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

Survivorship, syrup consumption, and cluster temperatures of honey bees were kept in hoarding cages with different numbers of bees. Cages with either 50, 100, 150, or 200 bees each were monitored over 4-6 weeks in incubators with 12h/12h 30° C/15° C temperature cycles to induce clustering. Survivorship and syrup consumption rates per bee were not different among the bee density groups, but cluster temperatures were. Cluster temperatures ranged from 0.45°C above incubator temperature in the 50 bee cages to 4.05° C in the 200 bee cages over the 1st 7 days, with each additional bee adding on average 0.02° C to cluster temperature. In another set of experiments, cages were established with about 200 bees each, and imidacloprid added to the syrup at 0, 5, 20, and 100 ppb. Imidacloprid in the syrup did not affect bee survivorship but it did reduce syrup consumption per bee, with bees fed 100 ppb imidacloprid syrup consuming on average 631 mg per bee over 28 days while average consumption among the other groups ranged from 853 to 914 mg. Cluster temperature was affected by imidacloprid treatment: bees fed 5 ppb imidacloprid syrup had higher cluster temperatures over the 1st 10 days, 4.17° C above incubator temperature, than either bees fed 100 ppb syrup or control (2.35 and 3.19° C, respectively). [ABSTRACT FROM AUTHOR]