Refuges of genetic variation: controlling crop pest evolution
Pests evolve resistance to our pesticides at an alarming rate. However, evolutionary theory tells us how we can slow the rate at which genes for pesticide resistance spread: by providing refuges where non-resistant insects thrive.
Insects persistently nibble away at crops in the fields and at the narrow profit margin they offer. What's a farmer to do? Spray, of course. Or, plant crops that have been genetically engineered to produce their own pesticides.
But evolutionary theory tells us that these solutions will not work indefinitely. Pest insects have short generation times and large population sizes which means that they evolve quickly. If pesticides are widely applied, or if fields are widely planted with pesticide-producing plants, insects resistant to the pesticide will evolve. Some degree of resistance has been documented for every major class of insecticide used in agriculture.1
The diagram below illustrates how refugia slow down the evolution of pesticide-resistant pests by allowing non-resistant pest strains to survive.
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).
When two heterozygous pests mate, only one in four offspring (on average) will be homozygous recessive (rr) and therefore resistant to the pesticide.
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.
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1 Committee on Strategies for the Management of Pesticide Resistant Pest Populations (1986). Pesticide Resistance Strategies and Tactics for Management. National Academy Press, Washington DC.
Pesticide photo courtesy of USDA, photo by Jeff Vanuga; Potato beetle photo courtesy of USDA, photo by Scott Bauer;
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