Action of pyrethroids

Abstract

Natural pyrethrins, also called pyrethrum or 'pyrethrum extract, have long been known as insecticides. They are extracted from the flowers of Chrysanthemum cinerariaefolium Vis which contain six insecticidally active components, of which pyrethrin I and pyrethrin II are the most active (Head 1973, Elliott & Janes, 1973). Pyrethrins are widely used for domestic purposes and for the protection of stored food, but for general pest control in agriculture they are applied on a limited scale only. The total world production does not exceed about 350 tons per yr: cf. organophosphates, about 100,000 tons (Glynne-Jones, 1973: Fest & Schmidt. 1973). Their Limited use is due in part to economic reasons. Other factors restricting their use are their low photostability and high (bio) degradability, and pyrethrins are usually applied in combination with metabolic synergists. Apart from these problems, the pyrethrins possess excellent insecticidal properties combined with remarkably low mammalian toxicity. Their insect (topical) to mammalian (oral) toxicity ratio is much better than for the majority of other organic insecticides (Elliott, 1977). In mammals and other animals pyrethrins are rapidly metabolized and practically no residues are left in the biosphere. These favourable properties have stimulated the search for synthetic insecticides based on the structure of the pyrethrins. The production of synthetic pyrethroids on a commercial basis started with allethrin and cyclethrin around 1950 (O'Brien, 1967). These early compounds, however, still lacked sufficient stability and they are less effective against many species of insects than the natural product. At the moment the total yearly production of allethrin is some 100 tons. In the last decennium great progress has been made, notably by Ellion and co-workers, in the development of more stable and highly active pyrethroids and in the near future more widespread application of these compounds may be expected. Because of the complex molecular structure, the most important features of the pyrethroid molecule will be described briefly before the mode of action is discussed. Excellent reviews on the structural requirements of insecticidal active pyrethroids can be found elsewhere (Elliott, 1971, 1976a, b, 1977; Elliott eI al., 1974a). Most of the information presented in the next section is derived from these reviews. GENERAL FEATURES OF THE PVRETHROID MOI.ECLI,I.