For antagonists and mutualists: the paradox of insect toxic secondary metabolites in nectar and pollen

The plant kingdom produces an extraordinary diversity of secondary metabolites and the majority of the literature supports a defensive ecological role for them, particularly against invertebrate herbivores (antagonists). Plants also produce secondary compounds in floral nectar and pollen and these are often similar to those produced for defense against invertebrates elsewhere in the plant. This is largely because the chemical armoury within a single plant species is typically restricted to a few biochemical pathways and limited chemical products but how their occurrence in floral rewards is regulated to mediate both defence and enhanced pollination is not well understood. Several phytochemicals are reviewed here comparing the defensive function alongside their benefit to flower visiting mutualists. These include caffeine, aconitine, nicotine, thymol, linalool, lupanine and grayanotoxins comparing the evidence for their defensive function with their impacts on pollinators, their behaviour and well-being. Drivers of adaptation and the evolution of floral traits are discussed in the context of recent studies. Ultimately more research is required that helps determine the impacts of floral chemicals in free flying bees, and how compounds are metabolized, sequestered or excreted by flower feeding insects to understand how they may then affect the pollinators or their parasites. More work is also required on how plants regulate nectar and pollen chemistry to better understand how secondary metabolites and their defensive and pollinator supporting functions are controlled, evolve and adapt.