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Genetic engineering vs. evolution
April 2014

Corn rootworm larva

The western corn rootworm chews into a plant.

In the late 1990s, a new weapon in the fight against agricultural pests was introduced: Bt corn. The new maize variety was genetically engineered to carry genes from the bacterium Bacillus thurinigiensis (hence the moniker "Bt") that cause the crop to produce an all-natural pesticide. This meant that growers could get good yields from their cornfields without spraying on so many toxins. Since then, many farmers have jumped on this bandwagon. In 2012, more than 69 million hectares were planted with Bt crops — an area about the size of Texas! There has been much debate over the risks of this technological advance, but now it appears that the downfall of Bt corn might be the very problem that it was supposed to solve in the first place: agricultural pests, in particular the western corn rootworm. These beetle larvae eat the roots of corn plants potentially ruining the crop. In recent years, more and more larvae that are resistant to the effects of the Bt toxin have been showing up in fields and chewing their way into plants. How and why did this happen? It all comes down to evolution.

Where's the evolution?

The evolution of resistance by natural selection is a common problem with any pesticide — whether sprayed on or produced by the plant itself. The large population sizes of insect pests provide many possible chances for a mutation to arise that confers on the insect some level of resistance to the pesticide. The insect carrying that mutation is likely to leave behind more offspring than other insects. And since those offspring are also likely to be resistant, the mutation can spread quickly through the population.

However, when Bt corn was introduced to the market, it came with a planting strategy designed to significantly slow the evolution of resistance to Bt. The idea is based on the fact that resistance isn't all or nothing. Pests can be just a little resistant to a pesticide or a lot resistant to it. Resistance genes are most effective when the animal carries two copies of them (i.e., is homozygous) and not very effective against high dose pesticides when only one copy is present (i.e., in heterozygotes). To slow the evolution of resistance, farmers were directed to plant a field of regular corn vulnerable to the pests (i.e., a "refuge") alongside their Bt fields. This vulnerable field would support a large population of susceptible insects. Meanwhile, in the Bt fields, the only insects to survive would be the few insects that carry two copies of the resistant genes. When it comes time to mate, the few resistant beetles would be unlikely to find each other in a sea of other rootworm beetles and would most likely mate with one of the many vulnerable individuals around. The offspring of these matings would carry only one copy of the resistance gene and so would be only a little resistant — allowing them to be killed by Bt. By keeping the resistant genes "diluted" in a large pool of vulnerable genes, regulators hoped to slow the evolution of a large, resistant population.

The plan was a good one — but there were two problems. First, many farmers, as many as 25% of them, stopped complying with the refuge guidelines and either didn't plant fields with vulnerable corn or didn't devote the recommended number of fields to this crop. Second, the Bt plants were not producing a particularly high dose of Bt — at least not from the perspective of the hardy western corn rootworm. The low dose of pesticide made it easier for insects to survive with lower levels of resistance — and this contributed to the spread of resistance genes in the population. The more insects survive to reproduce after exposure to a pesticide, the more quickly full resistance will evolve and the more quickly the pest population will reach its usual size.

Such lapses seem to have resulted in the rapid evolution of pesticide resistance in the western corn rootworm. More and more Bt cornfields in Iowa, the center of the Corn Belt, are showing signs of rootworm damage. And in monitored fields planted with a new variety of Bt corn, it took just three or four years for resistant rootworms to show up. This closely matches lab experiments in which resistant rootworm strains evolve after just three generations of living on a Bt crop.

To try to beat the rootworm's march towards resistance, seed companies have produced corn varieties with slightly different versions of the Bt toxin. If a population of rootworms is resistant to Bt pesticide 1, then the farmer could just switch to corn engineered to produce Bt pesticide 2 — or so the reasoning goes. In practice, research has revealed that some of these toxins are so similar to one another that insects resistant to Bt pesticide 1 will also be at least partially resistant to Bt pesticide 2, giving them a big leg up in the evolution of full resistance. This phenomenon is known as cross-resistance.

