Final answer:
Chemical repellents applied in breeding areas are the least effective method for eliminating insect breeding sites since they do not address the root problem. Other methods like sanitation, removing stagnant water, and sealing cracks are more direct in eliminating breeding grounds for insects.
Step-by-step explanation:
In answering the question "Which of the following methods is least effective in eliminating places where insects breed?", the least effective method mentioned is (c) Chemical repellents applied in breeding areas. While chemical repellents might deter insects temporarily, they do not eliminate the breeding sites themselves. In comparison, regular sanitation and cleaning (a), removing stagnant water (b), and sealing entry points and cracks (d) all serve to directly remove or block the opportunities for insects to breed effectively.
Mechanical and physical control methods, such as sanitation, can effectively limit insect populations by removing the environmental support they need to survive. Biological control, which may include introducing species that prey upon pests, also serves as an effective way to manage pest populations while protecting surroundings and lowering costs. Long-term reliance on chemical control, however, can lead to the evolution of resistance in target species, increased costs, and potential non-target impacts, making them less desirable as a primary method of pest control.