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2021 Mosquito Surveillance

Adult Mosquitoes

Culex erythrothorax, a mosquito that breeds in lakes and ponds with tules, was the most frequently collected mosquito in June.  The high number of tule mosquitoes this month reflects a large, earlier-that-normal seasonal emergence of this species in Pacifica. Adults of this species are usually present from around April through October. Seasonal helicopter treatments for this mosquito begin in July. Mosquito abundance overall has increased in June but counts of Culex pipiens, the northern house mosquito and typically the most abundant mosquito in San Mateo County, remain below average for this time of year.

 

Carbon-dioxide trap data for June

This table shows the average number of mosquitoes collected per trap per night during June 2021 compared to the five-year average collection data. Data are shown for the six most common species found in San Mateo County

SpeciesJune 20215-year June average
Culex erythrothorax14.92.2
Culex pipiens4.913.1
Culiseta incidens1.71.8
Aedes washinoi1.51.2
Aedes sierrensis0.70.6
Culex tarsalis0.30.9

This chart shows the average number of mosquitoes collected per trap per night by month in 2021. Data are shown for the six most common species found in San Mateo County.

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This chart shows the average number of mosquitoes collected per trap per night by month in 2021. Data are shown for the six most common species found in San Mateo County.

This chart shows the 5-year average number of mosquitoes collected per trap night by month for the previous 5 years. Data are shown for the six most common species found in San Mateo County.

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This chart shows the 5-year average number of mosquitoes collected per trap night by month for the previous 5 years. Data are shown for the six most common species found in San Mateo County.

 

Larval Mosquitoes

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 72 larval samples submitted to the laboratory. The most frequently occurring species in larval samples was Culiseta incidens, the cool weather mosquito, present in 38 of the 72 larval samples. This mosquito is found year-round in a variety of sources, especially backyard fountains and fish ponds. It is not known to transmit diseases. Culex pipiens, the northern house mosquito, was also collected frequently this past month, present in 33 of the 72 samples. This is the most common mosquito in San Mateo County, is a vector of West Nile Virus, and is found year-round.  While the laboratory only identified 33 samples with Culex pipiens, the District expects this mosquito to be more abundant than represented here. Culex pipiens often breed in sources that are difficult to sample, such as underground basins and drainlines. District seasonal staff are currently hard at work, treating catch basins across the county in order to control mosquito populations. .

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Number and percentage of larval samples containing each mosquito species collected in larval samples in June 2021. Larval samples commonly contain multiple species.
 
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