Mosquito surveillance data reports the abundance of various
species of mosquitoes in San Mateo County.
Different mosquito species have different seasonal life cycles,
bite different hosts at different times of day, and transmit
different diseases. It’s important for District staff to know
what mosquito species are present in order to minimize annoyance
and, more importantly, disease risk to humans and domestic
animals.
These graphs track the number of mosquitoes of each species
(vertical axis) by month (horizontal axis). These are frequently
compared to a five-year average of each mosquito species by
month, allowing staff to determine whether we have more, less, or
about the same mosquito populations as most years.
For more information about how the District traps mosquitoes for
disease and population surveillance, see Mosquito Trapping and Pools.
During July, larval surveillance focused on back yard sources
like fountains, ponds, and containers in addition to catch
basins. A technician uses a dipper to take a sample of the water
and visually inspects it for mosquito larvae. If larvae are
present, the technician transports the sample to the laboratory
for counts and identification. This July, 43 larval samples were
submitted to the laboratory. The most frequently occurring
species in larval samples was Culiseta incidens, the
cool weather mosquito, present in 29 of the 43 samples.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the
month of July compared to the five-year average for the six most
common mosquito species in San Mateo County. Culex
erythrothorax, the tule mosquito, was the most abundant
mosquito collected this month and was collected in above average
numbers. This species breeds among tule plants (cattails) in
fresh water. Most of the Cx. erythrothorax mosquitoes
collected in July came from Mills Field in San Bruno.
During June, larval surveillance focused on back yard sources
like fountains, ponds, and containers in addition to catch
basins. A technician uses a dipper to take a sample of the water
and visually inspects it for mosquito larvae. If larvae are
present, the technician transports the sample to the laboratory
for counts and identification. This June, 79 larval samples were
submitted to the laboratory. The most frequently occurring
species in larval samples was Culiseta incidens, the
cool weather mosquito, present in 50 of the 79 samples.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the
month of June compared to the five-year average for the six most
common mosquito species in San Mateo County. Culex
pipiens, the northern house mosquito, was the most abundant
mosquito collected this month and was collected in above average
numbers. This species breeds in urban and suburban sources,
especially in warm weather. A considerable amount of Cx.
During May, larval surveillance continued in remaining marshes
and seasonal impounds while technicians also increased
inspections of back yard and underground sources that can create
mosquito problems throughout the summer. A technician uses a
dipper to take a sample of the water and visually inspects it for
mosquito larvae. If larvae are present, the technician transports
the sample to the laboratory for counts and identification. This
May, 97 larval samples were submitted to the laboratory.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the
month of May compared to the five-year average for the six most
common mosquito species in San Mateo County. Aedes
washinoi, the fresh water mosquito, was the most abundant
mosquito collected this month. This species breeds in fresh water
impounds and marshes. It emerged in high numbers this spring,
especially from sources in Mills Field, near San Francisco
International Airport. These mosquito sources are now under
control.
Surveillance for mosquito larvae in seasonal impounds and marsh
sources continues, as many are still holding water. A technician
uses a dipper to take a sample of the water and visually inspects
it for mosquito larvae. If larvae are present, the technician
transports the sample to the laboratory for counts and
identification. This April, 120 larval samples were submitted to
the laboratory. The most frequently occurring species in larval
samples was Culiseta incidens, the cool weather
mosquito, present in 52 of the 120 samples.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the
month of April compared to the five-year average for the six most
common mosquito species in San Mateo County. Adult mosquito
abundances rose in April, which is typical. Aedes
sierrensis, the western tree-hole mosquito, was the most
abundant mosquito collected this month. This species breeds in
tree-holes that hold seasonal water, which are numerous because
of the higher than average rainfall this winter.
Surveillance for mosquito larvae in seasonal impounds and marsh
sources continues to be a priority, as on-and-off rainfall has
kept them full of water. A technician uses a dipper to take a
sample of the water and visually inspects it for mosquito larvae.
If larvae are present, the technician transports the sample to
the laboratory for counts and identification. This March, 120
larval samples were submitted to the laboratory. The most
frequently occurring species in larval samples was Culiseta
inornata, the winter marsh mosquito, present in 42 of the 91
samples.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the
month of March compared to the five-year average for the six most
common mosquito species in San Mateo County. Counts for all
species are averaging below five per trap night, which is
consistent with the five-year average. Culex pipiens
abundance is substantially lower than average (0.43 compared to
4.25). Typically, adult mosquito abundances for most species
begin to rise in April, and remain elevated until fall.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the
month of February compared to the five-year average for the six
most common mosquito species in San Mateo County. Counts for all
species are averaging below five per trap night, which is
consistent with the five-year average. The numbers are expected
to remain low until spring.
Surveillance for mosquito larvae is a priority in the winter, as
seasonal water sources have filled with rain water. A technician
uses a dipper to take a sample of the water and visually inspects
it for mosquito larvae. If larvae are present, the technician
transports the sample to the laboratory for counts and
identification. This February, 91 larval samples were submitted
to the laboratory. The most frequently occurring species in
larval samples was Culiseta inornata, the winter marsh
mosquito, present in 35 of the 91 samples.
Surveillance for mosquito larvae is a priority in the winter, as
seasonal water sources have filled with rain water. A technician
uses a dipper to take a sample of the water and visually inspects
it for mosquito larvae. If larvae are present, the technician
transports the sample to the laboratory for counts and
identification. This January, 207 larval samples were submitted
to the laboratory. The most frequently occurring species in
larval samples was Culiseta incidens, present in 78 of
the 207 samples.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the
month of January compared to the five-year average for the six
most common mosquito species in San Mateo County. Counts for all
species are averaging below five per trap night, which is
consistent with the five-year average. The numbers are expected
to remain low until spring.
Surveillance of water for mosquito larvae increases after autumn
rainfall, when marshes, impounds and natural ponds fill with
water. A technician uses a dipper to take a sample of the water
and visually inspects it for mosquito larvae. If larvae are
present, the technician transports the sample to the laboratory
for counts and identification. This December, 41 larval samples
were submitted to the laboratory. The most frequently occurring
species in larval samples was Cs. incidens, from 21 out
of 41 samples.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the
month of December compared to the five-year average for the six
most common mosquito species in San Mateo County. Counts for all
species are averaging below five per trap night, which is
consistent with the five-year average. The numbers are expected
to remain low until spring.
These table and graphs show the average number of mosquitoes
collected per CO2 trap per night during the month of
November compared to the five-year average for the six most
common mosquito species in San Mateo County. Counts for all
species continue to decline as we progress into the cooler
season. All species are near or slightly below average for this
time of year.
The table and graphs below show the average number of mosquitoes
collected per CO2 trap per night during the month of
October, compared to the five-year average for the six most
common mosquito species in San Mateo County. Counts for all
species are averaging below ten per trap, which is consistent
with the five-year average. Although Culex pipiens
had a late peak in September, it has returned to normal levels,
and we expect it to further decrease through the colder months.
These table and graphs show the average number of mosquitoes
collected per CO2 trap per night during the month of
July and August compared to the five-year average for the six
most common mosquito species in San Mateo County. The peak of
Cx. erythrothorax, the tule mosquito, in July and August
has subsided. Although the abundance is still above average, it
should continue to lower as autumn progresses. Cx.
pipiens, the northern house mosquito was collected in
atypically high numbers during September. The high numbers of
Cx.
