Resistance defined by WHO The ability in a strain of some organism to tolerate doses of a toxicant which would prove lethal majority of individuals in a normal population of the same species.

Resistance defined by A.W.A. Brown Any population, within a species normally susceptible to a given insecticide, that is no longer controlled by the pesticide in the area concerned

Tolerance Higher doses needed to effect species A vs. B Interspecies term

Cross-resistance Resistance to one compound following the development of resistance to another due to the presence of a mechanism that gives protection against both compounds. Compounds are usually "similar".

Negative Cross-resistance Increased susceptibility to one compound following the development of resistance to another compound Multiple resistance (or multi-resistance) A population with resistance to different compounds due to individuals having two or more mechanisms of resistance

Frequency of resistance genes prior to insecticide selection

Hypothetical change in resistance gene frequency following selection

How resistance evolves Increasing insecticide selection

How resistance evolves Increasing insecticide selection

Hypothetical changes in gene frequency from prior to insecticide use (A), initial use (B), moderate use (C), heavy use (D) and long term heavy use (E) ABCD E

Learn More! Pesticides, the Environment, and Human Health (Entom. 3070) Fall Insecticide Toxicology (Entom. 4900) Spring 2011.