Which of the following is a method of pest monitoring?

Pest monitoring is a critical aspect of integrated pest management (IPM) that involves regularly checking for the presence and levels of pest populations to make informed decisions on how to manage them. Trapping is a widely recognized method used for this purpose. It allows for the capture and observation of pests, providing valuable data on their life cycle, population dynamics, and behavior. This information can help determine whether pest control measures are needed and guide the timing and method of any interventions.

The other options, while they may contribute to pest management, do not specifically pertain to monitoring. Soil treatment, for example, focuses on treating existing pest problems rather than assessing pest populations. Planting repellent species can serve as a preventative strategy but is not a method for monitoring pests. Similarly, applying pesticides preemptively does not involve the observation or assessment of pest activity and could result in unnecessary pesticide use. Therefore, trapping stands out as the method that aligns directly with the goals of pest monitoring.