More than 150 years ago people of central Eurasia were using dried, crushed, daisy-like flowers from the genus Chrysanthemum to treat body lice. They were onto something. The crushed powder, called pyrethrum, contains components, called pyrethrins, which have an almost immediate paralyzing affect on insects. Pyrethroids are, essentially, man-made versions of pyrethrins. Permethrin is one of those man-made copies of the natural insecticide.
Two species of chrysanthemum flowers, C. cinerariaefolium and C. cineum, provide the pyrethrins for the commercial production of insecticides. Used in indoor and outdoor bug foggers, head lice treatments and flea sprays, pyrethrins are one of the least toxic treatments for pest insects around the home and garden. They are, however, highly toxic to aquatic life and to beneficial insects, such as bees. Commercial use of pyrethrins is limited since they break down quickly when exposed to light or water. Researchers have developed synthetic pyrethroids to create longer lasting pesticides.
A synthetic pyrethroid, known as permethrin, was registered for use on cotton in 1979. Since then, it has become one of the most widely used pyrethroids in the United States, available for agricultural and home garden use against a broad spectrum of insect pests. Unlike the botanically derived pyrethrins, permethrin persist longer in the environment, continuing its insecticidal properties for up to 12 weeks. Like its natural counterpart, permethrin is highly toxic to aquatic creatures and bees.
Sodium channels in nerve cell membranes allow charged atoms, ions, to pass through, changing the electrical charge of the cell, which releases neurotransmitters. Neurotransmitters communicate, through electrical messages, with other nerve cells to produce an action, such as moving a leg or flying away. In insects, pyrethrin and permethrin interfere with that communication by allowing the ion channels to stay open longer, ultimately paralyzing the insect. Since it does not degrade as quickly as natural pyrethrin, permethrin also acts as a gut poison to insects that eat permethrin-treated plants.
Both the natural and synthetic versions of chrysanthemum based insecticides threaten fish, mollusks, amphibians and non-target insects. Despite this risk, pyrethrins and permethrins are widely used because they break down quickly in the environment and have few toxic effects on mammals and birds. In 2013, the U.S. Environmental Protection Agency instituted labeling initiatives for manufacturers of pyrethrins and permethrin to ensure that professional and residential users apply the products in ways that minimize run-off and contamination to water bodies. Read and follow those instructions when applying and storing these insecticides.