Refuges of Genetic Variation: Controlling Crop Pest Evolution (2 of 2)

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Is there any way that we can slow the spread of resistant genes? Evolutionary theory points to an answer: we can provide havens for non-resistant insects (and their non-resistant genes!). These havens are called refugia—they are fields without pesticides (sprayed or plant-produced) located near fields planted with pesticide-producing crops.

The diagram below illustrates how refugia slow down the evolution of pesticide-resistant pests by allowing non-resistant pest strains to survive.

Effect of refugia on pesticide efficacy

Refugia may be particularly important in slowing the spread of insects resistant to the pesticide Bt (produced by a gene in the bacterium Bacillus thuringiensis). As one of the few pesticides used by organic farmers, Bt kills a small subset of insects and does not harm many beneficial organisms. However, resistance to this pesticide has become an imminent threat as corn and cotton genetically engineered to produce their own Bt fill more and more fields. These genetically engineered crops increase the selective pressure for Bt resistance on insect populations. If Bt resistant insects become common, organic farmers will have lost one of the few pesticides they are able to use.

Refugia slow the evolution of widespread Bt resistance by providing havens in which the non-resistant insects survive. The allele for Bt resistance happens to be recessive—that means that the resistant allele can be masked by the dominant non-resistant allele. So if a resistant insect (rr) surviving in the Bt-producing field mates with a non-resistant insect (RR) surviving in the refuge, all of their offspring will be non-resistant (Rr).

Offspring of homozygous pair

When two heterozygous pests mate, only one in four offspring (on average) will be homozygous recessive (rr) and therefore resistant to the pesticide.

Offspring of heterozygous pair

By keeping refuges for the non-resistant alleles, we can prevent many of the resistant alleles from being expressed. More insects will be vulnerable to Bt and the spread of the resistant allele will slow.

Only by understanding evolutionary theory and by recognizing how our short-term solutions are likely to affect evolving insect populations can we figure out ways to control the evolution of resistance.





Read more about refugia from the Cooperative Extension Service, Kansas State University.

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