Pyrethroid insecticides were first developed over 40 years ago at a time when concerns were growing about the persistent nature of organochlorine pesticides (also known as chlorinated hydrocarbons), such as DDT. Organochlorine residues were found to be accumulating in the environment and in some unexpected places in the food chain, sometimes where the pesticide itself had rarely been used, and safer alternatives were needed.
The pyrethroids were developed by a team of scientists at Rothamsted Research, led by Michael Elliott. They identified the most active components of pyrethrum, a natural (though relatively weak) insecticide mixture extracted from the chrysanthemum flowers Chrysanthemum cinerariaefolium and C. coccineum, and modified their structures to improve their activity against insects.
Pyrethroids are derived from natural insecticides in flowers. Image: iStock
From pyrethrum came the pyrethroids, which were more effective at lower application rates than other classes of insecticides; less persistent in the environment and less prone to bioaccumulate in organisms; and safer than their predecessors with very low mammalian toxicity.
Pyrethroids work by targeting the sodium channels in the neuronal membranes of insects. These channels open in response to brief membrane depolarisations and generate the electrical currents that are required for nerve signalling. Pyrethroids act by holding the channels open, which disrupts the normal functioning of the nervous system and eventually paralyses the insect.
The first generation of pyrethroids were developed in the 1960s. They included bioallethrin and tetramethrin, which were more active than the natural pyrethrum, but like pyrethrum were volatile and easily degraded by sunlight
The Rothamsted team then developed a second generation in the 1970s, notably permethrin, cypermethrin and deltamethrin, with improved light stability that made them much more suitable for use in agriculture.
Pyrethroids still account for up to 17% of global insecticide sales – a market worth more than $7Bn each year – and have made a significant contribution to the UK economy. Furthermore, pyrethrum-based products because of their natural origin can also be used for organic food manufacture.
Michael Elliott’s discoveries earned him a CBE and the Queen’s Award for Technological Achievement on two occasions.