Safe sites of pollen placement: a conflict of interest between plants and bees?

Plant stigmas and bee pollinators are competitors for pollen. Pollen placed on a pollinator's body can be picked up by conspecific stigmas or it can be collected by the pollinator as food. Hypothetically, one solution is for pollen to be placed on 'safe sites' on the pollinator's body, sites where the pollinator cannot easily remove it, leaving the pollen for stigmas. We compared 14 sites on the bumblebee body in terms of the ability of the bee to groom off fluorescent power, a dust that may be thought of as analogous to pollen. The safest sites were along the midline of the dorsal thorax, the dorsal abdomen, and the ventral abdomen. Next, we counted how much pollen is borne on the 14 sites by bees visiting one nectariferous and three nectarless Pedicularis species. In the four species, only 7, 26, 28, and 30% of pollen found on the bees were on safe sites. Finally, we observed that the 14 sites were contacted by stigmas of the four Pedicularis species; none of the most contacted sites were safe sites. Across all four Pedicularis species, pollen is mainly positioned on sites of the bee body that were beneficial for both the plant and the bee, not on sites detrimental to either of them. Our analysis showed that the conflict of interest between flowers and bees can be solved by cooperation. Pedicularis pollen is placed where it strengthens the mutualism between plants and pollinators.