Resistance

Abstract

Resistance to insecticides is defined by the World Health Organization (WHO) as ‘development of an ability in a strain of some organism to tolerate doses of a toxicant that would prove lethal to a majority of individuals in a normal population of the same species’ (WHO, cited in Ferrari 1996). Ferrari (1996) states that ‘resistance has a genetic composition of a population as a direct result of the selective effects of a toxicant’. Bossard et al. (1998) proclaim ‘a response of an organism or a population to a toxicant that enables the organism or population to withstand future toxicant exposures better, because gene amplification which may confer resistance does not require selection (Devonshire and Field 1991) and other individual responses to sublethal exposures are included’ to be a better definition of resistance. They suggest much of the insecticide resistance ascribed to cat fleas possibly to be simply a variation in flea susceptibility.