Letter to the editor "The resilience of the beehive".

A recent quantitative weight of evidence (QWoE) assessment of higher tier studies on the toxicity and risks of neonicotinoids in honeybees by Solomon and Stephenson reported a colony-level no-observed-adverse effect concentration (NOAEC) of 25 µg/kg (ppb) for imidacloprid and clothianidin. The toxicity of these insecticides to honeybees is however known to be reinforced with chronic exposure, and extrapolation of time-to lethal-effect toxicity plots compiled from published studies indicate that an imidacloprid level of 0.25 ppb, i.e. one-hundredth of the reported colony NOAEC, would kill a large proportion of bees nearing the end of their life. This huge discrepancy points to the impressive resilience of beehives in counteracting lethal effects of neonicotinoids, as long as the colony remains otherwise healthy with a productive queen that is able to maintain the colony population. The explicit connection between innate immunity loss and the neonicotinoids leading to infestation with a wide variety of pathogens appears to be the decisive factor that ultimately bring down stressed colonies.