Larval Control

A number of interventions targeting the larval stages of the mosquito are used in developing countries. While these interventions can be popular, the evidence for their effectiveness is generally weak and there is a critical need for more rigorous evaluation of the ecological settings in which they might have an impact on reducing malaria transmission.

Source reduction: The burrow pit resulting from the activity of this brickmaker in Kisian (western Kenya) is a potential breeding site for Anopheles larvae. Filling the burrow pit would remove a source of Anopheles adults.

Source Reduction

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.

Fogging (area spraying) using a machine mounted on the back of a truck.

Other Vector Control Interventions

Fogging or area spraying is primarily reserved for emergency situations such as epidemics. Fogging, which is effective in reducing the density of pest mosquitoes in the United States, has not been shown to be effective in any malaria-endemic areas. Fogging and area sprays must be properly timed to coincide with the time of peak adult mosquito activity, because resting mosquitoes are often found in areas that are difficult for the insecticide to reach (e.g., under leaves, in small crevices).

Personal protection measures include the use of window screens, ITNs, and repellents (such as DEET) and wearing light-colored clothes, long pants, and long-sleeved shirts. Well-constructed houses with window screens are effective for preventing biting by mosquitoes that bite indoors and may have contributed to the elimination of malaria from the United States and Europe.

Sterile male release has been successfully applied in several small-scale areas. However, the need for large numbers of mosquitoes for release makes this approach impractical for most areas. Genetic modification of malaria vectors aims to develop mosquitoes that are refractory to the parasite. This approach is still many years from application in field settings.