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History of Pyrethroids

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 World War II, 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 chrysanthemum extract. The first analog showed greater activity than the natural product and also longer efficacy in the field. These initial groups of synthetic “pyrethroids” showed a trend of increasing insecticidal activity with little change in their low mammalian toxicity.  However, they all remained 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 the 1970’s, suitable  field stability was obtained with the development of permethrin, cypermethrin and deltamethrin and these molecules were all registered by authorities across the world to later 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 replacement of many uses of organo-phosphates and carbamates which had dominated the markets after the demise of the organochlorine 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 to control insect pests and the diseases they transmit.

For more information on the truly fascinating history of pyrethroid chemistry that is summarized above see: