Insecticide Resistance vs Insect Tolerance and Susceptibility

by Pat Kelley Vice President

Insect Susceptibility: Although all insects can have resistant traits in their genes, with no previous exposure to an insecticide, nearly all of the population will generally be affected. Insect species that have not built up any genetic resistance to a particular insecticide are considered susceptible. Susceptible strains of insects will die at much lower dosages of an insecticide than resistant strains of that same insect. (Note: The USDA keeps active colonies of susceptible insect pests that were gathered in the 1950’s, prior to exposure to most insecticides, for the purposes of testing and possible reintroduction into the environment.)

Insecticide Resistance vs Insect Tolerance

Insect Tolerance: In order for us to understand insect tolerance let’s use an analogy in human terms of a respiratory and intestinal flu virus. Imagine a small hospital room with an elderly man in the bed who has many ailments. By his side is a young, healthy nurse who competes in triathlons during her free time. If a person sick with the flu walks into that room and coughs in their general direction, although they both may become sick, the elderly man is the more likely to pick up the virus because he has less cells to fight off the virus and he has a weaker immune system than the healthy nurse. The same is true of insects fighting off insecticides. Healthy, well fed insects can overcome exposure to an insecticide much better than insects that are stressed from drought or malnourishment. This is insect tolerance. Insect tolerance is not due to genetics, but is instead an inborn ability to survive a dose of insecticide without prior exposure. Tolerant insects may require a higher dose of insecticide before results are seen.

Insecticide Resistance: This has everything to do with genetics. After a dose of insecticide has been introduced into a large pest population, there is a small chance that a few individuals may have certain genes that assist them in overcoming the toxin. If all of the other insects in a population die from the exposure except the few that have these genes, then those few pass along the survival genes to all of the next generation. The susceptible insects are all gone and only the resistant ones survive. It can be thought of as an accelerated evolution. This typically occurs as a response to poor pest management practices over time. Resistant strains of insects require higher than normal doses of insecticide before they die. Eventually they may have no ill effects at all from a given insecticide.


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