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31 August 2017

Nectar robbers: how flowers discriminate against the wrong kind of bees

Toxic nectar is used to turn away short-tongued bees.

By Jason Murugesu

Civil rights issues are not just for humans. Ecologists at Kew Gardens have discovered that certain plant species use “toxic nectar” to repel the “wrong type” of bees from their flowers. 

The “wrong type” of bee in this case is a short-tongued bumblebee. They visit all types of plants and are termed generalists. In tough times however (austerity has affected us all) these bees must resort to robbing nectar from plants which have their nectar deeply hidden, and are usually only pollinated by long-tongued bees. Short-tongued bees, on the other hand, chew through the hood of the plants’ flowers to better access the nectar. This method is to the detriment of the plant as the bees bypass its reproductive structures. 

Ecologists call them nectar robbers.

But plants fight back, as scientists at Kew Gardens discovered. Over two summers, they studied two different species of Aconitum, more commonly known as Monkshood, and monitored the visitors they received. They used a newly-developed automated digital monitoring system called Rana, which detected and counted the number of bumblebees that visited the plants. 

The scientists found that over the 244 hours they monitored the plants, most of the bees who visited were a long-tongued species of bumblebee. Only 4 per cent of the plants’ visitors were the short-tongued nectar robbers, and even then most of the time, the robbers never actually docked onto the plants themselves. The reasons for this, the scientists believe are two-fold: economics and the “ecological paradox” that is toxic nectar.

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Foraging for nectar requires energy, so bees are cautious. They don’t want to waste energy looking for nectar in plants which may not have such a high concentration of the nutrient-rich liquid. As the nectar is hidden, they have to make a calculation as to whether each plant is worth searching through. It’s pot luck.

As Professor Phil Stevenson, Senior Research Leader at Kew, says “If the reward is worth the risk they will rob. Typically robbery may occur more when less other nectar is around.” 

Stevenson and co. discovered “toxic nectar” by observing that when the robber bees did dock, one species of plant was more robbed than the other. Using mass spectral analysis of each species’ nectar, the scientists determined that the plant that was robbed less had a higher concentration of an alkaloid called aconitine in its nectar- aka “toxic nectar”. 

This type of nectar however is a doubled-edged sword. The plant species which was robbed less also received significantly fewer visits by the long-tongued bees. The scientists concluded that plants that employ “toxic nectar” effectively may inadvertently repel the bees they are trying to attract as well. 

How have these plants learned to discriminate? This is due to a phenomenon known as specialist plant-pollination mutualism. The flowers of these plants will have evolved due to competition between pollinators. Bees with longer tongues have therefore encouraged genetic traits in the flowers, which allow the plant to protect itself from short-tongued visitors. 

The reason why “toxic nectar” is so effective is still unknown but Stevenson believes “it may be an adaptation of the plant. The pollinating 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 defence chemicals.” 

The short-tongued bees may be known as nectar robbers, but from another perspective, they are merely trying to make the most of what nature gave them. 

In all oppressive regimes, there are those that undermine the status quo, and in the British garden, it is the turn of these intrepid “nectar robbers”.

Still, studies have shown that climate change has affected the average tongue sizes of bees, so that more bees now have shorter tongues and are more likely to be generalists. One day the shunned minority may become the majority. Perhaps it is the oppressors we should be fearful for, after all.