Pyrethrins are insecticidal compounds that occur naturally in pyrethrum or chrysanthemum flowers (Chrysanthemum cinerariifolium and C. coccineum). Those flowers, which are grown mainly in Kenya, contain six toxins: pyrethrin I and II, jasmolin I and II, and cinerin I and II. Dried pyrethrum flowers and pyrethrum were used extensively as insecticides before World War II. They are potent insecticides with low mammalian toxicity, and they are relatively unstable under ultraviolet radiation and therefore degrade in the environment quickly (Matsumura, 1985; O’Brien, 1967). Use of pyrethrins declined after World War II with the advent of synthetic insecticides, such as DDT. In the 1960s, however, as concern about the harmful effects of some of those synthetic insecticides and their persistence in the environment grew, interest in the pyrethrins was renewed. A large number of pyrethrin derivatives, called pyrethroids, have since been synthesized and tested for insecticidal potency, mammalian toxicity, and biodegradability (Elliott, 1977). At least 2 dozen pyrethroids are in use today, and they make up one of the most popular classes of substances for control of agricultural and household insect pests. The rest of this section deals mainly with pyrethroids.
Two pyrethroids, permethrin and d-phenothrin, were used in the Gulf War. They were sprayed on military clothing to repel and kill flies and mosquitoes (Schreck and Kline, 1989; Schreck et al., 1986; Sholdt et al., 1989). Permethrin has also been used topically for treatment of head lice and scabies in humans (Asakawa et al., 1996; Facts and Comparisons, 2001; Fuortes, 1999; Llewellyn et al., 1996).
Pyrethrins and pyrethroids are esters of alcohols and acids (Elliott, 1977; Matsumura, 1985; O’Brien, 1967). Pyrethrins have excellent insecticidal properties, including a higher potency for insects than for mammals, but because they are relatively unstable in ultraviolet radiation, frequent application is necessary and can become expensive. Therefore, many pyrethroids with increased photostability have been synthesized (Elliott, 1977). Those pyrethroids may be divided into two large categories: type I pyrethroids, which do not contain a cyano moiety at the α position; and type II pyrethroids, which do contain an α-cyano group. Permethrin and phenothrin, the pyrethroid insecticides in this class used in the Gulf War, are type I pyrethroids. Other type I pyrethroids are resmethrin, allethrin, and cismethrin. Examples of type II pyrethroids are cypermethrin, fenvalerate, deltamethrin, and cyphenothrin (Ecobichon, 2001). The structures of pyrethrin I, permethrin, and d-phenothrin are shown in Figure 3.3.
Pyrethroids have two or three chiral carbon atoms, and the commercial pyrethroids are mixtures of four or eight isomers. Insecticidal activity varies greatly among isomers of a pyrethroid. For example, the most potent allethrin isomer against houseflies is d-allethronyl d-trans-chrysanthemate, and the least potent allethrin isomer is l-allethronyl l-trans-chrysanthemate; they differ in potency by a factor of 150.
Pyrethroids are hydrophobic compounds that are absorbed and distributed after ingestion by mammals and inhalation exposure (ATSDR, 2001b). Dermal absorption is