Insects have evolved three major mechanisms to overcome toxicants; 1) biochemical resistance; 2) physiological resistance; and 3) behavioral resistance.
- Biochemical resistance: In this form of resistance, an insecticide is detoxified by one or more enzymes before it can reach its site of action. Mixed-function oxidases or other enzymes are involved.
- Physiological resistance: Physiological resistance is any form of resistance that reduces toxicity through changes in basic physiology. In this form of resistance, the chemical is not broken down into a less toxic form, rather the insect accommodates the chemical by altering one or more physiological functions. This may involve:
- reduced neuronal sensitivity to insecticides e.g., knockdown resistance (kdr) in house flies to DDT and pyrethroids.
- altered acetylcholinesterase. Acetylcholinesterase that is less sensitive to inhibition by OP and carbamate insecticides has been documented in resistant strains of several insect, tick and mite species. E.g. Anopheles mosquito’s resistant to OP and carbamates have been shown to have an altered acetylcholinesterase.
- decreased penetration of the insecticide through the body wall through modification of the structure or composition of the cuticle. e.g., additional waxy layers in resistant strains.
- increased excretion or sequestration of insecticide. e.g. DDT storage in body fat, preventing it from reaching the site of action.
- Behavioral resistance: involves changes in behavior by which insects avoid insecticides. Ex. In mosquito, Anopheles gambiae, an endophillic strain (indoor dwelling) was susceptible to DDT sprays applied to the indoor walls. An exophillic strain not inhabiting indoors became dominant because its behavior allowed it to avoid exposure to the insecticide.
Cross-resistance: ability of an insect with resistance to one insecticide to resist other insecticides.