Introduction to Pyrethroid Insecticides: Chemical Structures, Properties, Mode of Action and Use

Abstract

During the 1920s, pyrethrin was studied because of its potential as a precursor for synthetic organic pesticides. The first pyrethroid pesticide, allethrin, was identified in 1949. It is a type I pyrethroid because of a carboxylic ester of cyclopropane. Type II was created with the addition of a cyano group in α position. Some phenylacetic 3-phenoxybenzyl esters missing the cyclopropane but with the cyano group are also considered type II. In the 1970s, pyrethroids transitioned from mere household products to pest control agents in agriculture. Later, pyrethroids have replaced organophosphate pesticides in most of their applications the same way the latter had replaced organochlorinated pesticides before. Works on the optimisation of pyrethroids has granted them better photostability without compromising their biodegradability, as well as selective toxicity, metabolic routes of degradation and more effectivity, translating into the use of smaller amounts. Most pyrethroids present different isomers, each with different biological activity and, therefore, different toxicity. Pyrethroids account for a quarter of the pesticides used nowadays. Pyrethroids’ relative molecular mass is clearly above 300 g mol⁻¹; they are highly hydrophobic, photosensitive and get easily hydrolysed, with degradation times below 60 days. They are not persistent and mammals can metabolise them. However, pyrethroids have been proven to bioaccumulate in marine mammals and humans. Studies in mammals reported carcinogenic, neurotoxic and immunosuppressive properties and potential for reproductive toxicity mainly. Acceptable daily intake values and no observed adverse effect level values have been established at 0.02–0.07 mg (kg body weight)⁻¹ day⁻¹ and 1–7 mg (kg body weight)⁻¹ day⁻¹.