Hi there
It appears that the main reference is in German, which I don't read, but here's the context:

Pollen of Ranunculus has been shown to be toxic to the honey bee: honey bee workers foraging on Ranunculus for pollen suffered dramatic mortality, a phenomenon known as the “Bettlacher May sickness” (Morgenthaler and Maurizio 1941). Most species of the Ranunculaceae contain the toxin protoanemonin, which is poisonous to vertebrates and shows insecticidal and antimicrobial activity (reviewed in Jürgens and Dötterl 2004). 

Praz, C. J., Müller, A., & Dorn, S. (2008). Specialized bees fail to develop on non‐host pollen: do plants chemically protect their pollen. Ecology, 89(3), 795-804.

refs

Morgenthaler, O. , and A. Maurizio . 1941. Die “Bettlacher Maikrankheit”, eine Vergiftung durch Hahnenfuss-Pollen. Schweizerische Bienenzeitung 64: 538–542.

Jürgens, A. , and S. Dötterl . 2004. Chemical composition of anther volatiles in ranunculaceae: genera-specific profiles in Anemone, Aquilegia, Caltha, Pulsatilla, Ranunculus, and Trollius species. American Journal of Botany 91: 1969–1980.

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