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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Mosquito larvae collections from September 2020

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.

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September 2020 CO2 trap collections

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.

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Mosquito larvae collections from August 2020

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.

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August 2020 CO2 Trap Collections

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.

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July 2020 CO2 trap collections

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.

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Mosquito Larvae Collections from June 2020

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.

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June 2020 CO2 Trap Collections

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.

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Mosquito larval collections from May 2020

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.

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May 2020 CO2 Trap Collections

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.

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Mosquito Larval Collections from April 2020

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.

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April 2020 CO2 Trap Results

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.

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March 2020 CO2 Trap 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. The numbers are expected to remain low until after spring rains.

Species

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Mosquito Larval Collections from March 2020

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.

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Mosquito Larval Collections from February 2020

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.

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February 2020 CO2 Trap 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.

Species

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Larval Mosquito Collections from January 2020

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.

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January 2020 CO2 Trap 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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December 2019 CO2 Trap 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 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.

 

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Larval Mosquito Collections from December 2019

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.

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November 2019 CO2 Trap Results

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.

 

Commands