The intersection of bee and flower sexes: pollen presence shapes sex-specific bee foraging associations in sunflower.

Foraging preferences are known to differ among bee taxa, and can also differ between male and female bees of the same species. Similarly, bees can prefer a specific flower sex, particularly if only one sex provides pollen. Such variation in foraging preferences could lead to divergent bee communities visiting different flower sexes of a plant species. We sampled bees visiting sunflowers to characterize bee species richness, abundance, and sex ratios on pollen-fertile and pollen-sterile cultivars. We asked whether female or male bees were more abundant on sunflowers, whether female bees were more abundant on pollen-fertile or pollen-sterile cultivars, and whether pollen presence predicted the sex of sampled bees. We further asked whether the bee community differed between pollen-fertile and pollen-sterile cultivars. Females of most bee species were more abundant on sunflowers compared to males, and females were usually more abundant on pollen-fertile cultivars. In three bee species, pollen presence was predictive of a bee's sex, with females more abundant on pollen-fertile cultivars than males. Further, the bee community differed significantly between pollen-fertile and pollen-sterile cultivars, with two bee species functioning as indicators for pollen-fertile sunflowers. Our results demonstrate that a bee's sex shapes foraging associations on sunflowers and influences abundance between pollen-fertile and pollen-sterile cultivars, and that pollen-fertile and pollen-sterile cultivars are visited by different bee communities. Bee sexes and flower pollen presence may be under-appreciated factors shaping pollination services in both agricultural and natural ecosystems, and could be important considerations for pollination of crops with pollen-fertile and pollen-sterile flowers.