Insecticide resistance most commonly arises under which condition?

Insecticide resistance most commonly arises when there are multiple generations of insects. This is primarily because the process of natural selection allows resistant individuals to survive and reproduce over time. When a pesticide is applied, it may kill the susceptible insects, but individuals with genetic mutations that confer resistance are more likely to survive. These resistant insects will then reproduce, passing on their resistance traits to the next generation.

As each generation is exposed to the insecticide, if the insect population has a history of exposure and there is a strong selection pressure from the chemicals, the proportion of resistant individuals can increase significantly. This repeated exposure creates an environment where resistance can develop and strengthen.

In contrast, practices such as spraying only when needed or alternating chemical classes can help manage and delay resistance by reducing the overall selection pressure on insect populations. Applying insecticide to all leaf surfaces, while it may increase efficacy in the short term, does not inherently contribute to resistance development as much as the ongoing generations of insects faced with the same treatment. Therefore, the frequency of generations interacting with pesticides is a critical factor in the evolution of insecticide resistance.