Pyrethroids toxicity introduction

Table 2 shows the relative toxicities of leading pyrethroids when applied topically to houseflies, deltamethrin being one of the most potent pyrethroids commercialised. More recent product introductions have been flumethrin and cyhalothrin. 1998 global sales of pyrethroids were 190m, they are particularly attractive products for tick control giving rapid clean-up and persistent efficacy. 

The lead compound, which initiated interest in the neonicotinoids, is a nitro-methylene heterocycle, nithiazine (Figure 1) [1]. Nifhiazine, with its unique nitromethylene moiety, showed low mammalian toxicity, but its insecticidal potency and field stability were inferior to commercial organophosphates and pyrethroids. Thus, nithiazine was not of commercial use for pest control. However, the introduction of 6-choloronicotinyl and 2-nitroimino-imidazolidine moieties led to the development of the first commercial neonicotinoid imidacloprid (Figure 1)... 

Prior to the introduction of synthetic pyrethroids, insect-resist agents were based mainly on speciality materials, such as polychloro chloromethyl sulfonamido diphenyl ether (PSCDs) (Eulan U33, Eulan WA New). The production of these compounds has been discontinued, however, because of a combination of environmental toxicity (149) and cost. One speciality insect-resist agent based on sulcofuron (Mitin FF), released in 1939 (153), retains a small market share for specific applications where cost is less important and where high resistance to wet conditions is required. It has also been used on wool insulation where it also has low volatility and provides resistance to rotting (154). 

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