Utility of fluctuating asymmetry as an indicator of pesticide stress in honeybees

Honeybee colonies are failing at record levels which threatens the ecological and economic services they provide. Pesticides are likely involved, but detecting their sublethal effects is difficult. Fluctuating asymmetry (FA) is considered an indicator of developmental stress, so here we tested whether FA is an indicator of sublethal pesticide exposure. Imidacloprid (a widely used insecticide) seriously damages honeybee foraging performance at sublethal concentrations. We assessed whether exposure to sublethal concentrations of imidacloprid changed honeybee FA. Additionally, we explored how the method of sampling honeybees influenced FA estimates. We sampled foragers, newly emerged, and within-hive bees treated with 0, 5, or 100 ppb imidacloprid. Landmark-based geometric morphometrics assessed forewing asymmetry. We found imidacloprid did not increase honeybee FA and FA was lower in foragers than other samples. Additionally, directional asymmetry (DA; which does not infer developmental stress) was higher in within-hive than newly emerged honeybees. Overall, FA is an unsuitable indicator of sublethal imidacloprid exposure in honeybees. However, to improve other future FA assessments, investigators should consider what sampling strategies are appropriate as sampling can impact the degree of FA detected. DA must also be adequately accounted for in FA estimates as it appears more prevalent and varying than previously thought.