This just out:

> In recent decades, a growing number of studies have documented the presence of compounds with deterrent or toxic properties in pollen and nectar, demonstrating that not only herbivores in the classical sense (i.e., consumers of vegetative tissue), but also pollinators, must confront plant defence compounds. In many instances, pollen and nectar appear to deter, harm or even kill pollinators, suggesting that plant chemical defence could play an important ecological role in plant–pollinator interactions. However, how defence compounds affect pollinator ecology and evolution remains mostly unknown. A variety of hypotheses have been put forward to explain the existence of toxic or deterrent nectar. 

> several pollen types frequently used by common generalist taxa contain defence compounds (e.g., Echium, Delphinium, Lupinus, Papaver, Solanum)

> Virtually all mutualistic relationships are influenced to some extent by antagonistic processes (Bronstein, 2001), and understanding how antagonists shape the evolution and ecology of pollination could greatly increase our understanding of this vital ecological process.

Rivest, S., & Forrest, J. R. (2019). Defence compounds in pollen: why do they occur and how do they affect the ecology and evolution of bees?. New Phytologist.

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