Question

1. Explain in a list of steps how the development of pesticide resistance in agricultural pests

(weeds, for example) occurs. List all the steps in the process, showing how this is an example of

evolution by natural selection. Be as thorough and detailed as possible.

2.Briefly propose a policy or a practice that would reduce the problem of pesticide resistance (and still

allow the use of pesticides).

3. Briefly (in 1-3 sentences) explain in biological evolutionary terms, why the policy or practice you

proposed would reduce pesticide resistance. (That is, the question is not asking about social or cultural

aspects of the policy's effectiveness).

Answer 1

Development of pesticide resistance - An evolutionary perspective:

Given the burgeoning global population, food security becomes an utmost priority to all nations. Pests pose an intrinsic impediment in fulfilling the nutritional need of the population. Hence, pesticides were developed worldwide to mitigate crop losses and damage caused by them. However,  the effectiveness of pesticides is threatened by the evolution of resistant pathogens, weeds and insect pests. Resistance to pesticides is an interesting case of evolutionary selective pressure on the pests.In other words, it is selective adaptation of these pests to the insecticides.

Repeated use of the same class of pesticides to control a pest can cause undesirable changes in the gene pool of a pest/weed leading to another form of artificial selection, pesticide resistance. When a pesticide is first used, a small proportion of the pest/weed 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. In response to resistance, farmers may increase pesticide quantities/frequency, which exacerbates the problem. Hence the pest/weed evolves in response to the selective pressure on its population - the pesticide.

Moreover, de novo mutations in the pest/weed population can also confer selective advantage against the pesticide that is applied. In other cases, a small number of individuals in the population may already possess the genetic robustness to overcome the pesticide toxicity. Hence, the genes of such member of the population are passed on down generations, and hence at some point the entire population becomes resistant to a particular pesticide.

Methods to mitigate pesticide resistance:

• Rational and minimal use of pestides. Utilization of pesticides only when there is utmost need.
• Integrated Pest management techniques
• Use of natural organic Biopesticides
• Rotating different classes of pesticides in different seasons, and minimal use of them.
• Rotation of crops
• Using grazing animals in some cases to reduce (some) weed populations.

Rotating different classes of pesticides in different seasons, and minimal use of them:

Different class of pesticides target different survival/evolutionary traits of the pest. Hence, this would definitely be more effective in controlling the pest as well as mitigating pesticide resistance. The use of a single pesticide over the years would just put a unidirectional selective pressure, which the pest can relatively easily come out of. Whereas, judicious use of different class of pesticides over differerent seasons/years, would definitely keep the evolutionary pressure high enough for the population to overcome it.