversification rates are found in a few angiosperm-feeding lineages that may have been favored by the radiations of their host taxa during the Cenozoic. Intriguingly, lineages-through-time plots exhibit signs of an early Cretaceous mass extinction, with a recovery starting first in angiosperm-associated clades. Hence, the oft-invoked assumption of herbivore diversification driven by the rise of flowering plants may overlook a Cretaceous global turnover in insect herbivore communities during the rapid displacement of gymnosperm- and pteridophyte-dominated floras by angiosperms.