Bumblebees are not deterred by ecologically relevant concentrations of nectar toxins

Abstract

Bees visit flowers to collect nectar and pollen that contain nutrients and simultaneously facilitate plant sexual reproduction. Paradoxically, nectar produced to attract pollinators often contains deterrent or toxic plant compounds associated with herbivore defence. The functional significance of these nectar toxins is not fully understood, but they may have a negative impact on pollinator behaviour and health, and, ultimately, plant pollination. This study investigates whether a generalist bumblebee, Bombus terrestris, can detect naturally occurring concentrations of nectar toxins. Using paired-choice experiments, we identified deterrence thresholds for five compounds found in the nectar of bee-pollinated plants: quinine, caffeine, nicotine, amygdalin and grayanotoxin. The deterrence threshold was determined when bumblebees significantly preferred a sucrose solution over a sucrose solution containing the compound. Bumblebees had the lowest deterrence threshold for the alkaloid quinine (0.01 mmol l(-1)); all other compounds had higher deterrence thresholds, above the natural concentration range in floral nectar. Our data, combined with previous work using honeybees, suggest that generalist bee species have poor acuity for the detection of nectar toxins. The fact that bees do not avoid nectar-relevant concentrations of these compounds likely indicates that it is difficult for them to learn to associate floral traits with the presence of toxins, thus maintaining this trait in plant populations.

Keywords: Behaviour; Bombus terrestris; Deterrence threshold; Grayanotoxin; Nectar toxin; Pollinator.

Fig. 1.

Mean (±s.e.m.) consumption (g), controlled for evaporation by Bombus terrestris of 0.5 mol l⁻¹ sucrose solution, with (light grey bars) or without (dark grey bars) one of five nectar toxins. Where bars are missing, assays were not completed because of limited availability of compounds. Asterisks indicate significant differences between consumption of two solutions at a given concentration according to least significant difference (LSD) post hoc comparisons (*P<0.05; **P<0.01; ***P<0.001). Black arrows represent naturally occurring concentrations of the compound in floral nectar.

Fig. 2.

Mean (±s.e.m.) (A) total consumption (g), controlled for evaporation of solutions at lowest concentration (0.001 mmol l⁻¹) for each nectar toxin, and (B) mortality of Bombus terrestris fed five different nectar toxins. Control bumblebees were fed 0.5 mol l⁻¹ sucrose in both solutions and so had no exposure to any toxin. N=40 bees per toxin per concentration. Different lowercase letters represent significant (P<0.05) differences in total consumption between compounds according to LSD post hoc comparison.