Simultaneous stressors : interactive effects of an immune challenge and dietary toxin can be detrimental to honeybees

Recent large-scale mortality of honeybee colonies is believed to be caused by multiple interactions between diseases, parasites, pesticide exposure, and other stress factors. To test whether a dual challenge has an additive effect in reducing survival, we experimentally stimulated the immune system of caged Apis mellifera scutellata workers from six colonies by injecting saline or Escherichia coli lipopolysaccharides (LPS), and additionally fed them the alkaloid nicotine (0 µM, 3 µM and 300 µM in 0.63 M sucrose). Workers did not increase their sucrose intake to compensate for the immune system activation, and those injected with E. coli LPS decreased their intake on the highest nicotine concentration. In the single challenges, injection and high nicotine doses negatively affected survival. All injected worker groups showed reduced survival. Without nicotine, survival of the saline and E. coli LPS worker groups was similar, but survival of E. coli LPS-challenged workers dropped below that of the saline groups when additionally challenged by nicotine, with bees dying earlier at higher nicotine concentrations. In the dual challenge of saline injection and dietary nicotine, a reduced effect on survival was observed, with lower mortality than expected from the summed mortalities due to the single challenges. However, additive and synergistic effects on survival were observed in workers simultaneously challenged by E. coli LPS and nicotine, indicating that interactive effects of simultaneous pathogen exposure and dietary toxin are detrimental to honeybee fitness.

近期，蜜蜂数量的大规模减少被认为是由多种疾病、寄生虫、农药暴露以及其他压力因素之间的相互作用所致。为了测试双重挑战是否具有累加效应，以降低生存率，本研究通过向六群Apis mellifera scutellata工蜂注射生理盐水或大肠杆菌脂多糖（LPS）来实验性地刺激其免疫系统，并额外给予它们尼古丁（0 µM、3 µM及300 µM，溶解于0.63 M蔗糖中）。工蜂并未增加其蔗糖摄入量以补偿免疫系统的激活，且在高尼古丁浓度下，注射大肠杆菌LPS的工蜂摄入量有所下降。在单独的挑战中，注射和高尼古丁剂量对生存率产生负面影响。所有注射工蜂组的生存率均有所下降。在没有尼古丁的情况下，生理盐水和注射大肠杆菌LPS的工蜂组生存率相似，但当额外受到尼古丁的挑战时，注射大肠杆菌LPS的工蜂组生存率低于生理盐水组，且在高尼古丁浓度下蜜蜂死亡时间更早。在生理盐水注射和饮食尼古丁的双重挑战中，观察到对生存率的降低作用，死亡率低于单独挑战的累加死亡率。然而，对于同时受到大肠杆菌LPS和尼古丁挑战的工蜂，观察到对生存率的累加和协同效应，表明同时暴露于病原体和饮食毒素的交互作用对蜜蜂的适应性有害。