Water that collects underneath buildings, in storm drains, vaults
and other underground sites in San Mateo County are very
significant sources of mosquito development. These sources are
the preferred habitat of the northern house mosquito (Culex
pipiens), which is an efficient vector of West Nile virus. Adults of Culex
pipiens are most abundant during the summer. However,
in San Mateo County, they can emerge throughout the year,
particularly in sheltered underground sources. A significant
amount of the District’s mosquito control operations are targeted
at mosquitoes developing in underground sources. These programs
include treatment of storm drains during the summer and utility
vaults in spring and fall.
Storm drain systems contain “catch basins,” which are the storm
water inlets to the system. The basins are designed to catch
sediment and have a sump area which is lower than the rest of the
drainage system. During winter months, storm drain systems are
flushed out regularly by winter rainstorms. However, during the
summer, water collects in the basins when residents water their
lawns or wash their cars. There are over 80,000 catch basins
within the District and each have the potential for producing a
vast numbers of mosquito larvae. Therefore the District hires
seasonal mosquito control technicians to help treat catch basins
during the summer months, applying a refined mineral oil in catch
basins from right-hand drive trucks.
Water that collects in utility vaults is another source of
mosquito development. District personnel treat these vaults
several times a year. The work is done on weekends in the early
morning to avoid traffic. Vaults are treated with time-release
briquettes that releases a hormone specific to mosquitoes over an
extended duration of time. Over 3,000 vaults are treated within
Water Under Buildings
Many of the buildings in San Mateo County are built on slab
foundations over bay fill. Over time, the ground beneath these
buildings begin to settle and large cavities are created that can
collect water from rain or broken pipes. This is known as
subsidence. Such cavities can produce enormous numbers of
mosquito larvae that can be difficult to reach and treat. The
best solution to this problem is filling the gaps around the
edges of the foundation with sand and gravel. This prevents adult
mosquitoes from entering the site to lay their eggs. Water can
also collect in the crawl space under buildings with perimeter
foundations. In this case, the installation of a sump pump and
diligent upkeep of the plumbing under the building will prevent
mosquito issues from arising.
Pools, Ponds, Fountains, and Other Neglections
Standing water around homes is a major breeding source for
mosquitoes. Fishponds, fountains, birdbaths, neglected swimming
pools and hot tubs can all produce mosquitoes under the right
Mosquitoes can also develop in discarded containers such as
flower pots or buckets. As little as an inch of water in the
bottom of a can is sufficient for mosquito development. Residents
are encouraged to empty any water-holding containers and/ or
store them upside down to prevent mosquitoes from developing.
Unused swimming and wading pools should be covered and/or drained
to prevent breeding.
Residents can request assistance from the District for
controlling mosquitoes in their own backyards. Many of these
sources can be treated without pesticides by simply stocking them
with mosquito fish. District staff will deliver fish, at no
additional charge, to any private pond within the District. San
Mateo County has over 2,300 backyard ponds, pools and fountains
that can be treated with fish each year.
Mosquitoes are found in both fresh and saltwater marshes in San
Mateo County. However, not all marshes produce mosquitoes.
Mosquito larvae are not strong swimmers and they cannot survive
in flowing water, deep open water, or in the presence of
significant populations of insect predators or fish. The District
works with property owners on new wetland restoration projects to
encourage design elements that will reduce mosquito development.
For existing marshes, the District encourages property owners to
maintain as much open water as possible and remove obstructions
that hinder water flow. For design guidance to minimize mosquito
production in fresh and salt water marshes, contact the District
at (650) 344-8592.
San Mateo County contains approximately 5,000 acres of salt marsh
along the shores of San Francisco Bay and the coastline. The
majority of these marshes are along the bay side of the county,
adjacent to homes and businesses. Much of the mosquito production
occurs in diked, reclaimed marshes and in low areas in the upper
marsh area that retain rainwater from winter rains. Four species
of mosquitoes are common in these areas: Aedes
squamiger, Ae. dorsalis, Culex tarsalis,
and Culiseta inornata. The first two, Ae.
squamiger and Ae. dorsalis, have the greatest
impact on local residents. They commonly travel great distances
(up to 25 and 15 miles respectively) inland and vigorously pursue
a blood meal in the cities adjacent to these marshes.
Ae. squamiger (the winter salt marsh mosquito)
hatch during winter months when water becomes trapped in the
marsh. Larvae of Ae. dorsalis, the summer salt
marsh mosquito, develop during summer months in depressions in
the high marsh which become flooded at extreme high tides. While
the winter salt marsh mosquito generally produces only one
generation each year, summer salt marsh mosquitoes can produce
multiple broods in a single summer.
Larvae of both Aedes species can reach millions of
mosquitoes per acre. Untreated salt marshes can generate billions
of adult mosquitoes in a single season. Adults are strong fliers
and will bite morning, noon and night, under all conditions. Once
emerged, adults of this species travel inland, often following
creeks, and then dispersing outward into the surrounding areas.
To prevent the emergence of adult salt marsh mosquitoes, Mosquito
Control Technicians inspect marshes throughout the county on a
weekly basis. When mosquito larvae are found, they are treated
with biorational materials that are specific to mosquitoes and do
not harm other organisms. These materials include two kinds of
bacteria and a growth regulator that prevents the immature
mosquitoes from completing development to the adult stage.
At least six species of mosquitoes develop in freshwater marshes.
Each type is specific to a season and type of habitat. Depending
on the species, typical larval sources include as forest pools,
grassy impounds or dense cattail marshes. Like salt marshes,
freshwater marshes are inspected regularly and treated with
biorational materials when mosquitoes are present.
During summer months, creeks can serve as a source of mosquito
development. Vector control technicians conduct regular
inspections of the creeks from May through October, treating
isolated pools of standing water as needed. Only those sections
of creeks that flow near residences or business are treated.
Inspections begin after the water level in the creeks has fallen
sufficiently to allow shallow pools of standing water to form
(usually in May). They use hand tools to open a walking path
through creek vegetation. They will then conduct monthly
inspections along this path for the rest of the summer. The
materials used in creeks are similar to those used to control
mosquito larvae in other sources. These materials are specific to
mosquito larvae and do not harm birds, fish, wildlife, or even
other insects. They include bacteria and a growth regulator that
prevents development of adult mosquitoes. The District does not
release mosquito fish in creeks.