Effects of bee density and sublethal imidacloprid exposure on cluster temperatures of caged honey bees

Authors: Meikle, William; Adamczyk, John; Weiss, Milagra; GREGORC, ALES - Mississippi State University

Submitted to: Apidologie
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/2018
Publication Date: 7/18/2018
Citation: Meikle, W.G., Adamczyk, J.J., Weiss, M., Gregorc, A. 2018. Effects of bee density and sublethal imidacloprid exposure on cluster temperatures of caged honey bees. Apidologie. 49(5):581-593. https://doi.org/10.1007/s13592-018-0585-z.
DOI: https://doi.org/10.1007/s13592-018-0585-z

Interpretive Summary: Honey bees are exposed to many kinds of agricultural chemicals such as fungicides and pesticides. However, bees are often exposed to low, sublethal doses, over a period of time, rather than high doses that kill the bees right away. Field studies of sublethal doses have shown that even very low doses, such as 5 ppb, can affect bee behavior, including the temperature in the colony cluster, and possibly colony health. Sometimes the results of field experiments are hard to interpret, because of the complexity of the environment where the bees live. In this study we exposed newly-emerged bees to sublethal concentrations of a neonicotinoid pesticide, imidacloprid, in the laboratory in bee cages, where we could control the environment, and we measured the temperature in their cluster when we lowered the temperature of the incubator. First, we conducted 2 experiments to determine the effects of different numbers of bees on cluster temperature and found that the number of bees is important. Then, we conducted 3 experiments using the imidacloprid. The bees fed imidacloprid at the high concentration, 100 ppb, ate less syrup with the pesticide, and had lower cluster temperatures. Bees fed the very low dose, 5 ppb, actually had higher cluster temperatures than the control bees. It may be that low concentrations stimulates bees. We do not know how this affects colony health.

Technical Abstract: Neonicotinoid pesticides such as imidacloprid are popular in commercial agriculture and the effects of field-relevant doses of those pesticides is of interest to beekeepers, pesticide manufacturers and applicators. Hive temperature has been found to be affected by exposure to imidacloprid in field trials so to explore that relationship, and to control some of the variability in field experiments, temperature was measured in clusters of bees kept in hoarding cages in a laboratory incubator programmed with a 12h/ 12h 30°C to 15°C temperature cycle. The 15°C phase induced bee clustering. Because the number of bees providing cluster heat as well as insulation would be affected by the number of bees in the cluster, cages with different densities of newly-emerged bees (50, 100, 150 or 200 bees per cage) were established and cluster temperature, as well as bee survivorship and daily syrup consumption, was measured over 4-6 weeks. Survivorship and syrup consumption rates per bee were not different among the different groups, but cluster temperatures were affected by bee density, with each additional bee adding about 0.02°C on average to the cluster temperature over the first 7 d. Using the same design, the experiment was conducted with two changes: initial bee density in all cages was set to about 200 bees per cage, and imidacloprid was added to the syrup at concentrations of 0, 5, 20 and 100 ppb. Bee survivorship was not affected by treatment. Syrup consumption was significantly affected, with bees fed syrup containing 100 ppb imidacloprid consuming less than bees in any other treatment. Cluster temperature was also affected by imidacloprid treatment: bees fed 5 ppb imidacloprid syrup had significantly higher cluster temperature than both bees fed 100 ppb syrup and the control bees fed no imidacloprid. Reasons for these behavior and biological difference among dosage are discussed.