Everyone knows about the birds and the bees but what most people don’t know is when plant-pollinator relationships are abused by pollinators, plants can create their own signature blend of toxin-infused nectar.
In 2017, Barlow and her team of researchers discovered that the story of plants and pollinators was more complex than a simple tale of two parties providing a service for each other. They found that nectar robbers forced Aconitum sp., to create the perfect blend of nectar and toxic alkaloids, where nectar robbers are deterred but not mutualistic pollinators that are essential for plant reproduction.
Mutualistic pollinators include long-tongued bumblebees, like Bombus hortorum, who have acquired resistance to the toxic nectar of Aconitum sp. over evolutionary time. This is a direct result of both species evolving in tandem to each other or co-evolving (Barlow et al., 2017) But just how toxic is this nectar and are seemingly alkaloid resistant bumblebees actually immune or does Aconitum sp. harm its pollinators in the name of self-defence? Is this desperate measure all in vain? Barlow and her team set-forth to find out.
Barlow spent two years at the Royal Botanic Gardens, Kew in the United Kingdom, observing the visitors for two species of Aconitum flowers, A.napellus and A.lycoctonum. Over the years, the team noticed visits from B.hortorum, the long-tongued pollinator, held true to our typical pollinator story by always making contact with the flower’s stamen or stigma when collecting nectar. However, a short-tongued bumblebee, known as Bombus terrestris, never once contacted the flowers of any two species. Instead B.terrestris used its mandibles to rip open the galea of the flower. Once pried open, this helmet-like structure oozed out the desired nectar (Barlow et al., 2017).
Overall, Barlow saw that nectar robbing B.terrestris steered clear the toxic plants, accounting for only 3.6% of the total visits to the flowers; whereas, long-tongued pollinator, B.hortorum accounted for 92.5% of total visits. This suggests that the desperate measures employed by Aconitum sp. was effective in deterring robbers without the loss of pollinators.
Next, Barlow and her team wanted to find out, just how resistant long-tongued bumblebees were to toxic alkaloids. In the laboratory, they feed nectar robbing, B.terrestris a sugar solution, pre-mixed with tiny amounts of alkaloids, to determine how much toxins it would take to dissuade the robbers from consuming the solution (Barlow et al., 2017). At 20 parts per million, the robbers completely avoided consumption of the tainted sugar. This amount of toxic alkaloids proved to be 10-20 times lower than what the long-tongued, B.hotorum could handle (Barlow et al., 2017).
From this, Barlow was able to conclude that the toxicity of nectar was a careful balancing act performed by Aconitum species. Long-tongued pollinators were not completely immune to toxic nectar but were more resistant than short-tongued robbers. Therefore, Aconitum sp. must be careful when selecting a concentration of toxic alkaloids. They must aim for the perfect blend of sugar and spice, where robbers are deterred but not mutualistic pollinators (Barlow et al., 2017).