Mosquitos & West Nile Virus

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West Nile Virus in Marin County

 "The best way to avoid the disease is to reduce exposure to and eliminate breeding grounds for mosquitoes. We also want to encourage the reporting of dead birds to the State’s West Nile Virus hotline." It apparently can take less than a week to become a flying mosquito.               

Spray Kills Millions of Honeybees


So do the following:
  Drain standing water twice weekly since it’s where mosquitoes lay eggs. This includes checking containers such as flower pots, tires, gutters, animal water bowls. ( saltwater marsh mosquitoes  can be reduced by creating tidal saltwater daily flooding of otherwise stagnant pools ) 

Use Microbial insecticides Bactimos®, Teknar® or Vectobac®.

• Dusk and dawn are when mosquitoes that carry the virus are most active so limit outdoor activities at this time or take appropriate precaution.

• Dress in long sleeves and long pants when outdoors especially at dusk and dawn.

DEET (I use 25%) is THE ONLY effective repellant Effective only for about 4 hours or less depending on wind, high temperature, high humidity, and sweating. When applying DEET, thoroughly apply the material to all exposed skin, including behind the ears.  Special formulations for children contain low concentrations of DEET in an oil-based medium that slowly releases the compound and limits its absorption through the skin. Mosquitoes will bite through lightweight clothing. ( I got bitten thru a teeshirt, worn thin )

• Maintain Screens on doors and windows.

• Contact the MSMVCD if you identify major sources of mosquitoes, so that they can then implement control measures.  Mosquitoes can migrate several miles from where they develop. 

Put mosquito-eating fish in ponds, fountains, birdbaths and other standing water around your home. These are available from the  Marin/Sonoma Mosquito and Vector Control
 District (MSMVCD( mosquito reports & news ).
Mosquito Larvae Mosquito Rafts Saltwater Mosquito Eggs
  Most people who become infected do not become ill. Up to 20% of those infected may develop flu-like symptoms with or without rash. Only about one in 150 people become severely ill, with symptoms that may include high fever, severe headache, and stiff neck may result in meningitis or encephalitis which can kill. 

People who are older than 50 years of age or have a suppressed immune system have a higher risk of becoming more severely ill from the virus. In California in 2012, there were 479 reported human cases throughout the state and 1,644 birds tested positive for the virus. Los Angeles County had the largest number of human incidents and Sacramento County had the largest number of infected dead birds.


The State of California and Marin County health departments are requesting that residents of Marin County continue to report dead birds to the State hotline. Birds of particular interest are: crows, ravens, magpies, jays and raptors (hawk or eagle). The birds must be dead only 24-48 hours to be acceptable for testing. So be sure it wasn't there the day before.

Contact the California Department of Health Services by submitting the information via .  Because the State is receiving a huge volume of calls, they are not able to respond in person to all callers.

Dispose of the dead bird, if the dead bird has not been picked up by five o’clock (5 p.m.) on the day it was reported: Avoid touching the dead bird with your bare hands. Ensure that your clothing does not come in contact with the dead bird or any blood, secretions or feces. Heavy duty, double, plastic bags should be used to pick up the bird, tightly seal the bags, and dispose in a trash container secure from children and animals. Afterwards, you should wash your hands thoroughly with warm water and soap. There is currently no evidence that the virus can be transmitted to humans that have taken these precautions.

The chemicals in the anti-mosquito pesticides - pyrethroids      

  The life cycles of mosquitoes vary widely from species to species. Some female mosquitoes lay single eggs on water surfaces; others lay single eggs on moist soil where later flooding is likely. Still other species lay batches of eggs, called rafts, 100 or more at a time on water surfaces. Eggs deposited on water surfaces usually hatch within a day or so, but eggs laid on soil surfaces do not hatch until flooding occurs, which may be months or even years later. Larvae, which are nearly invisible to the naked eye, hatch from eggs. Larvae molt three times to become 4th-stage larvae. Several days later, this larval form molts again to become a pupa. Adult mosquitoes emerge from pupae 1 to 2 days after that, with male mosquitoes always emerging first. In summer the entire life cycle, from egg to adult, may be completed in a week or less. 

Table 1. Checklist of Possible Mosquito Sources Around the Home.
Mosquito Sources What to Do to Reduce Mosquitoes
Ponds Stock pond with fish, or use Bacillus thuringiensis subsp. israelensis.
Remove excess vegetation.
Swimming pools Keep water off cover.
Maintain water quality at all times.
Tree holes Fill hole with sand or mortar.
Plastic pools Drain water when not in use, or cover so mosquitoes cannot lay eggs.
Containers Empty water.
Store in an inverted position.
Cover so mosquitoes cannot lay eggs.
Bird baths Change water at least once a week.
Standing water Eliminate by draining.
Fill in low areas.
Watering troughs Stock with fish, or change water weekly.
Cooler drains Prevent water from standing.
Street gutter or catch basins Keep litter and garden debris out of gutter.
Do not overwater yard.
Cesspool or septic tanks Seal and cover opening so mosquitoes can't lay eggs.
Roof gutters Clean once a year to remove debris which create pools.
Irrigated lawns or fields Avoid overirrigation.  Fix pooling leaks. 
Drain standing water.

County of Marin Department of Health and Human Services:

California Department of Health Services:

Centers for Disease Control:

National Pesticide Information Center (for information on insecticides including DEET):


the "New Jersey light trap" keeps track of adult mosquitos. The Marin / Sonoma district keeps 32 New Jersey traps scattered throughout the two county area.

This traps adults of some mosquitoes who are attracted to light. The trap operates on 110 volt current, with a 40 watt bulb and a fan that blows attracted insects into a killing jar at the bottom. A photocell turns the trap on at sunset and off during the day.

Culiseta inornata [green] develops huge numbers following the winter rains;

 Ochlerotatus sierrensis [blue], the "treehole mosquito", is only present in the spring and early summer;

Culex tarsalis
[red], the "encephalitis mosquito", occurs year-around, becoming frequent through the summer and fall.