Physical control involves modification of the environment to reduce or eliminate mosquito breeding places: drainage, diking, filling, leveling, or other engineering works. Cultural control aims for mosquito reduction through management of soils and vegetation that contribute to mosquito problems. Researchers are investigating the effects of conventional drainage and ditching practices on natural flow and fauna in salt marshes and studying ways to reclaim impermeable alkali soils, which are important sources of mosquitoes in some areas.