Ever since French beekeepers saw their bees dying as they collected pollen from treated sunflowers back in 1996, beekeepers have been concerned that their bees are being harmed the highly toxic neonicotinoid insecticides, with imidacloprid most widely used. The use of this class of insecticide has grown steadily ever since. Bee losses have become chronic as well. However, unlike the first case in France where bees were literally falling dead while gathering pollen, the widespread colony losses today are less explainable, often associated with outbreaks of a variety of diseases, and with very high winter colony mortality. So why blame the insecticides?
To see why the bees are dying, and why these pesticides are still being sold, we must examine the toxicology of the neonicotinoid chemicals as well as the history and science of pesticide regulation. The toxic nature of a chemical is characterized by its “LD50″ level. This is the amount of chemical that will kill half of the test organisms in short order. For many traditional pesticides (organophosphates), the LD50 level provides an adequate overall characterization of toxic effect. These pesticides tend to be short-lived and generally do not bio-accumulate in the target organism. If the dose doesn’t kill the organism, the toxic compound will be metabolized and excreted. Since the organophospate pesticides — which for many years made up the majority of pesticides sold — could be characterized easily with the single LD50 number, the culture of pesticide regulation largely accepts the acute LD50 as determinative for all toxic effects.