Pyrethroid

Allethrin

Permethrin

A pyrethroid is an organic compound similar to the natural pyrethrins produced by the flowers of pyrethrums (Chrysanthemum cinerariaefolium and C. coccineum). Pyrethroids now constitute the majority of commercial household insecticides. In the concentrations used in such products, they may also have insect repellent properties and are generally harmless to human beings in low doses but can harm sensitive individuals. They are usually broken apart by sunlight and the atmosphere in one or two days, and do not significantly affect groundwater quality.

Mode of action

Pyrethroids are axonic excitoxins the toxic effects of which are mediate through 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 a tiny hole which is shaped precisely to strip away the partially charged water molecules from a sodium ion and create a favorable way for 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 permamentaly depolarized, thereby paralyzing the organism.

Pesticide formulation

Pyrethroids are usually combined with piperonyl butoxide, a known inhibitor of key microsomal cytochrome P450 enzymes from metabolizing the pyrethroid, which would diminish its lethality.

History

Pyrethroids were introduced in the late 1900s by a team of Rothamsted Research scientists following the elucidation of the structures of pyrethrin I and II by Hermann Staudinger and Leopold Ružička in the 1920s. The pyrethroids represented a major advancement in the chemistry that would synthesize the analog of the natural version found in pyrethrum. Its insecticidal activity has relatively low mammalian toxicity and an unusually fast biodegradation. Their development coincided 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 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 itself has some insecticidal activity). 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 derivatives were followed with other proprietary compounds such as fenvalerate, lambda-cyhalothrin and beta-cyfluthrin. Most patents have now expired, making these compounds 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 an irritant to the skin and eyes, so special formulations such as capsule suspensions (CS) have been developed.

Classes of pyrethroids

(1R,3R)- or (+)-trans-chrysanthemic acid

Flucythrinate

The earliest pyrethoids are related to pyrethrin I and II by changing the alcohol group of the ester of chrysanthemic acid. This relatively modest change can lead to substantially altered activities. For example the 5-benzyl-3-furanyl ester called resmethrin is only weakly toxic to mammals (LD50 (rat, oral) = 2,000 mg/kg) but is 20-50x more effective than natural pyrethrum and is also readily biodegraded. Other commercially important esters include tetramethrin, allethrin, phenothrin, barthrin, dimethrin, and bioresmethrin. Another family of pyrethroids have altered acid fragment together with altered alcohol components. These require more elaborate organic synthesis. Members of this extensive class include the dichlorovinyl and dibromovinyl derivatives. Still others are tefluthrin, fenpropathrin, and bioethanomethrin.

Types

• Allethrin, the first pyrethroid synthesized (active ingredient of Raid)
• Bifenthrin, active ingredient of Talstar, Capture, Ortho Home Defense Max, and Bifenthrine
• Cyfluthrin, an active ingredient in Baygon, dichlorovinyl derivative of pyrethrin
• Cypermethrin, including the resolved isomer alpha-cypermethrin, dichlorovinyl derivative of pyrethrin
• Cyphenothrin, active ingredient of K2000 Insect spray sold in Israel and the Palestinian territories
• Deltamethrin, dibromovinyl derivative of pyrethrin
• Esfenvalerate
• Etofenprox
• Fenpropathrin
• Fenvalerate
• Flucythrinate
• Flumethrin
• Imiprothrin, active ingredient of Raid Ant & Roach Killer, synthesized to be an extremely fast-acting toxin by Sumitomo Chemical Co. in Canada.
• lambda-Cyhalothrin
• Metofluthrin
• Permethrin, dichlorovinyl derivative of pyrethrin
• Prallethrin, active ingredient in Baygon
• Resmethrin, active ingredient of Scourge
• Silafluofen
• Sumithrin, active ingredient of Anvil
• tau-Fluvalinate
• Tefluthrin
• Tetramethrin
• Tralomethrin
• Transfluthrin, active ingredient in Baygon

Environmental effects

Aside from the fact that they are also toxic to beneficial insects such as bees and dragonflies, pyrethroids are toxic to fish and other aquatic organisms. At extremely small levels, such as 2 parts per trillion, pyrethroids are lethal to mayflies, gadflies, and invertebrates that constitute the base of many aquatic and terrestrial food webs.

Pyrethroids have been found to be unaffected by secondary treatment systems at municipal wastewater treatment facilities in California. They appear in the effluent, usually at levels lethal to invertebrates.

Safety and effectiveness

Pyrethroids are considered safe to vertebrates, which have sufficient enzymes required for rapid breakdown. In terms of LD50 for rats Tefluthrin is the most toxic at 29 mg/kg. Pyrethroids are however highly toxic to cats because they do not have glucuronidase which serves in hepatic detoxifying metabolism pathways. Anaphylaxis has been reported after pyrethrum exposure, but allergic reaction to pyrethroids has not been documented. Increased sensitivity occurs following repeated exposure to cyanide, which is found in pyrethroids like beta-cyfluthrin (Multiple Chemical Sensitivity Awareness, J. Edward Hill, MD, President & Executive Committee Member, AMA).

Researchers "found a significant association between piperonyl butoxide (PBO) a common additive in pyrethroid formulations" ... "and delayed mental development at 36 months."

Resistance

Up until the 1950s, bedbugs were almost eradicated through the use of DDT. After the use of DDT for this purpose was banned, pyrethroids became more commonly used against bed bugs. As of 2010 nearly all populations of bedbugs have evolved nerve cells impervious to pyrethroids, and pyrethroids are no longer effective in combatting bedbug infestations.

Your last statement is simply untrue...developmental resistance to pyrethroids is temperal and often geographically associated with bedbugs...resistance is not fixed based on several current studies that show relatively good susceptibility to pyrethroids [See Yoon et al. (2008)Biochemical and Molecular Analysis of Deltamethrin Resistance in theCommon Bed Bug (Hemiptera: Cimicidae)J. Med. Entomol. 45(6): 1092-1101 (2008)]; This isn't saying resistance is not present...it is and where it is, it may well be that pyrethroids for those descrete populations are not working, but elsewhere this simply isn't fact. Even in areas of high, routine regular application of insecticides susceptibility can be sustained [See Steelman et al. (2008) Susceptibility of the Bed Bug Cimex lectularius L.(Heteroptera: Cimicidae) Collected in Poultry ProductionFacilities to Selected Insecticides, J. Agri. Urban. Entomol. 25(1): 41–51 (January 2008)] No one disputes resistance occuring, only the level and scope of resistance cannot be generalized as if every population sustains fixed consistent resistance to pyrethroids...even lab strains collected wild lose their resistance thresholds. It is just that researchers do not challenge lab populations as a matter of routine to sustain the threshold of resistance, because of the tremendous costs in time,money and insects (it takes a while to breed and sustain large numbers of insects for this enterprise) required to do so, nor do they routinely establish baseline resistance thresholds or resistance ratios on an ongoing basis...they generally use historic ones that may change in time and degree.13:46, 7 June 2013 (UTC)144.29.1.20 (talk)

External links

• Pyrethrins and pyrethroids on the EXTOXNET
• Multiple Chemical Sensitivity Awareness, J. Edward Hill, MD, President & Executive Committee Member, AMA
• Environmental Health Perspectives Journal Estrogenic Potential of Certain Pyrethroid Compounds...