The honey bees of the French countryside suffered a catastrophic die-off between 1994 and 1998. Unsurprisingly, the mass mortalities coincided with the introduction of several new-to-the-market agricultural insecticides. Environmentalists and farmers were quick to point the finger at one in particular: imidacloprid, a neonicotinoid produced primarily by the pharmaceutical giant BAYER. By 1999, enough pressure had mounted to result in a ban of imidacloprid by the French Ministry of Agriculture.

However, as reported in the Proceedings of the National Academy of Sciences, a new study is suggesting that imidacloprid wrongly took the brunt of the blame. It turns out, fipronil – not a neonicotinoid – may have actually been responsible for the die-off.

“We, therefore, postulate that fipronil, not imidacloprid, caused the mass mortalities of honey bees in France during the 1990s because it is lethal to honey bees in even trace doses due to its capacity to bioaccumulate and generate TRT [time-reinforced toxicity],” the study concludes.

Using a number of lab experiments and simulations to assess the potency of the two insecticides, researchers led by the University of Exeter in the UK discovered that imidacloprid does not appear to be strong enough to cause mass mortality in honey bees. It also doesn’t seem to bioaccumulate in individual bees.

On the other hand, fipronil appears more likely to accumulate in the bodies of the bees, while imidacloprid is fairly quickly eliminated. Over time, this allows fipronil to become more lethal. This subtle build up over time is perhaps part of the reason why it originally got off the hook so easily.

“Taken together, this evidence suggests that it is unlikely that even a sustained exposure to dietary imidacloprid at environmentally realistic levels can be the cause of mass fatalities,” the authors explain.

Fortunately, fipronil is now banned by the European Union, as is imidacloprid and two other neonicotinoids, although fipronil-treated seeds are still available in most countries. However, the wide debate about the many breeds of insecticides and their effects on bees continues to rage on.

None of this is to say that neonicotinoid exposure isn’t harmful to bee populations, either. Just last year, one of the biggest studies of its kind evaluated multiple real-world scenarios and the effects of neonicotinoid exposure on wild bee populations. The results, while not 100 percent definitive, showed that treating crops with the neonicotinoids clothianidin and thiamethoxam reduced the winter survival rate of honey bee colonies.

“It's reached a point where it's getting silly to deny that there is a link between these pesticides and harm to bees,” Professor Dave Goulson, a bumblebee ecologist at the University of Sussex, told IFLScience.

"There’s so much evidence now," he added.