Neonicotinoid - History

History

Nicotine acts as an insecticide but is also toxic to mammals. In fact, nicotine has a lower lethal dose for rats than flies. This spurred a scientific search for compounds that retain the insecticidal properties of nicotine but have selectively less effect on mammals, but initial investigation of nicotine related compounds (nicotinoids) as insecticides was not successful. The precursor to nithiazine was first synthesized by a chemist at Purdue University. Shell researchers found in screening that this precursor showed insecticide potential and refined it to develop nithiazine. Nithiazine was later found to be a postsynaptic acetylcholine receptor agonist, meaning it has the same mode of action as nicotine. Nithiazine does not act as an acetylcholinesterase inhibitor, in contrast to the organophosphate and carbamate insecticides. While nithiazine has the desired specificity (i.e. low mammalian toxicity), it is not photostable (it breaks down in sunlight), so it was not commercially viable. Neonicotinoids were developed after the lack of commercial success of nithiazine. The first commercial neonicotinoid, imidacloprid, was developed by Bayer.

Most neonicotinoids are water-soluble and break down slowly in the environment, so they can be taken up by the plant and provide protection from insects as the plant grows. During the late 1990s this class of pesticides, primarily imidacloprid, became widely used. Beginning in the early 2000s, two other neonicotinoids, clothianidin and thiamethoxam were in use as well. Currently, virtually all corn that is planted in the Midwestern United States is treated with one of these two insecticides and various fungicides. In addition, most soybean seeds are also treated with a neonicotinoid insecticide, usually thiamethoxam. Clothianidin is one of the most toxic substances known for honey bees.