The effectiveness of DDT insecticide as a residual spray is due to it being a:

The effectiveness of DDT insecticide as a residual spray is primarily attributed to its nature as a contact poison. Contact poisons work by adhering to the surface where they are applied and impacting insects through direct contact. DDT's design allows it to remain on surfaces for extended periods, meaning it continues to kill pests that come into contact with treated areas long after the initial application. This characteristic is crucial for managing insect populations effectively, particularly in situations where insects may not ingest the poison directly but may encounter it on surfaces such as walls, floors, and vegetation.

In contrast, systemic poisons are absorbed by the insect and may not be effective as a residual spray, as they require ingestion rather than contact. Repellents function to deter insects rather than eliminate them, which does not align with the goal of a residual insecticide like DDT. Ingestion poisons depend on insects consuming the chemical, which also doesn't apply to the intended use of DDT as a surface treatment. Thus, DDT works best as a contact poison, making it effective in long-term pest control strategies.