Which of the following is a biotic factor causing storage losses in grains?

To determine which factor is a biotic factor causing storage losses in grains, we need to understand the distinction between biotic and abiotic components in agricultural storage:

1. Biotic Factors: These refer to the living components of an ecosystem or environment that influence other organisms or agricultural produce. In the context of grain storage, biotic factors include living organisms that feed on, damage, or contaminate the grains. Examples include insects, rodents, birds, fungi, bacteria, and mites.
2. Abiotic Factors: These are the non-living physical and chemical elements of the environment. In grain storage, abiotic factors refer to environmental conditions that can deteriorate grain quality if not controlled. Examples include temperature, moisture content, humidity, and sunlight.

Let us evaluate each of the given options based on these definitions:
- Sunlight: This is a physical environmental factor, which is non-living and therefore abiotic.
- Temperature: This is a physical state of the environment, which is non-living and therefore abiotic.
- Moisture: Water content or humidity is a physical variable, which is non-living and therefore abiotic.
- Insects: Insects are living organisms that directly feed on grains, causing physical damage, weight loss, and quality degradation. Because they are living, they are classified as a biotic factor.

Thus, among the choices provided, insects are the biotic factor responsible for storage losses in grains.