A recent assessment of higher tier studies on the toxicity and risks of neonics in honeybees by Solomon and Stephenson reported a colony-level 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.
Drs Solomon and Stephenson responded to the letter from Dr. H Tennekes (“The Resilience of the Beehive” Journal of Toxicology and Environmental Health B 20: 316–386), as follows. Here we emphasize that our quantitative weight of evidence analyses were focused on the level of the honeybee colony. These colony-level responses include redundancy and resiliency as well as a number of possible sublethal effects of pesticides on the colony. We also note that the literature has shown that binding of neonicotinoid insecticides to the nicotinic acetylcholine receptor is reversible. The comments in this letter do not provide reasons to change our conclusions, that, as currently used in good agricultural practices as seed-treatments, imidacloprid, clothianidin, and thiamethoxam do not present significant risks to honeybees at the level of the colony.
Henk A. Tennekes (2018): Letter to the editor “The resilience of the beehive”,
Journal of Toxicology and Environmental Health, Part B, DOI: 10.1080/10937404.2017.1421425
Keith R Solomon & Gladys L Stephenson (2018): Response to Tennekes
(2018) “The Resilience of the Beehive” Journal of Toxicology and Environmental Health B 20: 316–386, Journal of Toxicology and Environmental Health, Part B