Which of the following causes a disease to occur in pests, reducing the pest population?

The correct answer is the pathogen, as it refers to biological agents such as bacteria, viruses, fungi, or other microorganisms that can infect and cause disease in pests. When a pathogen infects a pest, it can lead to a decline in the pest population due to illness, which can affect their reproduction, behavior, and survival rate. This natural approach can help manage pest populations without the need for chemical interventions, making it an important aspect of integrated pest management.

The other options, while they do interact with pest populations, do not specifically cause diseases in pests. Pesticides are chemical substances designed to kill or inhibit pest organisms but do not induce disease. Predators naturally control pest populations through predation but do not cause disease. Environmental changes can affect pest habitats and populations, but they do not directly result in diseases within the pests themselves.