PHONE
800-231-3236 (toll free)
707-285-2200 (office)
707-285-2210 (fax)
ADDRESS
595 Helman Lane
Cotati, California
94931-9736
HOURS
Monday through Friday
7:00AM to 3:30PM
"First, know well the mosquito" William B. Herms, 1909
Mosquito and vector control districts exist to prevent mosquito vector and pest populations from transmitting infectious diseases and to reduce their ability to cause annoyance and discomfort. To this end, the district administration sets up the program, laboratory-surveillance identifies the types of control needed, field operations undertakes the control activities, shop-facilities keeps all equipment ready for use, and public education keeps residents informed on the need for control and the methods used.
Insect control is carried out in many ways. A mosquito species lives in a unique ecological niche, and each situation presents an opportunity to practice the methods of integrated pest management (IPM). The first step is to look at the environmental conditions that keep a pest or vector population growing and healthy, and to decide which can be changed or eliminated to upset that balance.
Larval Source Reduction
It is far less costly and more effective to control mosquito populations as larvae, before they mature and disperse into the environment. The Marin / Sonoma district's primary emphasis is on cataloging, reducing and abating larval sources. Adult mosquito control is used only as a temporary measure, or when nothing else is possible.
Habitat modification
Based on their knowledge of a mosquito's biology, abatement district personnel change environmental
conditions to eliminate larval sources without the need for chemical agents. This might involve
removal of a past winter's mud and debris from a creek, cleaning weeds from ditches to allow
water to flow naturally and let mosquito fish find the larvae, emptying rain-filled containers
in a back yard, or restoration of a degraded marsh.
Until the 1960s, marshlands along the Petaluma River produced huge swarms of the salt marsh mosquito Ochlerotatus squamiger. Winter floods had raised the soil level and left more than a thousand small to large depressions filled with water during the winter, but dry during the warm summer. Oc. squamiger eggs require a dry summer dormancy before they can hatch. In the spring, females deposited their eggs around the edges of these drying pools.
Knowledge of Oc. squamiger's natural history provided a plan for long-lasting suppression.
If the sites were made unattractive, females would stop using them. District personnel dug more
than 100 miles of small ditches, 18 inches deep by 12 inches wide, connecting the depressions and
potholes with the surrounding tidal slough system. The renewed daily tidal exchange flooded the
depressions and removed thousands of acres of potential eggs beds. Within months, the Oc.
squamiger population dropped to a fraction of its earlier level.
When habitat modification is not possible, mosquito abatement districts resort to one of four alternative methods to control larvae: mosquito-eating fish, microbial insecticides, insect growth regulating hormone, or larvicidal oil.
Mosquito fish (Gambusia affinis)
Mosquito fish can eat 100
to 500 larvae per day. They play an important role in mosquito control in ponds, canals, irrigated
fields and some other freshwater sources. The fish live two to three years; they are live-bearing
and produce 3 to 4 broods each year.
Release of mosquito fish in open freshwater situations should only be done by certified vector technicians. Upon request, Marin / Sonoma personnel will stock ponds for residents. Mosquito fish are not a feasible control alternative for saltwater marsh mosquitoes because they cannot tolerate the changes of salinity or the shallowness of the North Bay area tidal wetlands.
Microbial insecticides
The product known as Bti (Bacillus thuringiensis
israeliensis) can be as effective as chemical insecticides. When the bacteria Bti encysts, it
produces a protein crystal toxic to mosquito and midge larvae Once the bacteria has been ingested,
the toxin disrupts the lining of the larvae's intestine. It has no effect on a vast array of other
aquatic organisms except midges in the same habitat. Bti strains are sold under the names Bactimos®,
Teknar® and Vectobac®.
Juvenile hormone
Methoprene (sold under the name Altosid®) is an insect growth regulator widely used by abatement districts to control mosquito larvae Methoprene mimics a natural juvenile hormone, and when present in the larval habitat it keeps immature insects from maturing into adults. Unable to metamorphose, the mosquitoes die in the pupal stage.
Vector control technicians sometimes use methoprene to reach larval sources that would otherwise be difficult or dangerous to treat. Pellets can be flushed down toilets into underground septic tanks known to be breeding house mosquitoes. The methoprene kills the mosquitoes without upsetting the septic system's bacterial digestive processes.
Larvicidal oils
Oils have been used for mosquito control for more than a century. The Marin / Sonoma district uses Golden Bear 1111®, a light viscosity oil that spreads quickly and evenly over the water surface, preventing larvae and pupae from obtaining oxygen through the surface film. Oils have always been used as a product of last resort for the control of mosquito pupae, since this stage does not feed but does require oxygen. The only other option would be draining the source. Closer surveillance and timing of other agents and techniques can greatly reduce the need for larvicidal oils.
Chemical larvicides
Costs and complexity of mosquito control have increased markedly since the passage of the Environmental Protection Act in 1969. The increasing number of governmental regulations and permitting bodies, rising costs of alternative chemicals, and the spreading resistance of many vector species to existing pesticides have almost completely changed or eliminated the use of chemical control agents.
Chlorinated hydrocarbons like DDT and Chlordane are very much a thing of the past, as are the use of Organophosphate and Carbamate insecticides. Chlorinated hydrocarbons were removed from the US market in 1964, and in 1987 the Marin / Sonoma district began a voluntary moratorium on the use of carbamates and organophosphates.
Chemical Control of Adult Mosquitoes
Chemical pesticides are seldom used by the district. The technique used for adult mosquito control is known as ultra-low volume (ULV) spray. A small quantity of the pesticide is atomized into micron size particles and broadcast in a fog that drifts into sites where the adult mosquitoes hide. At best control is achieved up to 300 feet away, but it does help reduce the numbers of biting mosquitoes to tolerable levels. In recent years the use of vehicle-mounted units has decreased in favor of small, hand-carried dispersal units. This allows a more precise application of the pesticide.
The pesticide used for ULV spraying is pyrethrum (sold as Pyrocide®), a naturally occurring substance harvested from two species of Old World chrysanthemums, or pyrethrum flowers. This material is the least toxic available for mosquito control, and it degrades into non-toxic by-products within 4 to 6 hours after spraying.
Helpful reference
The University of California at Davis has an online publication Mosquito Management Guidelines publication, stressing integrated pest management techniques. [U.C. Agric. & Nat. Resources Publ. 7451]