Traces of a neonicotinoid pesticide stimulate different honey bee colony activities, but do not increase colony size or longevity

Authors: Meikle, William; COLIN, T. - University Of Sydney; Adamczyk, John; Weiss, Milagra; BARRON, A.B. - Macquarie University

Submitted to: Ecotoxicology and Environmental Safety
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/13/2022
Publication Date: 1/17/2022
Citation: Meikle, W.G., Colin, T., Adamczyk Jr., J.J., Weiss, M., Barron, A. 2022. Traces of a neonicotinoid pesticide stimulate different honey bee colony activities, but do not increase colony size or longevity. Ecotoxicology and Environmental Safety. 231. Article 113202. https://doi.org/10.1016/j.ecoenv.2022.113202.
DOI: https://doi.org/10.1016/j.ecoenv.2022.113202

Interpretive Summary: Honey bees are often exposed to pesticides in agriculture, although the concentrations of the pesticides are usually low and sublethal. In this study we exposed honey bee colonies to very low levels of a pesticide, imidacloprid, which is in the neonicotinoid class of pesticides. We ran the same experiment each year for five years so we could also control for the natural variability in weather, which can affect flowering and thus colony health. We found that at a very low imidacloprid concentration of five parts per billion we could detect effects on the colony. Interestingly, some effects seemed to be positive, such as lower temperature variability and larger forager populations. However, if there were positive effects they did not affect colony growth or survivorship. We also measured hive CO2 concentrations, and found two things: 1) the more bees in the hive, the lower the average CO2 concentration, probably because so many bees can ventilate better; and 2) imidacloprid seemed to increase hive CO2 concentration among colonies with low populations. Increased CO2 concentration is consistent with increased metabolism, and so is better temperature control and more foraging activity.

Technical Abstract: How neonicotinoid contamination effects honey bees remains controversial. Studies have yielded contradictory results, and few have examined effects on colony development. Here we report the results of a comprehensive five-year study of the effects of the neonicotinoid imidacloprid on honey bee colonies. Colonies fed 5 ng/g (ppb) imidacloprid showed increased brood production, lower temperature variability, higher CO2 production and had more foragers compared to control colonies, but treatment did not affect adult bee numbers or average hive temperatures, and did not increase food stores, daily food acquisition or colony survivorship. These results suggest that imidacloprid contamination increased colony metabolism without improving colony productivity, and helps explain why some studies have reported no, or even positive, effects of neonicotinoids. Effect sizes were generally small but that could be attributed at least in part to variability in uncontrolled factors such as weather. We provide an explanation for the diverse effects of pesticide contamination on honey bees, and an improved understanding of how colonies are impacted.