Which of the following agents is responsible for causing diseases that reduce pest populations?

The correct choice is the pathogen, as it refers to bacteria, fungi, viruses, and other microorganisms that can invade and cause diseases in pest populations, effectively reducing their numbers. Pathogens can disrupt the biological functions of pests, leading to mortality or impaired reproduction, helping to manage pest populations naturally.

For example, certain fungi can infect insects, eventually leading to their death while also potentially providing further benefits to crop health by limiting pest outbreaks. Biological control methods often utilize pathogens as a means of pest management, allowing for a more environmentally sustainable approach compared to chemical pesticides.

In contrast, insecticides are specifically designed to kill insects directly. While they can effectively reduce pest populations, they do not cause diseases in the same way that pathogens do. Nematodes can affect pests, but they are typically used in a different context, such as biological control agents rather than inducing disease. Herbicides target unwanted plant species rather than pests, meaning they do not apply to this context. Therefore, pathogens are the agents responsible for causing diseases that lead to a natural reduction in pest populations.