In reference to Bill T's comments it seems to me that the articles being referred to are talking about parts-per-trillion exposure levels and not part-per-billion or parts-per-million. A translation from French to English (by the Yahoo web site translator) of part of the June 2009 INRA article being referred to says

" To evaluate the effect of a chronic toxicity, them bees are nourished during 10 days with a sugar syrup containing known concentrations of product: the ingestion of 1 picogramme per day is enough to kill a bee in 10 days (1 pg = 10-12 gram). Of more, the imidaclopride is degraded in six metabolites of which some are still more toxic. In addition, with the majority of insecticides, one observe sublétaux effects, i.e. that the bee does not die, but present behavioral disorders (confusion), physiological formations of the wings, reduction in growth…) or metabolic (hypoglycemia…). All these effects can in the long term to reflect itself on the survival of the colony. Since one can measure very low dose of imidaclopride in pollen, one sees that the biodisponible concentration is of the order of the microgram per kg of pollen. "

The parenthesis around  "1pg=10-12gram" are in the original article and have not been added by me.
                                                        Ernie Huber

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