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Title: Sublethal effects of Imidacloprid on honey bee colony growth and activity at three sites in the U.S.

item Meikle, William
item Adamczyk, John
item Weiss, Milagra
item GREGORC, ALES - Mississippi State University
item JOHNSON, DON - University Of Arkansas
item STEWART, SCOTT - University Of Tennessee
item ZAWISLAK, JON - University Of Arkansas
item Carroll, Mark
item LORENZ, GUS - University Of Arkansas

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2016
Publication Date: 12/28/2016
Citation: Meikle, W.G., Adamczyk, J.J., Weiss, M., Gregorc, A., Johnson, D.R., Stewart, S.D., Zawislak, J., Carroll, M.J., Lorenz, G.M. 2016. Sublethal effects of Imidacloprid on honey bee colony growth and activity at three sites in the U.S. PLoS One. 11(12):e0168603. doi:10.1371/journal.pone.0168603.

Interpretive Summary: Concerns have been raised among beekeepers and the general public about the effects of neonicotinoids, a class of insecticides used as systemic pesticides on honey bee health. Acute exposure to pesticides has clear negative effects on honey bees and on colonies, because bees often die from it, sometimes in large numbers. Sublethal effects are different and are more difficult to detect, because it can cause changes in bee behavior rather than survivorship. Neonicotinoid pesticides can directly impair honey bee learning and sensory capabilities, decrease foraging success, and can have indirect effects on bees by acting as repellents, and bees exposed to neonicotinoid pesticides have been found to be more susceptible to diseases. In this study we exposed colonies at 3 locations to very low concentrations of a neonicotinoid, imidacloprid, and then inspected hives for brood (all locations), hive frame weight (2 locations), and adult bees (one location). At one location we also kept the hives on scales so we could monitor weight continuously and placed temperature sensors inside the hive to monitor temperature continuously. We sampled bees and honey before and during all experiments to test for the pesticide. We also ran 2 laboratory cage studies to see how the same doses of imidacloprid affected bee survivorship and food consumption. We found that 1) imidacloprid last at least 7 months in stored honey; 2) a dose of 100 ppb in syrup fed to colonies in experiments without stored honey reduced the amount of capped brood and adult bees compared to untreated hives, but there were no effects if the hives had stored honey; 3) colonies fed 100 ppb syrup reduced their syrup consumption compared to other treatments and this effect was also observed in cage studies; and 4) continuous hive monitoring showed that foraging activity was higher in the 5 ppb treatment than either the 100 ppb treatment or the control during a nectar flow in Arizona, and temperature data indicated that thermoregulation in colonies fed 100 ppb was worse than in colonies in other treatment groups. Overall we did find treatment effects at low concentrations, but the effects were not always negative.

Technical Abstract: Field experiments in southern Arizona, central Arkansas and southern Mississippi were conducted to evaluate the effects of sublethal concentrations (0, 5, 20 and 100 ppb) of imidacloprid in sugar syrup on honey bee colony growth and activity. Response variables included discrete data from hive inspections (all experiments) and hourly hive weight and internal temperature data (Arizona experiments). Consumption rates per colony of 100 ppb imidacloprid syrup were significantly lower at two sites, an observation confirmed in laboratory studies of caged adult bees. Colonies fed syrup with 100 ppb imidacloprid had significantly lower adult bee masses, brood surface areas and average frame weights on average compared to colonies in the other treatment groups in Arizona. In contrast, no differences in capped brood surface area and average frame weight were observed among treatment groups in Arkansas, probably at least in part due to the abundance of alternative local nectar sources. Capped brood surface areas were significantly lower in Mississippi, also an environment rich in alternative forage, for colonies fed 5 ppb imidacloprid than control colonies, although some contamination of the control colonies was detected, probably due to robbing, so the results are difficult to interpret with confidence. Hive temperature data in Arizona indicated that colonies fed syrup containing 100 ppb imidacloprid exhibited significantly reduced temperature control during the winter than colonies fed syrup with 5 ppb. Hourly weight data showed that colonies fed 5 ppb imidacloprid exhibited significantly higher within-day weight amplitudes during a nectar flow in 2014, indicating more foraging activity, than either control colonies or those fed syrup with 100 ppb. Imidacloprid concentrations in stored honey corresponded well with concentrations in the syrup fed to the colonies and proved stable in the hive environment over 7 months after the end of treatment. These results showed that imidacloprid contamination can have long-term effects on honey bee colony growth and activity.