So what's the solution? One approach is to use corn varieties engineered to produce "pesticide pyramids" — i.e., multiple pesticides. The reasoning is that, while mutations conferring resistance to one pesticide are not particularly rare, an insect carrying mutations conferring resistance to several different pesticides is quite rare. So long as the pesticides don't exhibit cross-resistance, this means that crops producing pesticide pyramids would kill virtually all of the pests that attack it. The smaller the pest population that survives each generation, the more slowly a fully resistant strain will evolve.

Seed companies are beginning to produce these high-tech strains, but researchers recommend that growers return to older methods of pest control, even if they choose to plant the new pesticide-pyramid seeds as well. A tried-and-true approach involves crop rotation — disrupting the insect's life cycle by changing which crops are planted in which fields each year. This strategy keeps rootworm populations in check because rootworm beetles tend to lay their eggs in cornfields. If a field is planted with corn one year and soybeans the next, the larvae that hatch after the first year will all starve to death since they can't survive on soybeans and since they can't move far enough to find a cornfield. Pesticides can be a helpful method of pest control — but if there's anything that the recent history of agriculture has taught us, it's that evolution has a tendency to find its way around our most advanced technologies. Integrating new technologies with older methods of pest control may be the best approach for keeping our agricultural yields high over evolutionary time periods — which, in the case of crop pests, may be just a few growing seasons!


Read more about it

Primary literature:

  • Gassmann, A. J., Petzold-Maxwell, J. L., Clifton, E. H., Dunbar, M. W., Hoffmann, A. M., Ingber, D. A., and Keweshan, R. S., (2014). Field-evolved resistance by western corn rootworm to multiple Bacillus thuringiensis toxins in transgenic maize. Proceedings of the National Academy of Sciences USA. doi: 10.1073/pnas.1317179111
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News articles:

Understanding Evolution resources:

Discussion and extension questions

  1. Review the process of natural selection. Use the four steps described on that page to explain how Bt resistance became so common among rootworms feeding on Bt cornfields.
  2. Mutations are random in the sense that which mutations occur cannot be predicted based on which traits would be advantageous for the organism. Explain what this means with respect to the Bt-resistance mutations in the western corn rootworm.
  3. In your own words, explain how the presence of refugia can help slow the evolution of resistance to Bt.
  4. In your own words, explain how pesticide pyramids can help slow the evolution of resistance to Bt.
  5. Advanced: Imagine that Bt resistance is a dominant trait (instead of an incompletely dominant or recessive one) and that carrying a single mutation confers full resistance. In that situation, would refugia be a helpful strategy to help slow the evolution of resistance? Explain why or why not.
  6. Advanced: Imagine that Bt resistance is a dominant trait (instead of an incompletely dominant or recessive one) and that carrying a single mutation confers full resistance. In that situation, would pesticide pyramids be a helpful strategy to help slow the evolution of resistance? Explain why or why not.
  7. Advanced: Do you think it is possible for "resistance" to crop rotation to evolve — i.e., for traits to evolve that help the rootworms survive in areas where crops are rotated each season? Explain your reasoning, and if you answered yes, explain how such mutations might operate.



Related lessons and teaching resources



References

  • Gassmann, A. J., Petzold-Maxwell, J. L., Clifton, E. H., Dunbar, M. W., Hoffmann, A. M., Ingber, D. A., and Keweshan, R. S., (2014). Field-evolved resistance by western corn rootworm to multiple Bacillus thuringiensis toxins in transgenic maize. Proceedings of the National Academy of Sciences USA. doi: 10.1073/pnas.1317179111
  • Owens, B. (2014). Pests worm their way into genetically modified maize. Nature. doi:10.1038/nature.2014.14887
  • Pollack, A. (Nov 5, 2009). Rules on modified corn skirted, study says. The New York Times. Retrieved April 1, 2014 from The New York Times http://www.nytimes.com/2009/11/06/business/06corn.html


Photo by Marlin E. Rice, original source: http://www.plantmanagementnetwork.org/elements/view.aspx?ID=1928



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