The age of bees is disputed. The dispute probably eventually will be resolvable using "DNA clock" evidence, but so far, nobody has tried.
The oldest known stingless bee fossil was thought to be 74-94 million years old but was re-estimated to have been only 65-70 million years old. The oldest honeybee fossils are 22-25 million years old. Eusocialization is presumed already to have occurred by the times of those fossilizations since they were sterile "worker" bees.
M.S.Engel: New interpretation of the oldest fossil bee (Hymenoptera: Apidae), Amer. Museum Novitates #3296 (April 2000) 11 pp. (pdf)
C.D. Michener & D.A. Grimaldi: The oldest fossil bee: Apoid history, evolutionary stasis, and antiquity of social behavior, Proc. Nat'l. Acad. Sci USA 85,17 (Sept. 1988) 6424-6426.
But flowers presumably co-evolved with bees and are at least 130 million years old. About 100 small nests, apparently each collaboratively constructed 207-220 million years ago by early kinds of carpenter and miner bees to house 10-100 individuals, were found in Arizona's petrified forest in fossilized wood and sand. Chemical analysis showed they contained some of the organic compounds of the sort bees excrete from their Dufour glands to coat their nest walls. If so, then bees came before flowers.
Stephen T. Hasiotis (with contributions from others): Continental trace fossils, Soc. for Sedimentary Geology 2002, short course notes #51.
S.T.Hasiotis, R.F.Dubiel, P.T.Kay, T.M.Demko, K.Kowalska, D.McDaniel: Research Update on Hymenopteran Nests and Cocoons, Upper Triassic Chinle Formation, Petrified Forest National Park, Arizona http://nature.nps.gov/geology/paleontology/pub/grd3_3/pefo1.htm
If bees were already around before flowers, then that would make it much easier to understand why flowers appeared and diversified very suddenly (old puzzle noted by Darwin): The flowers suddenly saw how to take advantage of the bees (a pre-existing resource for them) and then both co-adapted rapidly. But it would make it much harder to understand: why are there not still some bee-types still around today which are not into flowers?
Engel (p.151) attacked Hasiotis' "fossil bee nests," claiming the chemical findings were "not diagnostic for bees at all" and these nests actually were probably "beetle galleries." Engel: "the bees are a derived, monophyletic group of the spheciform wasps and presumably arose sometime in the earliest mid-Cretaceous after the origin of angiosperms... between 125-90 million years ago... the diversification of bees must have coincided with the angiosperm radiations 130-90 Mya and [especially] 115-90... Bees arose after flowers."
Michael S. Engel: A monograph of the Baltic amber bees and evolution of the Apoidea (Hymenoptera), Bulletin of the American Museum of Natural History 259 (2001) 1-192. (pdf)
Conclusion: Eusocial bees are at least 65 million years old, but some kinds of bees with some degree of sociality may have existed as long as 220 million years ago (before flowers existed).
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