In this experiment, we chronically exposed whole mature bumblebee (Bombus terrestris) colonies to field-realistic levels of the neonicotinoid thiamethoxam (2.4ppb & 10ppb) over four weeks, and compared colony growth under laboratory conditions. We found no impact of insecticide exposure on colony weight gain, or the number or mass of sexuals produced…. As previous studies have reported pesticide effects on bumblebee colony growth, this may suggest that impacts on bumblebee colonies are more pronounced for colonies at an earlier stage in the reproductive cycle.
One of the major concerns about the impacts of pesticides on bees is that the pollination services they deliver may be affected. Although we find no impacts of thiamethoxam on the growth of mature bumblebee colonies in this study, previous work has found impacts on queen reproductive development(48) and ability to found colonies(47), and that worker bees exposed to thiamethoxam show reduced learning ability, reduced pollen collection, and differential visitation of flowers(42, 45). Therefore, although thiamethoxam may not cause impacts on colonies under laboratory conditions, it could have different impacts in a field setting where the ability to forage and collect pollen are critical for normal colony development(56), and where bees are often exposed to multiple pesticides simultaneously.
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