Avoidance, tolerance, or preference: Responses of the western honey bee (Apis mellifera) to xenobiotics

The ability of bees to detect and evaluate xenobiotics in nectar has implications for colony fitness. To delineate honey bee behavioral responses to xenobiotics in nectar when alternate food is available, we examined honey bees with a free-flight assay in a semi-field environment. We tested forager responses to natural phytochemicals and synthetic agrochemicals found in nectar. Phytochemicals included an alkaloid (caffeine), phenolics (p-coumaric acid, caffeic acid), and flavonoids (pinocembrin, chrysin, and naringenin); synthetic agrochemicals included herbicides (atrazine, glyphosate) and fungicides (chlorothalonil, prochloraz, boscalid). We found that foraging preferences were both chemical-dependent and dose-dependent. A preference is exhibited for p-coumaric acid at 10ug/ml and 100ug/ml, caffeic acid at 1ug/ml, and naringenin at 100ug/ml. Honey bees also display a preference for low concentrations of chrysin and prochloraz but avoid them at high concentration (prochloraz at 10000ng/ml and 100000ng/ml). This behavior may protect foragers from toxicity associated with high concentrations. They also avoid caffeine at 0.1ug/ml and 1ug/ml. However, honey bees did not avoid all potentially harmful xenobiotics; foragers displayed a significant preference for atrazine, glyphosate, and chlorothalonil. Prochloraz is the only fungicide that bees consistently avoid irrespective of concentration. The paradoxical preference displayed by foragers for low concentrations of herbicide and fungicide may expose bees to higher risks of adverse effects on colonies than previously considered due to cumulative effects of these compounds within the hive.