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Why can insect pests evolve resistance to the Bt toxin used in some genetically modified crops?

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CRISPR-Cas9; gene editing — gene editing The CRISPR-Cas9 gene-editing complex from the bacterium *Streptococcus pyogenes*.

Insect pests can evolve resistance to the Bt toxin used in genetically modified crops primarily due to the pressure of repeated exposure. When these crops are grown extensively, the constant presence of the toxin acts as a powerful selective force.1

In any large pest population, there may be individuals that possess rare genetic variations—mutations—that allow them to survive exposure to the toxin. Because the Bt-producing crops kill off the susceptible individuals, those few resistant pests survive, reproduce, and pass their resistance genes to the next generation. Over time, through the process of natural selection, these resistant individuals become more common, eventually rendering the toxin ineffective against that population.1

To manage this risk, scientists and farmers employ strategies such as planting "refuges"—areas of non-Bt crops near the Bt fields—to maintain a population of susceptible insects that can mate with any resistant ones, thereby diluting the resistance genes in the overall population.1