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 August, larval surveillance focused on backyard 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 August, 63 larval samples
were submitted to the laboratory. The most frequently occurring
species in larval samples was Culiseta incidens, the
cool weather mosquito, present in 33 of the 63 samples.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the
month of August compared to the five-year average for the six
most common mosquito species in San Mateo County. Culex
erythrothorax was the most frequently collected mosquito in
August, with numbers more than twice the five-year average. These
mosquitoes are collected in tule marshes, which will be treated
by helicopter. Culex pipiens, typically the most common
mosquito trapped in August, is at average levels this year.
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 was collected in above average numbers in
July, whereas Culex pipiens, typically the most common
mosquito in July, is well below average this year. Cx.
erythrothorax, the tule mosquito, is largely treated by
helicopter because the sites are often inaccessible by foot.
During the summer months, mosquito larval samples from seasonal
impounds decline. Summer samples are often collected from
residential areas, such as backyard fountains and fish ponds,
water under buildings, storm drains, other containers in yards,
and treeholes. Some are also collected from pockets along creeks.
This June there were 57 larval samples submitted to the
laboratory. The most frequently occurring species in larval
samples was Culiseta incidens, the cool weather mosquito, present
in 36 of the 57 larval 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
erythrothorax was the most frequently collected mosquito in
June, but numbers were only slightly above average. These
mosquitoes are collected in tule marshes, which will be treated
by helicopter. Culex pipiens, typically the most common
mosquito in June, is well below average this year.
Breeding sources of mosquito larvae are declining as spring
progresses and many impounds and marsh sources are drying.
Mosquito breeding shifts during warmer months primarily to
backyard ornamental ponds, fountains and containers, and creeks
with standing water pockets and underground sources like storm
drains or leaking pipes.
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. Counts for all
species are averaging below five per trap night, which is below
average for Culex pipiens at this time of year, but
consistent with the five-year average for other species. Adult
mosquito abundance will likely continue to rise as the daily
temperatures increase in the summer months.
Breeding sources of mosquito larvae are declining as spring
progresses and many impounds and marsh sources are drying.
Mosquito breeding shifts during warmer months primarily to
backyard ornamental ponds, fountains and containers, and creeks
with standing water pockets and underground sources like storm
drains or leaking pipes. This April there were 74 larval samples
submitted to the laboratory. The most frequently occurring
species in larval samples was Culiseta incidens, the
cool weather mosquito, present in 42 of the 74 larval samples.
The following table and graphs 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. Counts for all
species are averaging below five per trap night, which is
consistent with the five-year average. April was the first month
in 2020 the Aedes sierrensis, the western treehole
mosquito was collected. This mosquito is a vector of dog
heartworm and adults are usually present from spring to fall.
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. The numbers are expected
to remain low until after spring rains.
Surveillance for mosquito larvae is a priority in late winter and
early spring, as seasonal water sources fill 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 March, 109 larval samples were submitted
to the laboratory. The most frequently occurring species in
larval samples was Culiseta incidens, the cool weather
mosquito, present in 48 of the 109 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 February, 91 larval samples were submitted
to the laboratory. The most frequently occurring species in
larval samples was Culiseta incidens, the cool weather
mosquito, present in 38 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 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 January, 175 larval samples were submitted
to the laboratory. The most frequently occurring species in
larval samples was Aedes washinoi, present in 70 of the
175 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.
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 continue to decline as we progress into the cooler
season. 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 catch basins,
water under buildings, and residential containers in yards.
This December, 68 larval samples were collected in the field by
vector control technicians and identified by the lab. The most
frequently occurring species in larval samples was Aedes
washinoi, the woodland pond mosquito, found in 26 of the 68
samples. This seasonal mosquito species breeds in
shallow ground pools filled by winter rains and overflows from
rivers and streams. Culiseta incidens, the cool
weather mosquito, was the second most frequent species found,
present in 23 of the 68 samples. Both Cs.
The following table and graph 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. 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 catch basins,
water under buildings, and residential containers in yards.
This November, 50 larval samples were collected in the field by
vector control technicians and submitted to the lab. The most
frequently occurring species in larval samples was Culiseta
incidens, the cool weather mosquito, present in 38 of the 50
samples. This mosquito is present year-round in San Mateo County
and frequently collected from fish ponds, containers holding
water, and fresh water impounds. Culex pipiens, the
northern house mosquito, was also collected frequently this past
month. Culex pipiens is the most common mosquito in San
Mateo County.
The following table and graph 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. 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 catch basins, water under
buildings, and residential containers in yards.
