Pyrethroid

A pyrethroid is an organic compound similar to the natural pyrethrins, which are produced by the flowers of pyrethrums (Chrysanthemum cinerariaefolium and C. coccineum). Pyrethroids are used as commercial and household insecticides.

In household concentrations pyrethroids are generally harmless to humans. However, pyrethroids are toxic to insects such as bees, dragonflies, mayflies, gadflies, and some other invertebrates, including those that constitute the base of aquatic and terrestrial food webs. Pyrethroids are toxic to aquatic organisms, especially fish. They have been shown to be an effective control measure for malaria outbreaks, through indoor applications.

Pyrethroids are excitotoxic to axons. They act by preventing the closure of the voltage-gated sodium channels in the axonal membranes. The sodium channel is a membrane protein with a hydrophilic interior. This interior is shaped precisely to allow sodium ions to pass through the membrane, enter the axon, and propagate an action potential. When the toxin keeps the channels in their open state, the nerves cannot repolarize, leaving the axonal membrane permanently depolarized, thereby paralyzing the organism. Pyrethroids can be combined with the synergist piperonyl butoxide, a known inhibitor of microsomal P450 enzymes which are important in metabolizing the pyrethroid. By that means, the efficacy (lethality) of the pyrethroid is increased. It is likely that there are other mechanisms of intoxication also. Disruption of neuroendocrine activity is thought to contribute to their irreversible effects on insects, which indicates a pyrethroid action on voltage-gated calcium channels (and perhaps other voltage-gated channels more widely).

Pyrethroids are classified based on their mechanism of biological action, as they do not share a common chemical structure. Many are 2,2-dimethylcyclopropanecarboxylic acid derivatives, like chrysanthemic acid, esterified with an alcohol. However, the cyclopropyl ring does not occur in all pyrethroids. Fenvalerate, which was developed in 1972, is one such example and was the first commercialized pyrethroid without that group.

Pyrethroids which lack an α-cyano group are often classified as type I pyrethroids and those with it are called type II pyrethroids. Pyrethroids that have a common name starting with "cy" have a cyano group and are type II. Fenvalerate also contains an α-cyano group.

Some pyrethroids, like etofenprox, also lack the ester bond found in most other pyrethroids and have an ether bond in its place. Silafluofen is also classified as a pyrethroid and has a silicon atom in the place of the ester. Pyrethroids often have chiral centers and only certain stereoisomers work efficiently as insecticides.

Pyrethroids are toxic to insects such as bees, dragonflies, mayflies, gadflies, and some other invertebrates, including those that constitute the base of aquatic and terrestrial food webs. They are toxic to aquatic organisms including fish.

Pyrethroids are usually broken apart by sunlight and the atmosphere in one or two days, however when associated with sediment they can persist for some time.^{[better source needed]}