Pollen defenses negatively impact foraging and fitness in a generalist bee (Bombus impatiens: Apidae)

Plants may benefit from limiting the community of generalist floral
visitors if the species that remain are more effective pollinators and
less effective pollenivores. Plants can reduce access to pollen through
altered floral cues or morphological structures, but can also reduce
consumption through direct pollen defenses. We observed that Eucera
(Peponapis) pruinosa, a specialist bee on Cucurbita plants, collected pure
loads of pollen while generalist honey bees and bumble bees collected
negligible amounts of cucurbit pollen, even though all groups of bees
visited these flowers. Cucurbit flowers have no morphological adaptations
to limit pollen collection by bees, thus we assessed their potential for
physical, nutritional, and chemical pollen traits that might act as
defenses to limit pollen loss to generalist pollinators. Bumble bee
(Bombus impatiens) microcolonies experienced reduced pollen consumption,
mortality, and reproduction as well as increased stress responses when
exposed to nutritional and mechanical pollen defenses. These bees also
experienced physiological effects of these defenses in the form of hindgut
expansion and gut melanization. Chemical defenses alone increased the area
of gut melanization in larger bees and induced possible compensatory
feeding. Together, these results suggest that generalist bumble bees avoid
collecting cucurbit pollen due to the physiological costs of physical and
chemical pollen defenses.