In a recent paper, Henry et al. (2012; supplemental material April 20, 2012) address the important issue of honey- bee sublethal effects induced by systemic insecticides as a potential causal factor of colony collapse disorder (CCD). Henry et al. report a daily range of thiamethoxam exposure of 0.17–2.3 ng.bee−1.day−1. Henry et al. (2012) based their calculations on the method proposed by Rortais et al. (2005) assuming a thiamethoxam nectar content of 1.85 μg/kg1,2, and a winter oilseed rape sugar content of 10 to 30% (weight/weight) reportedly taken from Pierre et al. (1999). However, the range of thiamethoxam reported is incorrect. 



Henry et al. (2012) conclude their study with: “Our study clearly demonstrates that exposure of foragers to non-lethal but commonly encountered concentrations of thiamethoxam can impact forager survival, with potential contributions to collapse risk.” However in order to be commonly encountered, the dose of 1.34 ng.bee.day used by Henry et al. (2012) should be achievable four out of four weeks of flowering. It is clear that the dose applied in this study (Henry et al., 2012) could not possibly be reached within 1 day and without foragers flying at night on the first and second week of flowering, and could barely be reached within daytime in the third week of flowering. It is unlikely that free-flying bees would ever reach the daily dose of thiamethoxam that have been used by Henry et al. (2012), at least for 3 out of 4 weeks of flowering.



The ecological relevance of Henry et al.'s study is compromised by four main methodological issues. The daily range of thiamethoxam exposure is incorrectly estimated, the applied dose is uncommonly encountered, thiamethoxam is applied in an acute rather than a sub-chronical manner and the use of an incorrect formula falsely inflates the bees' homing failure rate.  Henry et al.'s (2012) data confirms that neonicotinoid insecticides modify the behavior of honeybees as has previously been reported (e.g., Guez et al., 2001, 2003) at non-lethal, albeit ecologically unrealistic concentrations. 



Guez, D. (2013). A common pesticide decreases foraging success and survival in honey bees: questioning the ecological relevance. Frontiers in physiology, 4.



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