Pyrethroid pesticides are synthetic analogues of pyrethrins, which are natural chemicals found in chrysanthemum flowers, in a natural environment, they were chemically unstable and broke down rapidly upon exposure to air and sunlight. Beginning in the 1970s, synthetic pyrethroids came into the market for agricultural purposes. Their desirable features are providing a quick knockdown of insects at low rates, relatively low mammalian toxicity and improved stability in outdoor environments, which has increased their marketability in agriculture. Pyrethroid pesticides are used to control a wide range of insects in public and commercial buildings, animal facilities, warehouses, agricultural fields, and greenhouses. They are also applied on livestock to control insects. In agriculture, cypermethrin, cyfluthrin, and deltamethrin have been used frequently on cotton. Pyrethroid insecticides are the most common active ingredients in commercially available insect sprays and are also used as structural termiticides. Formulations that are commercially available include aerosols, dips, emulsifiable concentrates, wettable powders, granules, and concentrates for ultra low volume applications targeting mosquitoes. Pyrethroids may be mixed with piperonyl butoxide, a synergist, which enhances the effect of the active ingredient. Their mode of action is interference with transmission of nerve impulses.

Development

Pyrethroids were introduced thirty years ago, by a team of Rothamsted Research scientists led by M. Elliott, and represented a major advancement in activity and relatively-low mammalian toxicity. Their development was especially timely with the identification of problems with DDT use. Their work consisted firstly of identifying the most active components of pyrethrum, extracted from East African chrysanthemum flowers and long known to have insecticidal properties. Pyrethrum rapidly knocks down flying insects, but has a low mammalian toxicity and negligible persistence - which is good for the environment but gives poor efficacy when applied in the field. Pyrethroids are essentially chemically stabilized forms of natural pyrethrum and belong to IRAC MoA group 3 (they interfere with sodium transport in insect nerve cells).

The 1st generation pyrethroids, developed in the 1960s, include bioallethrin, tetramethrin, resmethrin and bioresmethrin. They are more active than the natural pyrethrum, but are unstable in sunlight. Activity of pyrethrum and 1st generation pyrethroids is often enhanced by addition of the synergist piperonyl butoxide (which is not itself biologically active). With the 91/414/EEC review, many 1st generation compounds have not been included on Annex 1, probably because the market is simply not big enough to warrant the costs of re-registration (rather than any special concerns about safety).

By 1974, the Rothamsted team had discovered a 2nd generation of more persistent compounds notably: permethrin, cypermethrin and deltamethrin. They are substantially more resistant to degradation by light and air, thus making them suitable for use in agriculture, but they have significantly higher mammalian toxicities. Over the subsequent decades these were followed with other proprietary compounds such as fenvalerate, lambda-cyhalothrin and beta-cyfluthrin, but most patents have now expired, making them cheap and therefore popular (although permethrin and fenvalerate have not been re-registered under the 91/414/EEC process). One of the less desirable characteristics, especially of 2nd generation pyrethroids is that they can be irritant to the skin and eyes, so special formulations such as capsule suspensions (CS) have been developed.

Toxicity

Pyrethroid pesticides have low volatility, bind to soils, and are rarely detected in ground waters (USGS, 2007). Generally, they are not persistent in the environment due to their rapid degradation within days to several months. This class of pesticides has low toxicity in birds and mammals, but pyrethroids are highly toxic to fish and some aquatic invertebrates, so usage is restricted near water.

Human health effects from pyrethroid pesticides at low environmental doses or at biomonitored levels from low environmental exposures are unknown. Compared with other classes of insecticides such as organochlorines, organophosphorus, or carbamate pesticides, pyrethroid pesticides have less acute toxicity in animals and people. They are ranked as having moderate acute oral toxicity. Adverse effects from large doses are related to the action of pyrethroids on the nervous system, where these chemicals prolong sodium channel opening when a nerve cell is depolarized. Possible other additional actions on neuroreceptors and other ion channels may also explain some pyrethroid effects.