“We don’t serve your kind here” sounds like something one would hear in a diner in the 1950s, but flowers have their own way of sending this message as well.
While it seems a little hard to imagine, flowers can be discriminating when it comes to pollinators. In fact, certain plant species use toxic nectar to repel the wrong type of bee from its flowers.
As for what makes the bee the wrong type, it is all about tongue length, otherwise known as proboscides. Bees with short tongues are termed generalists as they tend to visit all types of plants and are only able to take advantage of shallow flowers like those in the daisy or aster family.
Long-tongued bees on the other hand are specialists, preferring a few deep-throated flower species. As they lap up the nectar, their bodies collect pollen to carry on to the next flower of the same species.
The problem is that in desperate times when other sources of nectar are not so readily available the short-tongued bees will chew through the hood of the plants’ deep flowers, bypassing its reproductive structures entirely.
Because of this exploitation, ecologists call these bees nectar robbers. But don’t feel too bad for the plants just yet, as scientists at Kew Gardens have discovered that they have their own method of fighting back.
The scientists studied two different species of monkshood over two summers and monitored which pollinators came to visit using a newly-developed automated digital monitoring system called Rana.
Over the 244 hours the plants were monitored, a majority of the pollinators were the long-tongued species of bumblebee. Only four percent were short-tongued bumblebees and even then, very few actually landed to attempt stealing some nectar.
Since foraging for nectar is an energy-consuming task, the bees are wary, as some plants do not have a high concentration of nectar. When the nectar is hidden, they have to make a guess as to which is worth the search.
“If the reward is worth the risk they will rob,” Professor Phil Stevenson, senior research leader at Kew told New Statesman. “Typically, robbery may occur when less other nectar is around.”
The ecologists at Kew Garden noticed that when the bees did land, they robbed one species more than the other. A mass spectral analysis of the two nectars revealed that the less robbed species had a higher concentration of the alkaloid aconitine in its nectar.
While the plant species was robbed less, it also had less visits from its desired pollinator as well, and the scientists concluded the toxic nectar may unintentionally drive away the bees it is trying to attract.
As for how the plant was able to favor one species over another in the first place is thanks to the phenomenon known as specialist plant-pollination mutualism.
“It may be an adaptation of the plant,” Stevenson told New Statesman. “The pollination bee (long-tongued bee) feeds on the poisonous plant more frequently than the robbing species and so is more likely to produce offspring with greater tolerance of the defense chemicals.”
While these specializations gives long-tongued bees an advantage, it also makes them more vulnerable to change as their dependence on fewer flowers can intertwine species’ risk of extinction.