Pollen chemical and mechanical defences restrict host-plant use by bees

Plants produce an array of chemical and mechanical defences that provide
protection against many herbivores and pathogens. Putatively defensive
compounds and structures can even occur in floral rewards: for example,
the pollen of some plant taxa contains toxic compounds or possesses
conspicuous spines. Yet little is known about whether pollen defences
restrict host-plant use by bees. In other words, do bees, like other
insect herbivores, tolerate the defences of their specific host-plants
while being harmed by non-host defences? To answer this question, we
compared the effects of a chemical defence from Lupinus (Fabaceae) pollen
and a putative mechanical defence (pollen spines) from Asteraceae pollen
on larval survival of nine bee species in the tribe Osmiini (Megachilidae)
varying in their pollen host use. We found that both types of pollen
defences reduce larval survival rate in some bee species. These
detrimental effects were, however, mediated by host-plant associations,
with bees being more tolerant of the pollen defences of their hosts,
relative to the defences of plant taxa exploited by other species. This
pattern strongly suggests that bees are adapted to the pollen defences of
their hosts, and that host-plant use by bees is constrained by their
ability to tolerate such defences.