Answer:
pests can evolve defenses against pesticides over time
Step-by-step explanation:
Pesticides are chemicals employed to kill pests generally. With continuous and indiscriminate use of pesticides, they become less effective because pests population often evolve a defense mechanism against them.
With each usage, more and more pest population survive. Those that survive gradually multiply and pass the survival gene to their offspring eventually giving rise to a population that is genetically insusceptible to the effects of the pesticide. This is generally referred to as evolution by natural selection.