Culex erythrothorax, a mosquito that breeds in lakes and ponds with tules, was the most frequently collected mosquito in September. The high number of tule mosquitoes this month reflects a large seasonal emergence of this species in Pacifica. The abundance of Cx. erythrothorax declined from August to September, but the number remains considerably above the five year average. Adults of this species are usually present from around April through October. Seasonal helicopter treatments for this mosquito began in July and a final treatment occurred in early October. Numbers of Culex pipiens, the northern house mosquito, increased in September, most likely due to the continued warm weather, and are at average levels of abundance for this time of year. Typically, these mosquitoes start a seasonal decline in October.
Carbon-dioxide trap data for September
This table shows the average number of mosquitoes collected per trap per night during September 2021 compared to the five-year average collection data. Data are shown for the six most common species found in San Mateo County
|Species||September 2021||5-year September average|
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.
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 freshwater 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 there were 68 larval samples submitted to the laboratory. The most frequently occurring species in larval samples was Culiseta incidens, the cool weather mosquito, present in 41 of the 68 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 37 of the 68 samples. This is the most common mosquito causing biting issues in San Mateo County, is a vector of West Nile virus, and is found year-round. District seasonal staff are currently hard at work, treating catch basins across the county in order to control mosquito populations.