Why Insect Populations Develop Resistance to an Insecticide

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Resistance in an insect population develops by the selection process. At first, only a very small portion of the population survives exposure to an insecticide. That new smaller population grows. It has a considerably higher portion of resistant individuals. When this population is exposed a second time to this pesticide in a second application, the same selection process occurs with a yet higher proportion of resistant individuals.

This process can be summarized this way. The degree to which resistance develops in an insect population depends upon both the proportion of the population that is already resistant and the strength of the resistance in each resistant insect. So, an insect population that has only a few individuals with a strong resistance to an insecticide  will selectively become a more highly resistant population by repeated applications of that pesticide.

The rate that insect resistance develops in a population is affected by how rapidly they reproduce, how highly resistant some individuals are, and their migration range.  The rate, timing and number of insecticide applications also affects the development of resistance in an insect population.

It is not the individual insect that changes when an insecticide is applied, but the population. So why have individual insects developed resistance?
  1. Some insects detoxify,destroy, or excrete the insecticide faster than others. Higher levels of enzymes to accomplish these tasks can be accomplished by regulation of the control of the genes or anabolic pathways, increasing the amount of these enzymes produced.   This is the most common way resistance occurs.
  2. Some insects may modify the binding site of the toxin resulting in the insecticide's effects are reduced. This is the second most common mechanism of resistance.
  3. Resistant insects may detect or recognize a danger and avoid the insecticide, leave the area or stop feeding, move to the underside of the sprayed leaves or move deeper into the crop foliage where the spray did not hit.
  4. They may absorb the insecticide more slowly than the susceptible insects because their cuticle was thicker or responded by growing thicker.
In nearly half the recorded case of resistant insects, they became resistant to two to five different chemical classes of chemicals.
  • Cross resistance is conferred by a single major genetic factor that differs between resistant and susceptible insects toward more than one chemical. Usually other chemical classes are also less toxic when this occurs.
  • Multiple resistance occurs when two resistance mechanisms are acquired independently from exposure to two different insecticides. Two separate genetic or gene-expression controls occur. 
The stability of the resistance observed can persist or decrease, depending on the frequency the that the insecticide is used. 
Managing Pesticide Resistance, Orchard Pest Management Online, Tree Fruit Research & Extension Center, Washington State University

The attached Video is an attempt to explain the development of resistance to insecticides directed toward the farmer or orchardist. Not only is it overly intricate, it places the advice within the framework of Integrated Pest Management, making the recommendations in the video exceedingly difficult to follow. For guidance on insecticide application, growers are best directed to the study materials for Certification in Integrated Pest Management, which based on sound science and requires a number of days or weeks study followed by a qualifying examination.

 Video: Mode of Action of Insecticides

In the literature cited below, all of the science regarding insect and insect population development of resistance to insecticides is presented from the perspective of pest management rather than from that of the biology of the insects undergoing development of resistance to insecticides.

References:

Wayne Buhler, Pesticide Environmental Stewardship, North Carolina State University Co-Operative Extension

Insecticide Resistance: Causes and Action,Mode of Action Initiative, USDA Southern Region, Integrated Management Center and the Insecticide Resistance Action Committee.

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