History of Pyrethroids

Pyrethroids have been used for over 70 years by farmers, governments, homeowners and commercial pest control operators.  Pyrethroids are widely used in products for agricultural, vector control (e.g. mosquitoes), animal health, stored grain, home and garden, and turf and ornamental markets.

Pyrethroids are a class of insecticides that have been designed to improve upon the insecticidally active natural products found in Chrysanthemum plant powders known as pyrethrum. They are highly effective causing very rapid death of pest insects.  When introduced in the 1970s and 80s, they were found to be as or more effective than the existing pool of insecticides and had lower mammalian toxicity making them safer for applicators and the public.  

Not only do they not bioaccumulate in the food chain like the organo-chlorine insecticides but they are also non-systemic which ensures they are not transported within plants – thus minimizing their effect on pollinators.  Consequently, they rapidly became the most widely used class of insecticides across the USA and the world and this popularity continues to the present day. Pyrethroids are very effective and therefore the amounts of chemical applied to create a given effect are much lower than many older-generation insecticides.

Despite the favorable  toxicity profile for humans, animals and birds relative to other insecticides, the extreme toxicity of pyrethroids to insects does mean that they need to be used carefully to ensure safety to non-target terrestrial insects in order to avoid impacts to pollinators and other beneficial organisms.

Most importantly, pyrethroids are highly toxic to aquatic insects and fish and this has led PWG, government scientists and regulators as well as academics to generate a remarkable body of data on these topics.

The aquatic fate and effects of pyrethroids is the most carefully researched and well understood area of insecticide science and PWG has tracked and documented all of this research (databases available from PWG upon request).  US EPA recently performed a registration review of the pyrethroids to ensure that any risks associated with their use were understood and mitigated to meet current, more stringent, regulations.

The EPA focused on aquatic fate and effects and mandated considerably more strict specifications for use to reduce any risks to acceptable levels. California State regulators have also recently added additional restrictions to pyrethroid use labels to ensure that the continued use of pyrethroids does not result in unacceptable risk.

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 web site at https://www.epa.gov/ingredients-used-pesticide-products/registration-review-pyrethrins-and-pyrethroids

To a chemist, pyrethroids are characterized with 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 birds guts, bloodstream and tissues and this prevents the accumulation of high residue concentrations.

Finely ground flower heads from two chrysanthemum species: the Caucasian pyrethrum or painted daisy (Chrysanthemum coccineum) and the Dalmatian pyrethrum (Chrysanthemum cinerariifolium) had been used many centuries ago in China to control domestic insect pests.  Its use spread along trade routes.

Around the beginning of the 20th century the extreme efficacy of the powder and its extracts was confirmed as were its limitations; namely, the extreme cost of production and the short persistence of effect outdoors.  At the time of the second world war, demand peaked and supply was a constraint; this stimulated the development of several classes of insecticides – organo-chlorines, organophosphates and carbamates.  In the same way work started to synthesize analogs of the pyrethrins, the active substances in the extract.

The first analog showed greater activity than the natural product and also longer efficacy in the field.  However, the initial groups of synthetic “pyrethroids” showed a trend of increasing insecticidal activity with little change in their low mammalian toxicity.  However, they were all subject to fairly rapid breakdown in sunlight which limited their application to agriculture but stimulated the desire for further improvements in the chemistry.

As an interesting historical note, even the famous Rachel Carson saw the potential advantages of pyrethroids and made this recommendation in Silent Spring.
“The ultimate answer is to use less-toxic chemicals so that the public hazard from their misuse is greatly reduced. Such chemicals already exist: the pyrethrins, rotenone, ryania, and others derived from plant substances. Synthetic substitutes for the pyrethrins have recently been developed, and some of the producing countries stand ready to increase the output of the natural product as the market may require.”

In the1970’s, suitable  field stability was obtained with he development of permethrin, cypermethrin and deltamethrin and these molecules were all registered by authorities across the world to be joined by even more active molecules such as bifenthrin, fenvalerate, cyhalothrin and cyfluthrin.

Those registrations marked a gradual adoption by growers/users world-wide and the ultimate take-over of many uses of organo-phosphates and carbamates which had dominated the markets after the demise of the organo chlorine insecticides.Since then, pyrethroids have played a major role in the protection of world food supplies both while being grown or stored.

They have also played a major part in protecting human health – perhaps most notably when it was realized that pyrethroid-treated mosquito netting could have dramatic effects in reducing malaria transmission.

Despite their long history, the unique properties of pyrethroids in terms of their efficacy, low to medium persistence, non-systemicity, lack of bio-accumulation and relatively low mammalian toxicity mean that they are likely to remain important tools in man’s armory against pests and diseases for some time.

For more information on the truly fascinating history of pyrethroid chemistry see https://www.chemistryviews.org/details/ezine/11099733/Pyrethrum_History_of_a_Bio-Insecticide__Part_1/ and the five related articles accessed via that page.

I have taken the liberty to summarize  various parts of those articles in the text aboveAnother useful review can be found in ” Pyrethrins and Pyrethroid Insecticides”  Jerome  J. Schleier III and Robert K.  D. Peterson in Chapter 3 in Green Chemistry No.  1, Green Trends in Insect Control.  Eds o.  Lopez and J.  G.  Fernandez-Bolanos.  Royal Society of Chemistry.  2011. – accessible via
https://www.montana.edu/rkdpeterson/documents/Schleier%20III%20and%20Peterson%202011%20Pyrethrins%20and%20Pyrethroids%20Chapter.pdf