This article is a part of the Green Thumbery series, where everyday gardening meets history and science.
The days have already taken on a chill here in Zone 7, so it’s almost time to wind down Green Thumbery for the year. We do have a few weeks left as the harvests come in, so it seemed like the right time to turn our attention to the pollinators who help our gardens. In recent years, honeybees have attracted a lot of attention as people are concerned about their declining population in relation to the their ability to pollinate certain plants and help with food production. This concern has spurred several corporate social responsibility campaigns designed to raise awareness and “help” the bees. But their role as pollinators seems to be a happy evolutionary accident that plants have taken advantage of. Bees do not visit flowers primarily to pollinate plants, but to get pollen to feed their offspring. Pollen is a major source of protein for bee larvae. The conflict that is created by the bees’ need for pollen and the plants’ need for pollen seems to be resolved by unkempt bees.
Bees actively collect pollen by gathering it from anthers with their forelegs or mouthparts. However, they also groom themselves to remove any pollen on their body that they may have picked up by brushing up against anthers during an active collection. This pollen is stored in the transport organs where it is effectively locked away and not released until they get back to the hive. This, as you can imagine, is problematic for plants whose pollination is helped by having bees transfer pollen between flowers. Pollen is expensive to produce, and flowering plants have developed several mechanisms to reduce pollen loss, including a toxic pollen that cannot be digested, pollen that is “hidden” in poricidal anthers, and guiding bees strategically so that pollen is deposited in places on the bees’ bodies that they cannot reach.
Just as you might not be able to reach that itchy spot between your shoulder blades, bees have trouble grooming some parts of their bodies. They can’t fully reach behind their heads and some places along their back that correspond to their chest and abdomen segments. These areas are known as “safe sites” (for pollen) because even after grooming, patches of pollen can be found in a dorsal line in these spots.
Researchers confirmed these safe sites by putting the insects in transparent plastic jars where the floor was covered in pollen and sealing the jar with a foam plug. Pollen from two plant species was used to rule out potential adherence issues due to size or structure of the pollen. The contained bees’ constant flight activity stirred up the pollen, mixing it well and the insects were coated within minutes. They were then transferred to a new, clean jar and allowed to groom themselves for 30 minutes. After this time, they were put in a refrigerator and then freeze-killed so their bodies could be photographed with a dissecting microscope. Pollen was also counted by removing the pollen with tape and then sticking it to black cardboard.
To test whether plants could access bees’ safe sites, an experiment was set up at the Botanical Gardens in Dusseldorf where two species of bees—a bumblebee (Bombus terretris) and a honeybee (Apis mellifera)—were enticed to visit flowers to collect pollen. The anthers of these flowers were marked with a white UV luminous pigment. After one visit, the bees were caught, freeze-killed, and then photographed for comparison.
The results revealed that following grooming, pollen was left on the dorsal waist, dorsal abdomen, and dorsal thorax. While bees wiped their heads and antenna with their forelegs, they did not reach the area between the antennae. And while they used their middle legs to clean the dorsal and lateral thorax, the bumblebees were unable to reach the space between their wings and the honeybees had trouble reaching a large area behind their heads. Furthermore, for both species, the transfer of pigment from the anthers included the safe sites.
This distribution of pollen is explainable when you consider the structure of the bee and where its legs can reach, as these are the primary means by which the insect transfers pollen into the transport organs. Remember, their primary goal is to get the pollen back to the hive, not to use it themselves, so they’re looking to capture as much of it as they can reach and pack it away for flight. The areas that they can reach, like the flanks, have no safe sites, while the ares that are less accessible, like the waist area, will have the most “free” pollen.
The relationship between the flower, pollen, and the bee is partly influenced by the morphology of the flower. Most bee-pollinated plants place pollen on the dorsal side of the because it is harder to remove for transport. Flowering plants have a vested interest in making this difficult for bees because the efficient distribution of pollen means less pollen production is required and fewer anthers are required—both of which carry a heavy energy cost.
Bees are known to groom in flight, and it’s not known whether this grooming is more efficient and gets more pollen from safe sites. We also don’t yet know how much pollen from safe sites are actually transferred to other flowers. But the evidence does suggest that for creatures that evoke a sense of order and precision, a little sloppiness may go a long way.
Did you plant bee-friendly flowers this year? Tell us about it. Comments have been disabled on Anthropology in Practice, but you can always join the community on Facebook.
Koch L, Lunau K, Wester P (2017) To be on the safe site – Ungroomed spots on the bee’s body and their importance for pollination. PLoS ONE 12(9): e0182522. https://doi.org/10.1371/journal.pone.0182522
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