In agriculture, one of the things we worry about the most is pesticide resistance. When a pest like an insect, weed, or disease is no longer effectively controlled by a pesticide, growers may be left with few – if any – alternatives. The concept of Integrated Pest Management (IPM), which takes a systems approach to pest management by creating environments that are hostile to pests and beneficial to crops, began as a reaction to the loss of efficacy of pesticides developed after World War II. But why do certain pesticides stop working? What’s the harm in spraying the same pesticide over and over if it worked well the last time? The reason goes all the way back to a fundamental of biological science: evolution by way of natural selection.
Charles Darwin (1809 – 1882) was an English naturalist, geologist, and biologist. In the 1800s, people knew that if you selected for certain traits in an animal, you could increase the chances that those traits would be passed to their offspring and become dominant traits. After all, people had been breeding different kinds of dogs this way for hundreds of years, and dog breeding was very popular in England. What they didn’t understand is how something like that could happen in the wild without human intervention. Leopards and lions look about as similar as Dobermans and dachshunds, but nobody had spent years in Africa and South America trying to give lions spots. Charles Darwin, along with Alfred Wallace, proposed a solution to this question that they called “natural selection.”
How to build a better leopard
To understand natural selection, you first need to know about DNA. DNA contains the instructions to make any complex form of life. During development, DNA gets copied a lot. Sometimes, mistakes get made. If a major error occurs, it could have a drastic effect on the final product. These major errors result in mutations. Mutations come about by random chance. Depending on the environment where the animal lives, these mutations may be helpful or harmful to the animal’s survival. If the mutation increases the animal’s likelihood that it will survive long enough to reproduce, they can pass that mutation to its offspring. If the mutation helps the family line survive and reproduce better than other members of its species that don’t have the mutation, then the mutation becomes a dominant trait. This process, where an animal’s environment selects which genetic traits will become dominant in time and which will be pruned, is called natural selection. This is often summarized as “survival of the fittest.” In this case, “fittest” doesn’t mean “strongest” but instead means whichever individual “fits” its environment the best. Mutation is random; natural selection is not.
Leopard spots work as a form of camouflage by breaking up the leopard’s shape when viewed through foliage. Now imagine a time long ago when leopards didn’t have spots until a genetic mutation caused a leopard to be born with something resembling spots. It probably didn’t look like leopard spots today, but it must’ve made them better than other leopards at hiding from prey. Because they are better able to compete than other leopards for food, they are more likely to be healthy, defend their territory from rivals, and live long enough to reproduce and give these traits to its offspring. In time, the spotted leopards outcompete the unspotted leopards until all leopards have spots.
What does this have to do with pesticides?
Insects, fungi, and bacteria are just as influenced by the forces of mutation and selection as anything else. Let’s focus on insects. When you spray an insecticide, that product is designed to kill the target pest in a particular way. It may be a neurotoxin, or disrupt the insect’s reproductive system, or it may kill by some other manner. This method of control is called the Mode of Action. Most members of a group of insects will by successfully controlled by the mode of action. However, in any given population of insects, mutations may occur. Many of these mutations will have zero effect on how well the mode of action of a pesticide will affect the insect. However, over time, the chances increase that a mutation will occur that does make the insect less susceptible, or even resistant, to the pesticide’s mode of action.
Should this occur, spraying the insecticide will kill most of the insects but will not affect the resistant insect. In the example image above, the red color indicates an insect that is resistant to the applied pesticide’s mode of action. After the spray, most of the insects are successfully controlled but the red insect is not. It now has dramatically reduced competition for mates and is more likely to be able to pass down its genetic resistance to its offspring. Over time, the resistant insect becomes the dominant population and the pesticide is now effectively useless. Unintentionally, the grower has been selecting for the mutated insect to thrive.
So how do we break this cycle? Remember that beneficial genetic mutations are relatively rare occurrences. It is statistically unlikely that any individual insect will be randomly born resistant to a mode of action, and it is even more unlikely that any individual insect will be born resistant to multiple modes of action. If we spray a product with one mode of action, we will kill the insects that are not resistant and leave the ones that are. However, if we then rotate to a pesticide with a different mode of action, we will be able to finish the job since it is highly unlikely that the insect population will be resistant to both.