Whether a picogram is or is not, can or cannot be equivalent to 1 ppt, it is a trillionth of a gram and quantity that could not be detected in pollen until very recently (Behaviour of imidacloprid in fields. Toxicity for honey bees Bonmatin, JM; Moineau, I; Charvet, R; Colin, ME; Fleche, C; Bengsch, ER in Environmental chemistry: green chemistry and pollutants in ecosystems (Ed. Lichtfouse, E; Schwarzbauer, J; Robert, D; Lichtfouse, E; Schwarzbauer, J; Robert, D) (2005).
As to the quantity consumed, I quote: 
« Depuis que l’on sait mesurer de très faibles doses d’imidaclopride dans le pollen, on voit que la concentration biodisponible est de l’ordre du microgramme par kg de pollen. Soit, pour une nourrice qui consomme, en 10 jours, 60 mg de pollen, une dose ingérée de 60 pg. »

“Once we were able to measure very low doses of imidacloprid in pollen, we saw that the concentration bio-available is of the order of
one microgram per kg of pollen.
A nurse bee that consumes 60 mg of pollen in 10 days, will have an intake of 60 pg.”

Which M. Bezunces concludes to be a lethal dose. His estimation of the qty of pollen ingested by the nurse bee is supported by, "Modes of honeybees exposure to systemic insecticides:
estimated amounts of contaminated pollen and nectar consumed by different categories of bees
Agnès RORTAIS, Gérard ARNOLD, Marie-Pierre HALM,
Frédérique TOUFFET-BRIENS   Apidologie 36 (2005) 71–83
© INRA/DIB-AGIB/ EDP Sciences, 2005"  
which found 65 grammes over 10 days for a nurse bee. 
 I must conclude that American honey bees are more resistant than our puny French bees. That would explain why imidacloprid is banned over here but not in the USA.
Bil Harley

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