Determination of exposure levels of honey bees foraging on flowers of mature citrus trees previously treated with imidacloprid

Field and tunnel cage studies were undertaken to determine the extent to which honey bees foraging on citrus blossoms were exposed to imidacloprid and its metabolites when citrus trees were treated with soil applications of the insecticide. Imidacloprid, imidacloprid olefin and 5-OH imidacloprid were detected in nectar and pollen sampled from the flowers of citrus trees treated with imidacloprid up to 232 days prior to bloom.  In tunnel studies, where foraging was restricted exclusively to citrus, imidacloprid residues in nectar extracted from flowers and from bee crops were similar (< 10 ng ml^-1) and below the established no observed effect limit; however, the residue levels were about 3-fold higher in nectar sampled from comb.  Concentrations of imidacloprid in nectar were higher in trees treated with higher application rates.  Based on published bioassay data, the imidacloprid concentrations in the floral nectar did not surpass levels that would compromise foraging activity under normal use conditions for imidacloprid.  Further research is needed to assess the impact of elevated levels of imidacloprid within stored nectar in the comb.