Title: insecticide | Description, Modes of Action, Types, & Environmental Impacts | Britannica
Headings: 
Insecticide
chemical substance
Modes of penetration
Synthetic insecticides
Chlorinated hydrocarbons
Organophosphates
Carbamates
Environmental contamination and resistance

Content: Consequently, the use of such chemicals began to be restricted in the 1960s and banned outright in the 1970s in many countries. Cases of insecticide poisoning of humans also occur occasionally, and the use of one common organophosphate, parathion, was drastically curtailed in the United States in 1991 owing to its toxic effects on farm labourers who were directly exposed to it. Another problem with insecticides is the tendency of some target insect populations to develop resistance as their susceptible members are killed off and those resistant strains that survive multiply, eventually perhaps to form a majority of the population. Resistance denotes a formerly susceptible insect population that can no longer be controlled by a pesticide at normally recommended rates. Hundreds of species of harmful insects have acquired resistance to different synthetic organic pesticides, and strains that become resistant to one insecticide may also be resistant to a second that has a similar mode of action to the first. Once resistance has developed, it tends to persist in the absence of the pesticide for varying amounts of time, depending on the type of resistance and the species of pest. Insecticides may also encourage the growth of harmful insect populations by eliminating the natural enemies that previously held them in check. The nonspecific nature of broad-spectrum chemicals makes them more likely to have such unintended effects on the abundance of both harmful and beneficial insects. Because of the problems associated with the heavy use of some chemical insecticides, current insect-control practice combines their use with biological methods in an approach called integrated control. In this approach, a minimal use of insecticide may be combined with the use of pest-resistant crop varieties;