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Why have some insect pests evolved resistance to Bt toxin?

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Rows of genetically modified barley plants grow in an outdoor test plot in Germany, with a yellow marker labeled 3 and protective netting overhead. — genetically modified organism Genetically modified crops are assessed for their impact on environmental factors such as soil quality.

Insect pests have evolved resistance to Bt toxin primarily as a result of repeated exposure to the protein. When crops are genetically modified to express this toxin continuously, insect populations are subjected to constant selective pressure, which favors the survival and reproduction of individuals that possess natural resistance.1

This development is a significant concern because it threatens to render one of the most environmentally benign insecticides ineffective. Certain populations of moths and cotton pests have already acquired such resistance. To mitigate this risk, agricultural experts utilize integrated pest management strategies, such as planting "refuges"—plots of non-Bt crops near Bt fields—to ensure that a local population of insects remains susceptible to the toxin, thereby slowing the spread of resistance genes throughout the population.1