Many insects are attracted to excreta because they provide rich organic material and water, both of which are essential for the insects' development. The most important groups from a health point of view are mosquitos, houseflies, blowflies and cockroaches.
Some mosquitos, particularly Culex pipiens and some species of Anopheles, breed in polluted water, including that found in some pit latrines. Unlike flies, mosquitos are not deterred by low light levels, so keeping excreta in a dark place does not prevent them from breeding. Possible solutions are to keep the pit fully sealed or to cover the surface of the liquid with a film that prevents mosquito larvae from breathing. Oil and proprietary chemicals have been used effectively but may contaminate the groundwater. An alternative is to use small plastic balls that float on the surface producing a mechanical cover to the liquid. Fortunately many latrines only have a free water surface for a short period immediately after starting up or emptying. After that a layer of scum forms on the water surface preventing further mosquito breeding.
Horseflies and blowflies
These are medium- to large-sized flies that are attracted to human food as well as faeces and refuse. The three larval stages are found in excreta or mixtures of excreta and decaying vegetable matter. Solid, moist and fermenting material is most suitable for the breeding of houseflies, but the larvae of the blowfly prefer more liquid faeces and may liquefy masses of faecal material (Feachem et al., 1983). Open pit latrines are ideal breeding places.
Flies use both sight and smell to find food. This is an important consideration when designing latrines since not only must excreta be stored in a dark place but any ventilation holes must be screened.
Cockroaches are attracted to latrines by the moisture and organic matter; they are then likely to transmit disease by carrying pathogenic organisms on their bodies. Provided that a site has a continuous food supply the cockroaches tend to remain where they are. Accordingly latrines should be sited as far as possible from where food is stored and prepared, to prevent migration of cockroaches from one to the other.
Rats look upon excreta as a food source. If they come in contact with excreta and then with food intended for human consumption there is a possibility of their transmitting disease. In Nepal, there has been a problem with rats burrowing into double-pit latrines through the holes left in the pit walls. Not only does this create a possible transmission route for disease but the rats deposit large volumes of soil in the pit which rapidly fill it. A full lining of the top 0.5 - 1.0 m of any pit should prevent rats from entering.