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December 2021 Newsletter

A grey rat pokes its nose through a hole in cardboard

Rodent Inspections

Did you know the District offers no-charge rodent inspections?  Our certified technicians can conduct a comprehensive exterior inspection and generate a custom report with details of how to keep rats and mice from entering your house. Learn about rodent prevention here. Fill out a service request here for a rodent inspection.


2021 Mosquito Surveillance

Adult Mosquitoes

Line graph showing the numbers of adult mosquitoes collected in CO2 traps from January through November 0f 2021
Average number of adult mosquitoes collected by species and month for 2021

Abundance of all adult mosquito species are low. Culex erythrothorax remains substantially higher than the five year average but has dropped considerably over the last several months after a peak in August. Cx. erythrothorax, the tule mosquito, breeds in cattails around fresh water lakes and ponds and prefers to feed on birds. The number of adult mosquitoes is expected to remain low until spring. Read more about mosquito surveillance here.

Larval Mosquitoes

Person in blue short sleeve shirt with braided hair as seen from the side while looking into a compound scope. There are many small tubes nearby, and the person is using a pair of tweezers to move something under the scope.
Vector Ecologist Dr. Tara Roth uses a compound scope to view samples.

This November, 208 larval samples were collected in the field by vector control technicians and submitted to the lab for identification. Heavy rain in October filled marshes and impounds, triggering mosquito larvae activity of seasonal species. Twelve mosquito species were detected in larval samples during the month of November, a substantial increase from five species detected in October. The most frequently occurring species in larval samples was Culiseta incidens, present in 111 of the 208 samples. This mosquito is present year-round in San Mateo County and frequently collected from fish ponds, containers holding water, and fresh water impounds. Culex pipiens, the northern house mosquito, was also collected frequently this past month, in 89 of the 208 samples. Culex pipiens is the most common mosquito in San Mateo County. It breeds in organically enriched water from a variety of sources, such as catch basins, unmaintained swimming pools, and leaks under buildings. Culex mosquitoes are of particular concern for mosquito control operations because they are potential vectors of West Nile virus. Read more about mosquito surveillance here.


2021 West Nile Virus Data

West Nile virus activity is generally very low during the winter months. For up-to-date information about West Nile virus in California during the winter, visit CDPH's website.


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
Service requests by type and month in 2021

The total number of all types of service requests was above average this November, with 160 compared to a five-year average of 135. However, the monthly total number of requests continues to drop, which is typical as winter approaches. Rodent service requests were the most abundant during the month of November, followed by mosquito related calls. November is the first month of 2021 that mosquito service requests were above average, likely because an overall dry year kept mosquito levels low until fall rains caused concerns of mosquito breeding in standing water.

Read more about service requests here.

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