Pollenkitt is associated with the collectability of Malvoideae pollen for corbiculate bees

Pollen grains of Malvoideae (Malvaceae) which corbiculate bees cannot collect constitute a floral filter that excludes pollen-collecting bumble bees and honey bees from exploiting pollen resources. Although large, spiny pollen grains are in fact harder to compact for collection by corbiculate bees, pollen morphology (e.g., grain diameter, spine length) is not by itself a reliable indicator of pollen collectability. In this study, we discovered that two Malvoideae species, Anoda cristata and Malope trifida, possess large, spiny pollen grains that can be groomed and collected by corbiculate bees. To gain insight into the underlying cause of collectability of Malvoideae pollen, we tested pollen adhesion to bumble bee setae and found that significantly less of the collectable pollen grains of A. cristata and M. trifida adhere to bees’ setae compared to uncollectable pollen grains of Hibiscus trionum. As the primary mediator of pollen adhesion is pollenkitt, a viscous lipid-rich substance covering pollen of zoophilous plants, we examined the surface of uncollectable and collectable Malvoideae pollen using cryo-SEM. Fresh pollen grains were abundantly covered with pollenkitt that also coated the long spines and formed liquid bridges between the grains. Washing pollen with hexane removed all pollenkitt, whereas washing pollen with water only removed pollenkitt on the collectable pollen grains of M. trifida, but not the uncollectable pollen grains of Hibiscus syriacus. We hypothesise that pollenkitt composition differs between Malvoideae species with uncollectable and collectable pollen. Specific pollenkitt properties might elicit excessive viscidity which affects adhesion to insect visitors but prevents pollen collection by corbiculate bees.