The role of toxic nectar secondary compounds in driving differential bumble bee preferences for milkweed flowers

While morphological differences such as tongue length are often featured as drivers of pollinator floral preferences, differences in chemical detection and tolerance to secondary compounds may also play a role. We sought to better understand the role of secondary compounds in floral preference by examining visitation of milkweed flowers, which can contain toxic cardenolides in their nectar, by bumble bees (Bombusspp.), some of their most abundant and important pollinators. We examine bumble bee species visitation of common milkweed (Asclepias syriaca) compared to other flowers in the field and test whether observed preferences may be influenced by avoidance and tolerance of cardenolides, as measured by the cardenolide ouabain, in the lab. We reveal that common milkweed is visited predominantly by one bumble bee species,Bombus griseocollis, in a ratio much higher than the abundance of this species in the community. We confirmed the presence and toxicity of cardenolides inA. syriacanectar. Lab experiments revealed thatB. griseocollis, compared to the common bumble beesB. impatiensandB. bimaculatus, exhibit greater avoidance of cardenolides, but only at levels that start to induce illness, whereas the other species exhibit either no or reduced avoidance of cardenolides, resulting in illness and mortality in these bees. Toxicity experiments reveal thatB. griseocollisalso has a substantially higher tolerance for cardenolides thanB. impatiens.Together, these results support a potential evolutionary association betweenB. griseocollisand milkweed that may involve increased ability to both detect and tolerate milkweed cardenolides.