Pyrethroid Chemistry Explained
Early synthetic work to improve on the natural pyrethrum products isolated from chrysanthemums developed a class of chemicals typified by Allethrin that showed high insecticidal activity but, although their stability to light in the field was improved, it was not sufficient for wide scale agricultural use.
The breakthrough came in the 1970’s with the development of much more photostable analogs such as permethrin, cypermethrin and deltamethrin.
Further chemical refinement led to the development and registration of compounds such as bifenthrin, cyfluthrin, cyhalothrin, tefluthrin, fenpropathrin and fenvalerate. Collectively, these molecules constitute the major members of the pyrethroid class of chemistry today.
For a full list of USA-registered pyrethroids, visit the EPA website.
To a chemist, pyrethroids are characterized by an “ester” bond – an organic acid linked to an organic alcohol and this is the secret of their relatively rapid degradation in the environment, animal stomachs and living tissue.
Nature has evolved many microbial and animal enzymes that are very effective at breaking these ester linkages – and once the ester bond is broken, neither part of the molecule is insecticidal. Fortunately though, their other defining characteristic is their extremely high “lipophilicity” which simply means that intact pyrethroids dissolve very readily in fatty tissues and this is a characteristic that ensures they rapidly get to the sites in insects where their effect is nearly instantaneous to block the central nervous system transmitting information to produce symptoms of lost coordination and paralysis which are known as "the knockdown effect".
It is true that organochlorine insecticides (e.g. DDT) are also highly lipophilic and this leads to their well-reported behavior of steadily building up concentrations in bird and animal fatty tissues. It might therefore be expected that pyrethroids would behave in the same way. However, again, the ester bond in pyrethroids is the secret as to why they DO NOT bioaccumulate like DDT. This is because the bond is readily broken in animals, fish and birds such that the chemical is continually broken down in the animal and bird guts, bloodstream and tissues and this prevents the accumulation of high residue concentrations.
For all the Pyrethroid Chemistries, click here.
