Mosquitoes, insects and other pests (such as birds, frogs, snakes) are undesirable elements to have in a rainwater harvesting system and must be kept out of all pipe systems that lead to the water storage vessel and the vessel itself.

Mosquitoes are known to be responsible for the spread of many diseases including Dengue Fever and Ross River Fever. Mosquitoes breed in water and all breeding habitats must be eliminated.

Legislation broadly says that where pipes hold water (in "wet" systems) they must be screened with a non-corrosive screen of not more than 1mm aperture to prevent the entry of mosquitoes and vermin.