The wonder of bumblebees laden with pollen apparently defying gravity as they forage through the summer air increasingly belongs to the tales we tell our children about these amazing creatures rather than to their own experience. As bee populations collapse, some have thought that other species of smaller bees can take over the work of the bumblebees.
But that appears to be a false hope. It turns out that smaller bees actually rob the plants of pollen, which is an important protein source for the bees' offspring, but fail to incidentally transfer the pollen from the male to the female parts of the plants.
reports Matt Koski, the lead author in a team of University of Virginia and College of Wooster (Ohio) biologists who studied the visits of several species of bees to plants ranging from Alabama to Minnesota. Bumblebees earn their name from the loud hum they generate, which actually arises from their secret tool for harvesting pollen. Plants often stash their pollen away to make it difficult for bees to access. The bumblebee produces high-frequency vibrations that effectively blast the pollen out of storage where it clings to the bees' fuzzy hairs.
“We were surprised to find that some of the small pollinators were actually detrimental to the plants they visited, rather than beneficial,”
The UVA team tracked the grains of pollen transported away and the pollen deposited by bumblebees, a medium sized bee species, and two smaller species of bees. They found that the bumblebees often left some pollen behind where it could fertilize the flowers, paying for their meals by helping the flowers to create seeds. Importantly, bumblebees frequently visited the female phase of the flowers, improving the fertilization efficiency.
Conversely, the medium size and smaller bees acted as "pollen thieves." They took away pollen without succeeding to transfer it to the plants' stigmas; as a result, their visits actually reduced the plants' fertility. Once the pollen has been "stolen," the plants potentially have lost their chances to successfully produce seeds.
This study points to the need to conserve important pollinator species by protecting their habitat, reducing pesticide threats, and by controlling both climate change and the introduction of invasive species.