The good people of Puckett, MS are like any other biological population; they are not all the same. All the folks I’ve met in Puckett are the good friendly type, but they probably do have a few of that other sort there as well.
What does this have to do with insecticide resistance? Insect populations are not all the same either. Insects of the same species can differ from one another in many ways, and one is in their susceptibility to specific insecticides. When a new insecticide is first marketed it may be highly effective against a particular insect pest. But if there are a few insects in the population that are not controlled by that insecticide, these are the ones that survive and breed to produce the next generation. Over time this process of selection for insecticide resistance—killing the ones that are susceptible and leaving the ones that are resistant--can result in a population of insects that can no longer be controlled with that insecticide.
How is it that some insects have the ability to survive exposure to a new insecticide in the first place? If they have never been exposed to that insecticide before, how can they already have a mechanism that allows them to survive it now? They may never have been exposed to this particular toxic chemical, but their ancestors have been exposed to plenty of others.
Plants produce a large array of toxic chemicals for the specific purpose of preventing insects from eating them. This evolutionary arms race has been going on between plants and insects for millions of years. The plant develops secondary chemicals that make insects that feed on them sick, or repel them from feeding in the first place, and then some insects develop ways to overcome that defense. Butterfly milkweed contains compounds that are toxic to most insects, but Monarch butterfly caterpillars can eat these plants, filter out the toxins and store them in their bodies to use for their own defense. Because insects already have a wide range of mechanisms to cope with toxins produced by plants, they can adapt and use some of these mechanisms to cope with toxins produced by humans.
Insects develop resistance to insecticides in several ways. They can develop the ability to breakdown the toxic molecule inside their bodies before it does them harm, or there can be differences in the skin, or integument, of the insect that prevent them from absorbing the insecticide into their bodies. Some insects can adopt behavioral changes that keep them from being exposed to the insecticide. For example, cockroaches sometimes develop aversion to bait products. This happens when there is a small portion of the roach population that just does not like that bait and will not eat it. Some folks don’t like broccoli; insects have taste preferences too.
Keep in mind the development of resistance must happen genetically over time at the population level and it usually takes many generations for resistance to develop in a high enough portion of the population that the insecticide no longer works. This phenomenon of resistance is not restricted to insects and insecticides. Weeds develop resistance to herbicides; fungi develop resistance to fungicides; bacteria develop resistance to antibiotics; rats develop resistance to rodenticides; and so on.
Blake Layton, Extension Entomology Specialist, Mississippi State University Extension Service.
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