The puzzle of monarch butterflies (Danaus plexippus) and their association with plants containing pyrrolizidine alkaloids

1. The chemical defences of monarch butterflies involve two kinds of secondary plant metabolites: cardiac glycosides (CGs) that are obtained from larval hostplants, and pyrrolizidine alkaloids (PAs) that are gathered by adults usually independent from feeding behaviour. While monarchs and CGs have received great attention, monarchs and PAs appear largely ignored in science as well as by the public. 2. We review and discuss PA‐pharmacophagy specifically for Danaus plexippus and its closest relatives and aim to encourage research and future consideration of this peculiar type of insect‐plant relationship, which represents an intrinsic but surprisingly understudied aspect of the lives of adult monarchs.