Environmental manipulation refers to changes made to the environment that have to be repeated and are often timed to correspond with the vector breeding cycle. This includes clearing water plants, flushing streams, and causing fluctuations in the water-level of potential breeding sites.
When choosing appropriate techniques for environmental manipulation, it is important to consider whether the vector breeding grounds are created by agriculture, waste water, irrigation, etc and the resources available (in terms of community support and manpower, and financial resources). Community members can be easily involved in environmental manipulation, as many of the methods available are easily done with resources (shovels, machetes, etc) that are already on-hand.
Changing microhabitats within standing bodies of water
Some vector species prefer to breed in shaded water, some in sunlit water. It is important to know the predominant vector species in a particular area before manipulating the environment so that no new breeding sites are created. In the case of malaria vectors, for example, that prefer shaded cooler water, clearing the aquatic vegetation that grows in or near the water will prevent vectors from breeding there. An increase in sunlight can also cause faster water evaporation, meaning that potential breeding sites will dry up quicker. Conversely, vectors that prefer to breed in warm sunlit water will be deterred if shrubs or trees are planted that provide shade over the water.
Vegetation that grows inside bodies of water provides habitat for early developmental stages of vectors. Some species, like Mansonia, use these plants for oxygen, while others use them to hide from predators such as larvivorous fish. In smaller breeding sites these water plants can be cleared by hand with simple equipment; for larger bodies of water herbicides or herbivorous fish can be used to destroy the plants.
This can be employed to clear away eggs, larvae, pupae or to change water salinity. Vectors, such as mosquitoes, breed on the edges of small, slowly moving streams, and a large gush of water can dislodge the developing stages of vectors, removing them from their habitat. Some vectors breed in areas close to seawater, such as marshes or lagoons; an increase in salinity will make the area unfavorable to that vector species. Flushing to change salinity is accomplished by letting in additional seawater.
Flushing streams and other bodies of water requires construction of a dam that can be opened to provide an amount of water large enough to wash away the developing vectors. Dam construction can be costly, meaning that more initial financial resources are required; however, once the dam in built, minimal maintenance or labor may be necessary for its upkeep.
Similarly to flushing streams, fluctuations in water level can wash way vectors in early stages of development. If the water level is decreased, this fluctuation can also strand larvae at the edge of a body of water. Constant fluctuations in water-level also make it difficult for plants to grow along the margins of a body of water. Fluctuations should be done regularly, at intervals between seven to ten days so that the larvae do not have a chance to mature into adults. The difference in water depth should be between 30-40 centimeters. In some instances water-level fluctuation can also increase crop yield by decreasing the amount of weeds that are able to grow, making this method appealing when the breeding site is associated with agriculture.
Other forms of manipulation
It is also possible to decrease the amount of larvae or pupae in a water body by steepening the shoreline, which reduces the amount of shallow water where vectors like to breed. This can also increase the flow of water, deterring vector breeding in that area.
Environmental Management for Malaria Control in the East Asia and Pacific Region (2004)Vector Control Methods for Use by Individuals and Communities (WHO, 1997)