Nicotine in floral nectar pharmacologically influences bumblebee learning of floral features

Abstract

Many plants defend themselves against herbivores by chemical deterrents in their tissues and the presence of such substances in floral nectar means that pollinators often encounter them when foraging. The effect of such substances on the foraging behaviour of pollinators is poorly understood. Using artificial flowers in tightly-controlled laboratory settings, we examined the effects of the alkaloid nicotine on bumblebee foraging performance. We found that bumblebees confronted simultaneously with two equally rewarded nicotine-containing and nicotine-free flower types are deterred only by unnaturally high nicotine concentrations. This deterrence disappears or even turns into attraction at lower nectar-relevant concentrations. The alkaloid has profound effects on learning in a dose-dependent manner. At a high natural dose, bees learn the colour of a nicotine-containing flower type more swiftly than a flower type with the same caloric value but without nicotine. Furthermore, after experiencing flowers containing nicotine in any tested concentration, increasing numbers of bumblebees stay more faithful to these flowers, even if they become a suboptimal choice in terms of reward. These results demonstrate that alkaloids enhance pollinator flower constancy, opening new perspectives in co-evolutionary process between plants and pollinators.

Figure 1

Bumblebee preference for nicotine-enriched nectars. Individual preferences (mean value of 100 choices) of bees (n = 60, 20 each nicotine concentration) for the flower type associated with nicotine-laced sucrose solution during a foraging choice test. Bees are deterred only by unnaturally high nicotine concentrations (50-ppm). This deterrence disappears or turns into attraction at lower nectar-relevant concentrations (1 and 2.5-ppm). Red dots represent individual bees, while the black bars indicate the mean value for each group. The grey line indicates random choice level. *P = 0.001.

Figure 2

Nicotine-laced nectar enhances the forward learning performance of bumblebees during foraging. Learning curve (mean choice accuracy ± SEM) across 50 visits of bees exposed to 1-ppm (n = 15), 2.5-ppm (n = 16) and 50-ppm (n = 14) nicotine and not exposed (n = 15). Individual bees were allowed to forage on two flower types, one unrewarding (water only) and one rewarding (sucrose solution only (controls) or sucrose solution laced with nicotine). After a rewarding flower had been visited for the first time by a bee, 50 consecutive choices were recorded. Each learning curve was generated from the mean choice accuracies (percentage correct choices made) for every consecutive block of 10 choices. Overall, nicotine at 2.5-ppm significantly enhanced the speed of learning (P = 0.015) while the other two tested doses did not (1-ppm: P = 0.1; 50-ppm: P = 0.26).

Figure 3

Nicotine previously encountered in nectar prompts bumblebees to make suboptimal choices in terms of reward. Reversal learning curve (mean choice accuracy ± SEM). After the forward learning phase, each bee underwent an individual reversal learning procedure in which the rewarding and unrewarding flower colours were switched and the rewarding flower for all groups provided sucrose solution (without nicotine). For each bee (n = 60) 70 consecutive visits were recorded. Learning curve for each group was generated from the mean choice accuracies (percentage correct choices made) for every consecutive block of 10 choices. Overall nicotine at 50-ppm strongly reduced the speed of learning (P < 0.001). A poorer performance than controls, yet not statistically significant, was also observed in bees previously exposed to the other two tested doses (1-ppm: P = 0.15, 2.5-ppm: P = 0.31).

Figure 4

Individual bee performance in the reversal learning test. Plot of individual bee performance (n = 60) in the reversal learning test in which the rewarding and unrewarding flower colours were switched with respect to the forward learning phase, and the rewarding flower for all groups provided sucrose solution (without nicotine). For each bee (n = 60) 70 consecutive visits were recorded (x-axis). Different colours represent different individual bees. Circles represent raw data of bee choices that were pooled to 3 large bins of sequential choices for clarity. Curves were fitted to the 70 visits completed by individual bees. The plots reveal the large variation among individual bees’ responses. Some bees were unaffected by nicotine but an increasing number of bees were very slow or completely failed to revert with increasing nicotine concentration encountred in the forward learning phase.