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What Is Pesticide Resistance? Part I
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While discussing GMO crops, this column alluded to pesticide resistance as a potential problem with GMO traits such as glyphosate (Roundup ®) tolerance and resistance to the Bt trait found in crops such as corn and cotton. Last week, Barton County ANR Extension Agent shared a column on herbicide resistance from Extension Agronomy.  But what exactly is resistance? Not necessarily to just pesticides but also crop resistance to something like a disease. And maybe the more important question is how or can it be prevented?  Today’s column tackles what is pesticide resistance and how does it develop.  
If you search the internet there are many very good definitions of pesticide resistance, however, many are long and cumbersome. This definition from greenfacts.org website is simple, concise, and suits the purpose of this column. Pesticide resistance is the genetically acquired ability of an organism to survive a pesticide application at doses that once killed most individuals of the same species. In English, the treatment that once was effective in controlling the pest no longer does and it is due to a change in the genetics of the organism. So how does this genetic change happen that renders the pesticide ineffective?
The short answer is “natural selection.” While a Palmer amaranth is Palmer amaranth there are slight genetic differences. The same hold true for any pest. A pesticide has what is termed a “mode of action” which is simply how the pesticide kills the target pest. There are various modes of actions for pesticides that target different sites and/or systems of the pest. For example some insecticides target the nervous system, some interrupt growth and development, and others disrupt energy flow. Within these broad categories, different chemistries act in different ways and on different targets. The same holds true for herbicides with over ten different modes of action. So how does resistance occur?
When the pest population is exposed to a pesticide the population contains minor differences in their genetics, their DNA. The result is variation in the response of the pest population to the treatment. While almost all individuals may be killed quickly, some will die but a bit more slowly, some will experience the negative effects of the pesticide but survive, and a few members of the population will be essentially immune to the toxin. The ones that are affected but survive and the ones unaffected will go ahead and reproduce. So over time if the same mode of action is used exclusively, or almost exclusively, the treatment “selects” for those organisms resistant or unaffected by the pesticide while eliminating susceptible pests. Natural selection in combination with the treatment selects against susceptible organisms and for resistant ones since only the resistant organisms can reproduce.
Crop breeders have used this characteristic but in reverse when they breed for resistance to pests for crop plants.  Next week, how can resistance be prevented or once it occurs how can it be managed?