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November 2021 Newsletter

Two small black insects near a penny
Termites are much smaller than a penny - this photo shows two termites.


The rains in October brought out termites!  Many people had questions about the little black insects they found around their homes, and you can learn more here. As a reminder, District staff are able to identify a wide variety of insects and other arthropods. Fill out a service request here for an insect identification and more information about whichever pest is 'bugging' you.



2021 West Nile Virus Data

Map of California with county outlines. Counties are colored blue (indicates WNV activity without human cases), green (WNV activity with human cases), or white (no WNV activity). San Mateo County is white (no WNV activity as of 10/28/21).

As of November 1, 2021, there have been 279 dead birds reported in San Mateo County. Of those, 71 have been suitable for testing and all have tested negative for West Nile virus (WNV). No mosquito pools or sentinel chickens have tested positive for West Nile virus in San Mateo County in 2021.

Statewide, there have been 92 human cases of WNV in 2021, including 6 WNV deaths. Throughout California, 4,866 dead birds have been reported and 1,610 have been tested with 209 (13%) dead birds positive for WNV. Additionally, 2,256 mosquito samples have tested positive for WNV. Statewide, 88 sentinel chickens have tested positive.

Read more about WNV here.



Sentinel Chickens

A blue background with brown soil ground. Three dark brown chickens peck at the ground.
This year's chickens were Rhode Island Reds.

In early November, the District said goodbye to the two sentinel flocks of chickens who have been adopted out to their retirement home on a farm in Santa Rosa, CA. Chickens are an important part of the surveillance program for West Nile virus, western equine encephalitis virus and Saint Louis encephalitis virus. This year, the District housed one flock in the city of San Mateo and one in the city of East Palo Alto. The chickens live in these coops from April, when virus season begins, to the end of October, when virus season ends. Over this time they are tested every two weeks for antibodies to these mosquito-borne viruses. The District is very grateful to the chicken adopters who provide a forever home for the birds once their service is complete.


2021 Mosquito Surveillance

Adult Mosquitoes

Line graph showing the numbers of adult mosquitoes collected in CO2 traps from January through October 0f 2021

Culex erythrothorax, a mosquito that breeds in lakes and ponds with tules, was the most frequently collected mosquito in October.  The high number of tule mosquitoes this month reflects a large seasonal emergence of this species in Pacifica.  The numbers of Cx. erythrothorax continue to decline since a peak in August, but the abundance remains well above the five year average in October of 2.74 mosquitoes per trap per night. Adults of this species are usually present from April through October. Seasonal helicopter treatments for this mosquito began in July and a final treatment occurred in October. Numbers of Culex pipiens, the northern house mosquito, started a seasonal decline in October. Adult Cx. pipiens have low abundance through the winter, but remain present because of continued breeding in sheltered, warm underground sources such as water collecting beneath buildings. Read more about mosquito surveillance here.

Larval Mosquitoes

Side view of a person with short hair and a tan colored baseball hat and blue surgical mask on. The person appears to be standing in a field, with a bright yellow vest and holding a metal stick that has a white plastic cup on the end of the stick. The person is looking in the cup.
Technician Evan checks a water sample collected using a dipper.

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 counting and identification. This October, 42 larval samples were submitted to the laboratory. The most frequently occurring species in larval samples was Culiseta incidens, present in 30 of the 42 samples. This mosquito is present year-round in San Mateo County and often collected from backyard sources such as fish ponds, containers holding water, and fresh water impounds. Culex pipiens, was also collected frequently this past month, in 26 of the 42 larval 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 target this mosquito with the catch basin treatment program, which wrapped up during the month of October. Read more about mosquito surveillance here.



2021 Service Requests

Line graph showing the numbers of service requests by category for months in 2021 - please contact for the data used to produce this chart

The total number of service requests is below average for this month of the year (219 compared to an average of 280). This is the lowest number of service requests in the month of October in the past five years. The number of yellowjacket and other wasp requests has steeply declined from a peak in July, which is a normal seasonal change as yellowjacket and paper wasp activity ceases for the cooler months. However, they remained the most frequent category of request throughout October. Mosquito service requests were lower than average in October, which is consistent with most of 2021. Other categories of service requests are at typical levels for October.

Read more about service requests here.

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