How Do Insects Become Resistant to Insecticides?

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There are now insects resistant to every synthetic pesticide that is used. In a given insect population there may be a few individuals that carry genes for resistance to the insecticide under consideration, having evolved before that insecticide has been applied, having arisen from a random mutation in the past. In the natural environment these insects having this mutation will be compromised, being weaker and producing fewer progeny.

Use of the considered insecticide results in the preferential survival and breeding of the mutants and the  fatality or failure to thrive relative to the mutant individuals. Subsequent application results in a population shift to a higher mutant to native ratio. Repeated applications of insecticide result in selection for the resistant individuals who pass on resistance to their offspring. Ultimately, the insecticide that had been effective at reducing the insect population size, no longer controls the now resistant population.

Resistance probably results from a combination of several factors:

  • Many species produce large broods, increasing the probability of of mutations and ensures the rapid expansion of resistant populations.
  • Humans often rely on insecticides exclusively for insect control. This increases selection pressure resulting in resistance that has no other component to control the resistant population.
  • Insecticides that fail to degrade quickly, contribute to selection for resistant strains even after they are no longer being applied.
  • In response to resistance, managers may increase dosage and frequency, each of which exacerbates the development of resistance.
  • Some insecticides are toxic toward species that feed on or compete with the target insect.This allows the insect population to expand, often causing more insecticide application, often called a "pesticide treadmill".
  • Insect predators and parasites generally have smaller populations and are less likely to evolve resistance than are the insecticide's primary target such as mosquitoes.
  • Insects with limited diets are more likely to evolve resistance because they ar exposed to higher insecticide concentrations.
  • Insects with shorter life cycles develop resistance more quickly than others.


Resistance to insecticides was first documented  by A.L. Melander in 1914 when scale-causing insects demonstrated resistance to an inorganic insecticide, a copper/lime combination. From 1914 to 1946 an additional 11 cases were recorded.

The development of organic insecticides starting with DDT gave hope that insecticide resistance was a "dead issue". However, by 1947 housefly resistance to DDT had evolved. With the introduction of every new insecticide class (cyclodienes, carbamates, formamidines, organophosphate, pyrethroids, and even Bacillus thurigensis) more cases of resistance surfaced within 2 to 20 years. In the US, studies have shown Drosophila melanogaster infesting orange groves were becoming resistant to malathion. The diamond-back moth evolved resistance to Bacillus thurigensis about 3 years after its introduction. The Colorado potato beetle has evolved resistance to 52 different compounds belonging to all major classes of insecticides.

https://en.m.wikipedia.org/wiki/Drosophila_melanogaster

 Diamond-back moth  en.wikipedia.org


Colorado Potato Beetle  Entymology at the University of Kentucky.

In the 1940's crop losses were at 7%, rising to 13% in the 1980's and 90's even though more insecticide was being used. Although the evolution of resistance is usually considered to be a result of increased pesticide use, insect populations can also adapt to (develop resistance to) non-chemical means of control. An example is the northern corn rootworm (Diabrotica barberi) that became adapted to the corn-soybean crop rotation by spending the year when the field is planted with soybeans in diapause (responding to the absence of corn roots available as an adverse environmental condition.

Corn Rootworm Larva  http://ento.psu.edu/extension/factsheets/corn-rootworm

http://ento.psu.edu/extension/factsheets/corn-rootworm

References:

Bacillus thurigensis, University of California San Diego

  1.  Miller GT (2004), Sustaining the Earth, 6th edition. Thompson Learning, Inc. Pacific Grove, California. Chapter 9, Pages 211-216.
  2.  Levine, E; Oloumi-Sadeghi, H; Fisher, JR (1992). "Discovery of multiyear diapause in Illinois and South Dakota Northern corn rootworm (Coleoptera: Cerambycidae) eggs and incidence of the prolonged diapause trait in Illinois". Journal of Economic Entomology. 85: 262–267. doi:10.1093/jee/85.1.262

Comments

  1. AnonymousMay 22, 2017 at 12:28 PM

    Very intriguing to learn that insects develop resistance in ways other than using pesticides!

    ReplyDelete
  2. AnonymousMay 22, 2017 at 12:28 PM

    This comment has been removed by the author.

    ReplyDelete

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