Insecticide resistance describes the decreased susceptibility of a Insect population to a Insecticide that was previously effective at controlling the Insect. Insect species evolve Insecticide resistance via natural selection: the most resistant organisms are the ones to survive and pass on their genetic traits to their offspring.
Manufacturers of Insecticides tend to prefer a definition that is dependent on failure of a product in a real situation, sometimes called field resistance. For example, the Insecticide Resistance Action Committee (IRAC) definition of insecticide resistance is 'a heritable change in the sensitivity of a Insect population that is reflected in the repeated failure of a product to achieve the expected level of control when used according to the label recommendation for that Insect species'.
Insecticide resistance is increasing in occurrence. Farmers in the USA lost 7% of their crops to Insects in the 1940s; over the 1980s and 1990s, the loss was 13%, even though more Insecticides were being used. Over 500 species of Insects have evolved a resistance to a Insecticide. Other sources estimate the number to be around 1000 species since 1945.