Impact of imidacloprid on honey bee hypopharyngeal glands at two levels: measurement reliability and laboratory versus field conditions

Abstract

The use of neonicotinoids has undoubtedly behavioral and physiological effects on bees. This work aimed to ring test the digital measurement of honey bees’ hypopharyngeal glands (HPGs), validate the method against the subjectivity of the assessors, and assess differences in the impact of sub-lethal levels of imidacloprid on honey bee HPG size between laboratory and field studies. Newly emerging bees from colonies fed with sugar solution and pollen paste, contaminated with imidacloprid at concentrations of 2 and 3 μg/kg, respectively, were maintained in either their own colonies or in laboratory cages. Digital measurements of HPG acini diameter were performed by 10 independent assessors at bees of days 10 and 15 post-emergence. We found that the reliability of measurements undertaken by 10 different independent assessors was greater than 95%, and there was no significant difference among them. Furthermore, we demonstrated that HPGs have a significantly larger diameter under field than in laboratory conditions and that 15-d-old bees had a significantly smaller acini diameter than 10-d-old workers in laboratory but not in field conditions, presumably due to the fact that laboratory bees did not perform any nursing tasks. Imidacloprid significantly decreased acini diameter of 10-d-old workers, both under laboratory and field conditions, presumably forcing them to become foragers earlier in life. We conclude that the digital measurement of HPG acini is a highly reliable method; it can be used in Tier 1 risk assessment process of any contaminant, and it can also be used as a proxy for higher Tier studies.