The chemistry, metabolism and residue analysis of synthetic pyrethroids

Abstract

The chemical properties and metabolism of the synthetic pyrethroids, phenothrin, permethrin, cypermethrin, decamethrin and fenvalenate are reviewed. With the exception of decamethrin, which is a single stereoisomer, all the other pyrethroids discussed are used as a mixture of isomers. These chemicals are all relatively non—volatile; they have very low solubility in water, and can all be hydrolysed at the ester linkage. They are all more stable to photolysis than the natural pyrethroids although phenothrin which contains an isobutenyl group is still relatively photolabile. The photostability of the other chemicals is such that they have been successfully developed as agricultural insecticides. When applied to the leaves of plants, the pyrethroids are not translocated to any significant extent. They are degraded on the plant surface, probably mainly by photodegradation to a wide range of products many of which are formed by ester cleavage and ring hydroxylation. These products are subsequently conjugated by the plant mainly as glycosides. When plants are grown in treated soil, again little translocation of chemical to aerial parts of the plant occurs. That trace of chemical which is translocated is not intact pyrethroid but is metabolites of the acidic half of the original molecule. In soil, the pyrethroids are readily degraded. The tine taken for loss of half of the chemical ranges from about 2-12 weeks depending on the structure of the compound and the soil type. Hydrolysis of the ester linkage is a major degradation route. The products are further modified by loss of the cyanhydrin where applicable and by hydroxylation. The intact pyrethroids can also by hydroxylated. Further degradation also takes place and extensive evolution of 14C02 has been observed from radiolabels incorporated in various positions of the molecules. Most of the degradation observed has been shown to be mediated by soil microorganisms. When dosed to rodents, the pyrethroids are rapidly metabolised and the metabolites are excreted in similar amounts in both the urine and faeces. A large number of metabolites have been identified. These are mainly composed of intact pyrethroids and hydrolysis products both of which may be hydroxylated in various positions. The metabolites may also be conjugated with sulphate glucuronide, taurine and other chemicals. In addition radiolabelled permethrin has been bsed to ruminants and hens. Again excretion was rapid and only small amounts of dosed radioactivity transferred to the meat, milk and eggs of the treated animals. Sensitive residue analysis techniques are available for the analysis of the pyrethroids. In general the methods involve an initial extraction with organic solvent, followed by a liquid-liquid partition and an adsorption chromatography ‘clean-up’ step. The final determination is by gas–liquid chromatography using an electron capture detector. Various gic columns may be used to give total or partial separation of the isomers or to avoid separation of isomers. In the case of phemothrin, due to its poor sensitivity to an EC detector, analysis procedures are itore complex. No publications are available describing analysis of the breakdown products of the pyrethroids but some of the methods suggested for parent compounds involve a hydrolysis step and these could be adapted for analysis of metabolites. © 1981 IUPAC