Mosquito Surveillance

Overview

How to Interpret Mosquito Surveillance Data

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.

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Larval mosquito collections from June 2019

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.

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June 2019 Carbon dioxide baited trap results

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.

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Mosquito Larval Collections from May 2019

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.

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May 2019 Carbon Dioxide-baited trap results

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.

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Mosquito Larval Collections from April 2019

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.

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April 2019 CO2 Trapping Results

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.

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Mosquito Larval Collections from March 2019

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.

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March 2019 CO2 trapping results

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.

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February 2019 CO2 Trapping Results

 

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.

 

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Mosquito Larval Collections from February 2019

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.

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Mosquito Larval Collections from January 2019

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.

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January 2019 CO2 Trapping Results

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.

Species

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Mosquito Larval Collections from December 2018

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.

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December 2018 CO2 Trapping Results

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. 

Species

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November 2018 CO2 Trap Results

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.

Species

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October 2018 CO2 Trap Results

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.

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CO2 Trapping in September 2018
Average number of adult mosquitoes per trap

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.

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CO2 Trapping in August 2018

Species

July 2018

5-year July average

August 2018

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July 2018 CO2 Trapping Report
Average adult mosquitoes collected per trap per night

These tables and graphs 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.

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CO2 Trapping in June 2018
Average adult mosquitoes collected per trap per night

These table and graphs 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. All species increased in abundance during June, except Aedes sierrensis, which kept consistent numbers from May. The number of collected Culex pipiens, the most abundant summer mosquito, remains below average. This is likely a result of pro-active seasonal catch basin control, utilizing mapping software in the field for more effective treatments.

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