It’s fall, why am I still seeing mosquitoes?



In Contra Costa County, there’s more than one mosquito season because we have more than one type of mosquito.

Contra Costa County is actually home to 23 known species of mosquitoes. Among these 23 different species, there are mosquitoes that have the ability to transmit the causative agents of diseases including West Nile virus, malaria, Western Equine Encephalitis, dog heartworm and St. Louis Encephalitis.

These 23 different species of mosquitoes also:

  • Can fly distances up to 20 miles
  • Are active winter, spring, summer or fall
  • Can develop from egg to adult in many different types of water including salt water, fresh water and polluted water
  • Can develop in many different sources of water including ponds, naturally occurring pools, creeks, tree holes, neglected swimming pools, buckets and other man-made sources, marshes and agricultural areas including pastures

In recent days, the Contra Costa Mosquito & Vector Control District has received reports from residents in Bethel Island, Knightsen and the Summer Lake area of Oakley, describing “vicious, day-biting mosquitoes.”

The District is aware of the mosquitoes in the area and District employees are working to locate and control the source of the mosquitoes.

We have identified the mosquitoes as irrigated pasture mosquitoes (Aedes nigromaculis and Aedes melanimon), which are known to be active in Contra Costa County primarily in summer and early fall and can fly up to 20 miles from irrigated agricultural areas in search of humans and large mammals including horses to bite.

We are also seeing an influx of western encephalitis mosquitoes (Culex tarsalis), which are capable of transmitting West Nile virus. These mosquitoes are known to be active in Contra Costa County from summer into early fall when pastures and fields are flooded for irrigation and duck hunting. So far, the District has not found any mosquitoes infected with West Nile virus.

As a precaution, it is important for residents to protect themselves:

  • Dump out any amount of standing water
  • Avoid being out when mosquitoes are present: typically Dawn and Dusk
  • Defend yourself against mosquitoes by wearing mosquito repellent

If you are being bitten by mosquitoes while on your own property, swat a mosquito, stick the dead mosquito in a clear baggie and contact the District to request a mosquito inspection. We need that sample to identify the mosquito species and to help direct us to the source of the mosquitoes.

We all play a part in reducing the number of mosquitoes in Contra Costa County. Every backyard can produce mosquitoes. That’s why mosquito control is in your hands and ours. And with 23 different species of mosquitoes in Contra Costa County, just because it’s fall, doesn’t mean mosquito season is over.

About Kevin

Councilmember - City of Oakley, Manager of Mainframe Operations and Optimization – USS-POSCO INDUSTRIES, Co-Founder and Board Member - Friends of Oakley A Community Foundation, Commissioner - Contra Costa Transportation Authority, Board Member - Tri Delta Transit, Transplan, San Joaquin Joint Powers Authority and RD 2137, Advisory Board – Opportunity Junction
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