Naturwissenschaften (2012) 99:153–158 indicating a mixed co-infection of the two species which is still quite common (Chen et al. 2008). Results A pronounced increase in Nosema growth was evident within individual bees reared in colonies exposed to a low, sub-lethal level of the pesticide imidacloprid. Two trials were conducted in this experiment. In the first trial, in July, ten newly emerged bees from each of three control colonies, four colonies feed high levels of imidacloprid, and two colonies feed with low levels of imidacloprid (inconsistent emergence of adult bees from brood combs in the low group resulted in an unbalanced design by treatment for this trial) were fed 1 million spores per milliliters of sugar syrup (ca. 333,333 spores per bee) and had Nosema spore counts quantified 12 days after emergence. Bees originating from colonies feed high and low levels of imidacloprid had higher Nosema spore counts than controls (ANOVA, F 0 7.34, d.f. 0 2, 70, P 0 0.0013; Fig. 1). In the second trial, in August, newly emerging bees from nine different colonies, distinct from the colonies used in trial one (three feed with high levels of imidacloprid, three low levels of imidacloprid, and three control colonies) were collected. Ten bees from each cage (colony) were examined for spore counts on day 12. While the presence of spores had the expected effect of increasing spore levels in Nosema spore feed bees (ANOVA, F 0 26.41, d.f. 0 2, 357, P < 0.0001), no difference was ob- served in the final spore counts in bees feed different doses of Nosema (Tukey’s HDS test; Fig. 2). We therefore combined the data from both dosage replicates for further analysis. Bees 155 Fig. 2 Total spore count in 12-day-old bees fed two different levels of Nosema spores following adult emergence. Columns with different letters are significantly different from each other (Tukey HSD test P<0.05) in trial 2 originating from colonies chronically exposed to imidacloprid, had higher levels of Nosema spores 12 days after being feed Nosema spores than bees originating from control colonies fed the same Nosema spore solution, consis- tent with the results from trial 1 (ANOVA, F0 14.85, d.f.0 4, 355, P<0.0001; Fig. 3). Our total colony sample size across the two trials was controls n 0 6, low n 0 5, and high n 0 7 distinct colonies tested of the 30 total colonies. A few colonies lost queens and or had limited brood which prevented inclusion in the laboratory studies. We verified pesticide exposure to colonies by measuring the weekly consumption of the treated protein patties and by analyzing the level of imidacloprid in stored bee bread and random-aged bees removed from colonies 1 week after the exposure period (Table 1). Daily protein patty consumption averaged 29.0 ± 0.84, 29.3 ± 0.78, and 31.1 ± 0.85 g for the Fig. 1 Total Nosema spore count in 12-day-old bees derived from colonies feed high, low, and no levels of imidacloprid (July, trial 1). Immature bees were removed from colonies, allowed to emerge, and feed sugar solution with 106 Nosema spores per milliliter of sugar syrup. Columns with different letters are significantly different from each other (Tukey HSD test P<0.05) Fig. 3 Total Nosema spore count in 12-day-old bees derived from colonies feed high, low, and no levels of imidacloprid (August, trial 2). Immature bees were removed from colonies, allowed to emerge, and feed sugar solution with known amounts of spores (see text for details). Columns with different letters are significantly different from each other (Tukey HSD test P<0.05)