Chronic exposure to field-realistic doses of imidacloprid resulted in biphasic negative effects on honey bee physiology.

There have been many investigations on the negative effects of imidacloprid (IMD) on honey bees. IMD is known to disrupt honey bee physiology and colony health at a relatively low concentration compared to other pesticides. In this study, honey bee colonies were chronically exposed to field-realistic concentrations (5, 20, and 100 ppb) of IMD, and the body weight, flight performance, carbohydrate reserve, and lipid contents of forager bees analyzed. Transcriptome analyses followed by quantitative PCR were also conducted for both nurse and forager bees to elucidate any changes in energy metabolism related to phenotypic disorders. The body weights of newly emerged and nurse bees showed decreasing tendencies as the IMD concentration increased. In forager bees, however, IMD induced a biphasic change in body weight: body weight was decreased at the lower concentrations (5 and 20 ppb) but increased at the higher concentration (100 ppb). Nevertheless, the flight capability of forager bees significantly decreased in a concentration-dependent manner. The effects of IMD on target gene transcription in forager bees showed biphasic patterns between low (5 and 20 ppb) and high (100 ppb) concentrations. Nurse bees showed typical features of premature transition to foragers in a concentration-dependent manner. When exposed to low concentrations, forager bees exhibited downregulation of genes involved in carbohydrate and lipid metabolism and in the insulin/insulin-like growth factor signaling pathway, upregulation of transporter activity, and a dose-dependent body weight reduction, which were similar to insulin resistance and diabetic symptoms. However, increased lipid metabolism and decreased energy metabolism with body weight gain were observed at high IMD concentration. Considered together, these results suggest that field-realistic doses of IMD alter honey bee energy metabolism in distinctly different ways at low and high concentrations, both of which negatively affect honey bee colony health.