Pesticide residues, if present in air, soil and water can pose a serious threat to biological diversity and human health. After depositing in the environment, the pesticides start to break down and forms metabolites that are more or less toxic.

Answer: Residual insecticides remain effective where they are applied for some length of time. The length of time depends on the formulation (dust, liquid, etc.), the type of surface (soil, brick, wood, etc.) and the condition of the surface (wet, greasy, etc.). Termiticides are examples of insecticides with very long residual.