At issue is whether insects *change* in response to pesticides or if
the resistant individuals are already there. Entomologists seem to
agree they are already there "when a pesticide is first used".

> Repeated use of the same class of pesticides to control a pest can cause undesirable changes in the gene pool of a pest leading to another form of artificial selection, pesticide resistance. *When a pesticide is first used*, a small proportion of the pest population may survive exposure to the material due to their distinct genetic makeup. These individuals pass along the genes for resistance to the next generation. Subsequent uses of the pesticide increase the proportion of less-susceptible individuals in the population. Through this process of selection, the population gradually develops resistance to the pesticide. Worldwide, more than 500 species of insects, mites, and spiders have developed some level of pesticide resistance. The twospotted spider mite is a pest of most fruit crops and is notorious for rapidly developing resistance to miticides.

> Selection for resistance can occur if a small proportion of the insect population is able to survive treatment with insecticide. These rare resistant individuals can reproduce and pass on their resistance to the offspring. If an insecticide with the same mode of action is repeatedly used against this population, an even greater proportion will survive. Ultimately, the once-effective product no longer controls the resistant population.

Excerpt from "Fruit Crop Ecology and Management", Chapter 2: Managing
the Community of Pests and Beneficials by Larry Gut, Annemiek
Schilder, Rufus Isaacs and Patricia McManus.

* * *

> How Do Pests Become Resistant?
> Pest population is exposed to pesticide
> Some members of the pest population survive because of a genetic predisposition to be resistant to pesticide
> Surviving members of the pest population pass along the genetic resistant to their offspring

Pesticide Resistance
Stephen J. Toth, Jr.			Wayne G. Buhler
Department of Entomology		Department of Horticultural Science
North Carolina State University	North Carolina State University

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