The graph below shows the effect of the number of insects on the crops when an insecticide is used in each of the five years. Explain why the percentage of insects began to rise in the second year until almost reaching its original level.

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. Insect populations develop resistance to an insecticide through which a few mutants in the population may tolerate the chemical better, while the susceptible insects succumb to it. Pest species had been exposed to natural toxins long before agriculture began. For example, many plants produce phytotoxins to protect them from herbivores. As a result, co-evolution of herbivores and their host plants required development of the physiological capability to detoxify or tolerate poisons. Adaptation to pesticides comes with an evolutionary cost, usually decreasing relative fitness of organisms in the absence of pesticides. Resistant individuals often have reduced reproductive output, life expectancy, mobility, etc. Non-resistant individuals sometimes grow in frequency in the absence of pesticides Hence, the number of insects will rise in the second year as the insects slowly become resistant to the pesticide.