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IV. Larvicide

Larviciding is a control strategy, which involves the use of materials and equipment to control the larval or immature stage of mosquito development. Since larvae are relatively contained and limited to specific aquatic habitats, larviciding is a high priority approach to the IMM plan. The primary material used for larval control is Bacillus thurigiensis israelensis (Bti), which is a bacterial agent that acts specifically on mosquito and blackfly larvae and is harmless to all other animal and plant life. The use of Bti is considered biological control, since the agent is a naturally occurring bacterial agent and is not a conventionally synthesized chemical. The material can be applied as either a granule or as a liquid.

Another bacterial agent used as a biological control is Bacillus sphaericus. This material is applied in the form of a granule and is especially effective against Culex mosquitoes. It is primarily used in the spring and fall.

Another non-conventional material used to treat mosquito larvae is methoprene, under the trade name Altosid. This material is considered to be a biorational product. Altosid is formulated either as a liquid, pellet or briquet. Methoprene is an insect juvenile hormone and acts to keep the mosquito larva from developing beyond its larva stage. The immature mosquito dies either as a larva or pupa and does not emerge as an adult.

The larvicide program is divided into three categories of general breeding habitats: (A) Marshland, (B) Woodland, Pastures, and Vacant Lots, and (C) Roadside Septic Ditches.

A. Marshland

There are approximately 60 square miles of marshland along the southern border of the parish, which breed a variety of floodwater and permanent water mosquitoes. The most common species that breed in the marsh are Culex salinarius, Culex nigripalpus, and Anopheles crucians, which both produce about 16 generations a year. Culex salinarius is a competent vector for West Nile virus and St. Louis encephalitis and is the mosquito that bites at dusk and dawn. Extremely large numbers can be produced if left unchecked. Culex nigripalpus is a competent vector for St. Louis Encephalitis. Anopheles crucians can cause an extreme nuisance and will seek a blood meal throughout the night.

High tides and/or heavy rainfall following extended drought conditions will produce extremely large numbers of the salt marsh mosquito in many marshland locations. This mosquito is a primary vector for eastern equine encephalitis and will migrate as far as 20 miles from its breeding site in search of a blood meal.

The District has documented hundreds of marshland locations that breed mosquitoes. When mosquito breeding counts reach a specified threshold, as determined by analysis of inspection data, and depending on the species and size of area, larvicide treatments are performed by an agricultural fixed wing aircraft to reduce mosquito breeding. The aircraft can apply Bti as either a liquid or granule, depending on the circumstances. Evaluations of treatments have indicated excellent control with these applications. This allows the District to reduce the amount of use of conventional insecticides.


B. Woodland, Pastures, Vacant Lots

These sites are more inland from the marsh and in close proximity to area residents. Many of the sites are low areas in association with the Pearl, Tchefuncte, and Bogue Falaya River Basins. The primary means of control of floodwater mosquitoes that breed in swales, potholes, and depression areas filled with water, is by the use of Altosid XR Briquets (methoprene) which are about the size of a role of ôlifesaversö. The District has identified thousands of floodwater breeding sites throughout the parish. The briquets are applied in the early spring to known breeding sites as a pre-flood treatment or before larvae are present. When rainfall occurs and mosquito eggs hatch, the methoprene is slowly released from the briquets into the aquatic habitat and effectively kills the larvae. The briquets remain effective for about five months. A subsequent application is performed in the late summer or early fall.


C. Roadside Septic Ditches

This larvicide phase is directed at controlling the southern house mosquito, the primary vector for West Nile virus and St. Louis encephalitis. It is imperative that effective larval control be maintained in these areas, in order to reduce the potential risk for an outbreak of these diseases. Larvicide is the primary means of controlling this mosquito. To control and contain the breeding, the District owns and operates six modified right-hand drive trucks equipped with larvicide sprayers that apply the liquid formulation of Bti. The operation is a one-man process and is accomplished by an inspector driving on the shoulder of the road while treating the water in the ditch. Since the waste effluent from residential septic tanks and filter beds enters the ditches continuously, there is always optimum habitat for breeding, and the need for treatment is constant.

During peak breeding times, two additional trucks equipped with a granular formulation of Bti are used in addition to the five right-hand drive trucks that apply liquid Bti.

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