Source reduction is removal or permanent destruction of mosquito breeding sites. The larval habitats may be destroyed by filling depressions that collect water, by draining swamps, or by ditching marshy areas to remove standing water. Container-breeding mosquitoes are particularly susceptible to source reduction as people can be educated to remove or cover standing water in cans, cups, and rain barrels around houses. Mosquitoes that breed in irrigation water can be controlled through careful water management.
For some mosquito species, habitat elimination is not possible. For these species, chemical insecticides can be applied directly to the larval habitats. Other methods, which are less disruptive to the environment, are usually preferred:
- Oils may be applied to the water surface, suffocating the larvae and pupae. Most oils in use today are rapidly biodegraded.
- Toxins from the bacterium Bacillus thuringiensis var. israelensis (Bti) can be applied in the same way as chemical insecticides. They are very specific, affecting only mosquitoes, black flies, and midges.
- Insect growth regulators such as methroprene are specific to mosquitoes and can be applied in the same way as chemical insecticides.
- Potential biological control agents, such as fungi (e.g., Laegenidium giganteum) or mermithid nematodes (e.g., Romanomermis culicivorax), are less efficient for mosquito control and are not widely used. Likewise, mosquito fish (including Gambusia affinis) have largely been ineffective except in a few studies.
Theoretically, it seems that source reduction would be an ideal approach to mosquito control: eliminate mosquitoes before they reach the stage where they can transmit malaria. However, larval habitats may be small, widely dispersed, and transient. Anopheles gambiae, one of the primary vectors of malaria in Africa, breeds in numerous small pools of water that form due to rainfall. The larvae develop within a few days, escaping their aquatic environment before it dries out. It is difficult, if not impossible, to predict when and where the breeding sites will form, and to find and treat them before the adults emerge. Therefore, larval mosquito control for the prevention of malaria in Africa has not been attempted on a large scale.
More information available at Center for Disease Control and Prevention (cdc.gov)