The same evolutionary mechanism that produces antibiotic resistance in bacteria also produces pesticide resistance in insects (and in weeds, against herbicides). The four steps of natural selection apply in exactly the same way:
- Variation: a few insects in the field have a mutation that makes them less susceptible to a particular pesticide.
- Differential survival: when the field is sprayed with the pesticide, susceptible insects die. Resistant insects survive.
- Differential reproduction: the survivors reproduce. Their offspring inherit the resistance allele.
- Allele frequency change: after a few generations, most of the insect population in that field is resistant. The pesticide stops working.
Real examples
- DDT resistance in mosquitoes appeared within just a few years of DDT being introduced as an anti-malaria spray.
- Pyrethroid resistance in bedbugs has made standard chemical control much less effective.
- Glyphosate-resistant weeds (resistant to the herbicide "Roundup") are now widespread in farmland that has been repeatedly sprayed.
Slowing the spread
Strategies to manage pesticide resistance include:
- Rotate pesticides: use different chemicals with different modes of action, so insects cannot become resistant to all of them at once.
- Integrated pest management (IPM): combine pesticides with other methods (biological control with natural predators, crop rotation, resistant crop varieties) to reduce the selection pressure from any one pesticide.
- Genetically modified crops that produce their own insecticide (Bt crops) reduce the need for spraying, although insects are slowly becoming resistant to these too.