Most species never build up population sizes large enough to be considered pests.
Those species that do become pests are often responding to human changes to the environment, or have resulted when humans has introduced species into new areas. Chemical pesticides have been the principle method of pest control for the past half century. Pesticides were originally very effective at controlling pests. However, problems such as the evolution of resistance, development of secondary pests and ecotoxicological effects of pesticide exposures have limited the utility of pesticides. Integrated Pest Management combines these diverse pest control strategies, along with judicious use of chemical pesticides, to regulate pest populations at levels that are low enough so that economically important damage does not occur.