Which of the following is a method used to reduce pest populations biologically?

Introducing natural predators is a recognized method of biological control that effectively reduces pest populations. This approach involves bringing in organisms that naturally prey on the pests, which can help balance ecosystems and minimize pest outbreaks without relying on synthetic chemicals.

For example, ladybugs are often introduced to control aphid populations, as they feed on these pests. This method can lead to sustainable pest management by maintaining a healthier environment in which natural enemies can thrive, effectively reducing the need for chemical interventions.

The other choices—using chemical sprays, employing sticky traps, and applying granular pesticides—are methods of pest management that do not involve biological control. Chemical sprays and granular pesticides rely on synthetic substances to kill pests, while sticky traps are more of a physical control method that captures pests rather than establishing a natural predator-prey relationship. Each of these alternatives has its own risks and benefits but does not harness biological mechanisms in the same way that introducing natural predators does.