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Larval Source Management via Water Level Control

September 9, 2013 - 15:47 -- Jeff Juel

The global battle to eradicate malaria is haunted by what is known as “the pesticide treadmill”. Mosquitoes have demonstrated the ability to develop resistance to every new insecticide to which they are exposed – thus new insecticides will always be needed. The plasmodium parasite that causes malaria and is transmitted by Anopheles mosquitoes also develops resistance to the drugs that are used to treat people infected with malaria - another treadmill.

Historically, malaria control was mosquito control. Before insecticides were widely used; mosquito control included
dyking and drainage of flood plains, draining marshes, intelligent irrigation, and screening doorways and windows to
stop the mosquitoes entering houses. This approach resulted in the successful reduction and elimination of malaria many decades ago, notably in Palestine, parts of Italy and in the United States.

When the Tennessee Valley Authority (TVA) began developing hydroelectric dams and reservoirs to generate cheaper
power, dam construction led to large areas of standing water. This increased mosquito breeding and morbidity and mortality due to malaria. To respond to the malaria outbreaks, the TVA successfully controlled (and continues to control) mosquito populations by strategically releasing water at their dams to raise and lower water levels in the downstream reservoirs in ways that killed mosquito populations. The system developed by the TVA – Water Level Control
- alternately strands and washes away mosquito larvae. Once each week during the active mosquito season (May – October), the level of water in reservoirs is gradually raised by one foot (30cm) and then rapidly dropped by one foot to flush out larvae and disrupt mosquito breeding (Carl Kitchens, A Dam Problem: TVA’s Fight Against Malaria 1926-1951, April 23, 2012).

The pesticide treadmill is disheartening: The mosquitoes always win in the end. For contrast, many of the strategies
that worked in the past (including water level control) can be used to eradicate mosquitoes today and into the foreseeable future. Mosquitoes are mechanically/biologically vulnerable during their aquatic larval stage and they will never evolve to a condition where they do not have a vulnerable aquatic larval stage. Using water level control as a means of killing mosquito larvae in the drainage systems of our cities and farmland circumvents the pesticide treadmill.

Farms and developed areas within a river’s flood plain are commonly protected from flooding by levees. The lands
protected by levees require interior drainage systems and a means of passing the collected water through the levee. The interior runoff is normally passed through the levee using culverts with flap gates. Flap gates are one-way valves that allow water to pass downstream, but prevent water from back flowing through the culvert during floods.
If the river adjacent to a levee is subject to variations in water level due to tides, a flow control device known as a Variable Backflow Flap Gate, or VBFG, can be used to control backflow rather than prevent backflow. A VBFG consistently fills and then drains the lower reach of the drainage with the flooding and ebbing tides. This filling and draining can be used to dilute the mosquito larvae in the drainage system effectively and subsequently discharge them into the receiving body of water.

During the flood tide, the water level in the lower reach of the drainage system will rise to a prescribed level, at which time the VBFG automatically closes. There will be standing water in the lower reach for several hours during the remainder of the flood tide and for a portion of the following ebb tide. This standing water will appear to be a viable pool on which a female mosquito will deposit her eggs. Note that some species of mosquitoes deposit their eggs on mud or on when the water level is higher than normal such as during a runoff-producing storm. Later, when the VBFG opens during the ebb tide, the ditch will drain and a fraction of the mosquito eggs and mosquito larvae will pass downstream through the open flap gate into the tidally-influenced body of water. The VBFG operates unattended for years and closes consistently at a selected elevation on each flood tide. Drainage is improved and wetland habitats are preserved and enhanced - benefitting fish, amphibians, and waterfowl.

For contrast, vector control in the U.S. (beginning around 1900) was not green. Wetlands were routinely oiled at a rate of 30 gallons per acre and large quantities of toxic chemicals and DDT were also used to kill mosquito larvae. Natural wetlands were diked and drained, ditched, or filled out-right. Malaria was eradicated – but at a great cost to the natural wetland environment. The Mosquito Crusades were essentially a war on wetlands justified by the significant morbidity and mortality of malaria. The destruction of wetlands in the 1900s was tragic - but this need not be repeated.

Rather than destroying wetlands to reduce breeding sites for mosquitoes, VBFG’s enhance waterways and wetlands
and induce female mosquitoes to deposit their eggs in habitats where their progeny have virtually no chance of surviving.

This method of larval source management will not work everywhere; however it will work in many places where existing
dikes and the associated drainage infrastructure are located near tidally-influenced watercourses. Economic development will result in new flood control projects, and well-engineered flood control projects will reduce mosquito vectors. With VBFG’s, these flood control projects will preserve and enhance natural wetlands.

This scheme could be implemented as part of an integrated vector management program designed to reduce mosquito
populations to a point below the disease transmission threshold level. Implementation of such a scheme will reduce the breeding of mosquitoes automatically without human intervention (for decades), and with virtually no operational cost.

In many locations, the use of insecticides either as adulticides or larvicides will be significantly reduced. In addition, the natural wetlands will be preserved and the water quality improved for the benefit of flora and fauna.