Investigating the impacts of field-realistic exposure to a neonicotinoid pesticide on bumblebee foraging, homing ability and colony growth

The authors examined chronic exposure to thiamethoxam and its effect on bumble bee foraging, homing, and corresponding colony growth. RFID tags were used to monitor foraging activity, and the number of bees returning with pollen loads was also recorded. The number of individuals returning with pollen was recorded starting after day 5 of treatments, and homing trials began after 2 weeks of treatment. Exposure to thiamethoxam had no effect on the number of days that bees foraged, the daily number of foraging trips, or the number of bees foraging per colony. However, bumble bees exposed to thiamethoxam did perform significantly longer foraging trips compared to controls (68 vs. 55 min), and returned with pollen less frequently.

In the homing trials, control bees and bees exposed to thiamethoxam were released at 1 km or 2 km distances; researchers recorded the return rate and return time of individual bees. Interestingly, exposed bees had a better return rate than controls at both 1 km and 2 km distances. For the 1 km trials, the volume of sucrose consumed before the trial also contributed to return rate, and return time was best explained by nectar consumption only. The authors provide several theories for the positive effect of thiamethoxam exposure on homing ability: neonicotinoids could excite brain regions involved with navigation, the longer foraging trips of exposed bees could have included more time exploring the landscape than foraging, and potential selective impacts of exposure could leave the most fit individuals to survive through the trial. They also note that temperature and solar radiation can impact homing, and it would be interesting to test for interactive effects with pesticide exposure (Stanley et al. 2016).

While the researchers did not see significant effects on colony growth from treatment, the large confidence intervals suggested that larger sample sizes may be needed to fully address this question; on average the number of bees in control colonies were greater than the number of bees in treatment colonies – though this difference was not statistically significant. Notably this study did not measure the effect of thiamethoxam on the number of reproductive bumble bees (queens and males).

Overall, thiamethoxam exposure made the bumble bees less efficient pollen foragers over their entire foraging career, suggesting that impacts to pollination services could be exacerbated over time. The effects documented in this study were seen at very low exposures (2.4 ppb) while bees also had access to uncontaminated nectar in the field. This suggests that very low levels of thiamethoxam exposure can negatively affect bumble bees.

Stanley, D.A., A.L. Russell, S.J. Morrison, C. Rogers, and N.E. Raine
Journal of Applied Ecology
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