Aggressive Salt Marsh Mosquitoes and West Nile Virus Vector Arrive Early

Release date: 
Feb 27

Cotati, CA-While mosquitoes are present year-round in Marin and Sonoma counties, Vector Control Technicians are discovering an early emergence and high abundance of several species of mosquitoes this year. This anomaly is being attributed to the extreme drought followed by flooding and warmer weather, and could mean a longer and more aggressive West Nile virus season.

"There are over 20 different species of mosquitoes in our two counties, with different species emerging at different times during the year," stated Nizza Sequeira, Public Relations Director for the Marin/Sonoma Mosquito & Vector Control District. "Our biggest concern right now is the abundance of mosquitoes being detected so early, and the accelerated maturation process."

The mosquito populations being detected include two different salt marsh species, Aedes squamiger and Aedes dorsalis. These species can fly up to 20 miles, are extremely aggressive biters, can emerge as adults by the thousands, and cause extreme discomfort and potential injury to residents (including schools), visitors, livestock, and wildlife.  These mosquitoes are being found in high numbers in large areas such as wetlands and marshes adjacent to schools and subdivisions.

Along with the early arrival of the salt marsh mosquitoes is the Culex tarsalis mosquito, one of the current primary vectors of West Nile virus in California that can also vector other pathogens.  High mosquito populations coupled with an early arrival could mean an increase in West Nile virus activity this year.

Concerns are not only limited to the early emergence of mosquitoes, but also the threat of invasive species entering the two counties. Two invasive mosquito species, Aedes albopictus and Aedes aegypti, have already been detected in several counties in California. Aedes aegypti, which is capable of transmitting several viruses including dengue, chikungunya and yellow fever, have been detected as close as San Mateo. The District is actively conducting surveillance for these species in both Marin and Sonoma counties.

The District operates under an Integrated Vector Management Program.  In this program, preference is given to controlling mosquito populations in the aquatic, larval stage as opposed to flying, biting adults.

District officials are actively controlling mosquitoes in the larval stage and are asking residents to do the same by eliminating all sources of standing water on their properties. Being proactive will help reduce adult mosquito populations, thereby decreasing the incidence of discomfort and mosquito biting issues as well as the potential for West Nile virus transmission.   

Here are some useful tips to follow: 

  • Eliminate standing water in old tires, buckets, toys, trash or any other item that can hold water.
  • Cover rain barrels and other containers with a mosquito-proof screen (fine mesh-1/16 of an inch).
  • Check septic tank lids to ensure a tight seal, repair cracks, and screen vent pipes using a fine mesh screen (1/16 of an inch).
  • Clean out gutters to allow water to flow
  • Keeps screens tightly closed on windows and doors.
  • Report mosquito problems, neglected swimming pools, or any area that could be producing mosquitoes at or 1-800-231-3236.
  • Wear  mosquito repellent when outdoors at dusk and dawn. Use a repellent containing one of the following active ingredients: DEET, Picaridin, oil of lemon eucalyptus, or IR3535. 
  • Residents interested in staying informed about District activities are encouraged to follow MSMVCD on  twitter and  facebook.