Nicotine and floral constancy in bumblebees

A buff-tailed bumblebee, Bombus terrestris, visiting heather. Photo by Steve Slater. 

This week's paper is by David Baracchi from his postdoc in Lars Chittka's lab at Queen Mary University. Baracchi tackled the very cool combination of nectar chemistry and bee learning. Research with caffeine has indicated that the presence of caffeine in flower nectar enhances pollinator memory for that flower type, thereby increasing floral constancy (consistency in visits to one floral type over others) and thus pollination. In this paper they examined nicotine, which is present in the flowers of plants such as wild tobacco, Nicotiana attenuata. Baracchi studied the effect of nicotine in flower nectar on bee learning of flower colors, and fidelity to that color even after it was no longer rewarding. 

What they did

Baracchi and his colleagues did three experiments. In the first they assessed how the presence of nicotine at different concentrations affected foraging choices of bees. Individual bees were allowed to forage in an array of artificial flowers, half of which were blue and half violet. They tested 20 bees each with three nicotine concentrations, two natural concentrations (low natural and intermediate natural) and one unnaturally high concentration.

In the second experiment they examined the effect of nicotine on learning. Bees were trained to associate one flower color with a reward (sugar water alone or plus one of the three nicotine concentrations), and another color with just water. They then counted how many visits bees made to the rewarded versus unrewarded flower types.

In the third experiment they used the same bees from the second experiment but reversed the color/reward associations. So the flower color that had been rewarding was now unrewarding and vice versa. They examined how many visits bees made to the two flower types. 

What they found

In the first experiment, bees were actually attracted to nicotine-laced nectar at the low natural concentration, showed no preference at the intermediate natural concentration, and were deterred at the unnaturally high concentration. 

In the second experiment bees with the intermediate natural concentration of nicotine added to their rewarding flowers made more correct choices than bees with the low natural, unnaturally high, or no nicotine in nectar. Nicotine therefore appears to enhance memory (or preference for some other reason, this paper does not distinguish) for rewarding flower types but only at intermediate concentrations. 

In the third experiment when the color/reward pairings were reversed, bees made more visits to the now unrewarding flower types when they had been paired with nicotine. Especially at the highest concentration! Although the unnaturally high concentration did not appear to improve learning, it did cause the bees to remain faithful to that flower color even when it was no longer rewarding. The other two nicotine concentrations also enhanced fidelity to unrewarding flower types, in a dose-dependent manner, with the strongest effect at the highest concentration. 

The takeaway

I am fascinated by how compounds in flower nectar may have psychological effects on pollinators to manipulate their behavior to the plant's advantage. The dose dependent effects in this paper are very interesting. Baracchi and his colleagues show preference but no improved learning at very low concentrations, no preference but improved learning at intermediate concentrations, and no preference or improved learning but strong fidelity at high concentrations. I am interested in the different pollination requirements or conditions that could give a selective advantage to each of these concentrations.