Consider briefly the process of evolution and you might imagine a lumbering process, splitting lineages and bringing new species forth from old, or the gradual formation of morphological novelties like wings. While it’s true that evolutionary processes such as the formation of new species are generally slow by our standards, other effects of evolution that cannot be seen with the naked eye (but are no less important) can often develop over the course of a human lifetime.
In only a few decades, we have seen the proliferation of pesticide-resistant strains of a number of important crop pests through the differential reproduction of individuals bearing molecular traits that allow them to avoid succumbing to pesticides. Organisms, in this case crop pests, have some variable number of offspring each generation that can survive and reproduce themselves. If a lineage of crop pests has a greater number of viable offspring because it is better able to withstand the pesticides that killed other insects, its descendants will come to represent a larger portion of the population. The population of pests will then have, in aggregate, more of the attributes of the successful lineages, including those that lent increased reproductive success–in this case, pesticide resistance. This is the essence of evolution by natural selection.
It is easy to see why this is a subject of ongoing research. Agricultural pests are responsible for billions of dollars a year in crop damages, totaling 13% of the world’s crop production, and disease-carrying insects, such as mosquitoes, present ongoing challenges to human health. While developing alternative pest-control strategies is an active area for research, our primary means of controlling insect pests is still through the proper use of chemical pesticides. So then, what do we do when evolution is making our main methods for controlling insect populations less effective by the year, and what constitutes proper pesticide application?
One way we have tried to curb the success of resistance evolution is to limit the continuous exposure of insects to the same pesticides. In theory, this strategy of rotation prevents pests from continuously evolving resistance, thus prolonging the usefulness of insecticides. Or so the conventional wisdom goes. This is where Wade Lab researcher Dylan Siniard comes in. Siniard’s work uses techniques from quantitative genetics, which is essentially the measurement and characterization of traits that are infinitely variable, say height or weight in humans. Although pesticide resistance sounds like the kind of trait that an insect either has or doesn’t have, there is a lot of variability in the types and effectiveness of these adaptations. Using considerable sample sizes, Siniard characterized the susceptibility of six populations of the flour beetle, Tribolium castaneum, to a variety of pesticides. Then, using those data, he assessed how much variation in the resistance trait was present in each of the populations .
Finally, he tested each population to see if there were genetic correlations between the effects of different pesticides. Certain pesticides were found to have genetically correlated susceptibilities. This means that pests immune to certain pesticides were found to also be immune to others and vice-versa. Using the data from these assays, Dylan was able to model the evolutionary effects of using single pesticides and mixed-application techniques. Surprisingly, rotation was not always the best strategy: with some pesticides, rotating genetically positively correlated pesticides causes resistance to evolve even faster than it would without rotation. Rotating negatively correlated pesticides, however, greatly retarded evolution of pesticide resistance.
Dylan’s work is illuminating for a few reasons beyond the practical application of its findings. Nature doesn’t confine itself to the boundaries of our understanding, and this is an important reminder of this. While the conventional wisdom that rotation is good was partly supported by this work, it is only true to a point. In some cases, alternating years between spraying and not spraying prolonged susceptibility better than rotation of certain combinations of agents. Finally, this is but one example of a practical application of evolution and its study to a pressing economic problem, showing the importance of basic research in solving the problems of applied science.
Edited by Victoria Kohout and Clara Boothby