Supplemental data Effects of imidacloprid exposure on honey bee colony growth and activity

To better understand how exposure to a low (5 ppb) concentration of imidacloprid affects honey bee colonies we conducted five field experiments involving a total of 54 bee colonies. In each experiment, bee colonies in one treatment group were fed sugar syrup adulterated with 5ppb imidacloprid over six weeks, and colonies in the other group were fed unadulturated sugar syrup over the same time period. Treatments were intended to simulate nectar collection by the colonies. No commercial agriculture existed within 10 km of the sites. We collected data from hive assessments, including adult bee mass and mass of food resources, and the total surface area of capped brood; and we collected data from sensors, including continuous hive weight, internal temperature and, for two years of the study, internal CO2 concentration. We measured Varroa mite fall before and after treatment each year of the study, and for 3 years of the study we measured the dry weight of newly-emerged adult bees.