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.
Surveillance for mosquito larvae in seasonal impounds and marsh
sources continues to be a priority, as they continue to hold
water from winter rainfall. 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, 94
larval samples were submitted to the laboratory. The most
frequently occurring species in larval samples was Culiseta
incidens, the cool weather mosquito, present in 51 of the 94
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. The abundance of adult
mosquitoes is expected to remain low until spring.
Surveillance for mosquito larvae is a priority in the winter, 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.
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 abundance of adult
mosquitoes is expected to remain low until spring.
The heavy rains this month have filled seasonal water sources
with rain water, making surveillance for mosquito larvae a top
priority. This January, field staff collected 145 larval samples
that were submitted to the laboratory for identification.
The most frequently occurring species in the larval samples was
Culiseta incidens, the cool weather mosquito.
Culiseta incidens larvae are found year-round in a
variety of freshwater sources.
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 abundance of adult
mosquitoes is expected to remain low until spring.
Surveillance for mosquito larvae typically increases during the
month of December as seasonal rains begin and water sources begin
to accumulate. Through December 2020 there has not been a lot of
late autumn/early winter rain, so many impounds remain dry.
This December, 26 larval samples were submitted to the
laboratory. The most frequently occurring species in larval
samples was Culiseta incidens, the cool weather
mosquito, present in 9 of the 26 samples.
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 was the most frequently collected mosquito in
October, seen at a lower than average level this year compared to
the five-year average. These mosquitoes are controlled by the
seasonal catch basin program and often detected in property
inspections by technicians.
During October, larval surveillance focused on
backyard sources like fountains, fish 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 October, 53
larval samples were submitted to the laboratory. The most
frequently occurring species in larval samples was Culex pipiens,
the northern house mosquito, present in 33 of the 53 samples.
During September, larval surveillance focused on backyard sources
like fountains, fish ponds, and containers in addition to catch
basins and natural sources such as creeks and fresh water ponds.
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 September, 90 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
September 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 September, seen in
higher numbers this year compared to the five-year average. Much
of this increase is due to significantly elevated numbers of
these mosquitoes in marsh areas on the coast, coupled with
improvements to targeted surveillance of this species this
season.
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.
