Biphasic negative effects of chronic imidacloprid exposure on honeybee physiology

There have been many investigations on the effects of imidacloprid (IMD) on honeybees as a potential factor for colony collapse disorder. In this study, honeybee colonies were chronically exposed to field-realistic doses (5, 20, and 100 ppb) of IMD, and the body weight, flight performance, and carbohydrate reserve of forager bees were analyzed. Transcriptome analyses followed by quantitative PCR were also conducted to elucidate any changes in energy metabolisms. The body weights of newly emerged and nurse bees showed decreasing tendencies as IMD concentration increased. In forager bees, however, IMD induced a biphasic change in body weight: decrease at the low concentration range (5 and 20 ppb) but increase at the high concentration (100 ppb). Nevertheless, the flight capability of forager bees significantly decreased in a concentration-dependent manner. The effects of IMD on target genes in forager bees showed biphasic patterns, whereas nurse bees showed typical features of premature transition to foragers in a concentration-dependent manner. When exposed to the low concentrations, forager bees exhibited downregulation of the carbohydrate and lipid metabolism and the insulin/insulin-like growth factor signaling pathway, upregulation of transporter activity. However, increased lipid metabolism and decreased energy metabolism were observed in the high IMD concentration. Taken together, these results suggest that field-realistic doses of IMD alters honeybee energy metabolism in distinctly different mechanisms at low and high concentrations, both of which negatively affect honeybee colony health.