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Mosquito Awareness Week

California Mosquito Awareness Week is April 14-20, 2024. This week is dedicated to spreading awareness of mosquitoes and the diseases they transmit. In Placer County, Culex tarsalis mosquitoes can transmit West Nile virus during the summer months. It’s important for the public to protect themselves from mosquito bites by using EPA-registered mosquito repellent, dump and draining standing water and contact the Placer Mosquito and Vector Control District report mosquito activity.

Mosquitoes in Placer County

Mosquitoes are common flying insects that are found in most parts of the world. There are over 3,500 species of mosquitoes worldwide, 53 species in California and over 30 species in Placer County. Some species of mosquitoes can carry and transmit diseases to people and animals.   In Placer County, about five species are considered primary vectors of diseases like West Nile virus, Western equine encephalitis virus, dog heartworm, St. Louis encephalitis virus or malaria.  Our newest species found in 2019, Aedes aegypti, can carry other diseases like dengue, zika and chikungunya.  Other species may bite people, but do not commonly carry or transmit disease. These are considered nuisance mosquitoes. Only adult female mosquitoes bite and pass disease from one host to another. Check out the California Department of Health’s Guide to Important Mosquitoes in California. 

All mosquitoes have four life stages and need water to develop into adult mosquitoes.

  1. Egg:
    • Some adult female mosquitoes lay about 100-400 eggs in clusters called rafts that float on the water’s surface. Other species of mosquitoes lay eggs singly on the water’s surface, on a water-filled container’s side or even in moist soil. Depending on the mosquito species, eggs can hatch within a few days after being laid or can remain dormant until flooded with water.
  2. Larvae (larva, singular, larvae (pronounced “lar-vee”) plural)
    • Most mosquito larvae are found close to the water’s surface where they breathe through a tube called a siphon and feed on small organic particles and microorganisms. When startled, larvae can dive down to hide. Larvae can be found in a wide variety of standing water sources including neglected swimming pools, ditches, storm drains, rice fields, irrigated pastures, treeholes, log ponds, snow pools, ponds, artificial containers, potted plant trays and even discarded car tires. Larvae shed their skin or molt four times during the next several days or weeks. During the fourth molt, larvae change into pupae.
  3. Pupae (pupa, singular, pupae (pronounced “pew-pee”) plural)
    • Mosquito pupae are larger than larvae, breathe air through two tubes called trumpets and can swim but do not eat. During the pupal stage, the adult mosquito is growing inside the pupal case. When it’s fully developed, the pupal case splits and the mosquito emerges from the water as an adult.
  4. Adult
    • The newly emerged adult mosquito rests on the water’s surface until it is strong enough to fly away and start to feed. Female mosquitoes usually require a blood meal to lay eggs. Male mosquitoes do not feed on blood. Female mosquitoes usually mate only one time and start to look for a blood meal. The female mosquito uses her antennae to detect heat and carbon dioxide to find a suitable host to bite. After its bloodmeal, the female mosquito looks for a place to lay eggs.

Different species prefer different places to lay eggs. Moving or exposed water are generally not good places for the female mosquito to lay eggs. Still water with vegetation growing, hidden containers or treeholes are great places for a mosquito to lay eggs. Female mosquitoes will then get another blood meal and lay another brood of eggs several more times. Feeding on first an infected host and later an uninfected host is how female mosquitoes transmit or vector disease.

Mosquito Fast Facts

  • Some mosquitoes can develop in just three days in warm temperatures.

  • All mosquitoes need water to complete the life cycle.

  • Mosquitoes do not develop in grass or shrubbery, but adult mosquitoes rest in these areas during the day.

  • Only female mosquitoes bite.

  • Male and female mosquitoes feed on nectar and other liquids for nourishment.

  • Female mosquitoes are attracted to heat and carbon dioxide.

  • Female mosquitoes may live as long as three weeks during summer or several months over the winter.

  • Mosquitoes breed in standing fresh water often found around the home.

  • Mosquitoes can also breed in urban storm drains, septic seepage and agricultural irrigation.

  • Getting rid of standing water on your property can help eliminate mosquitoes.

Invasive Mosquitoes

Invasive mosquitoes, such as Aedes aegypti and Aedes albopictus, are mosquitoes that are non-native to California. These mosquitoes have been found circulating throughout the state and have the ability to transmit Zika, dengue, chikungunya and yellow fever. They aggressively bite during the day and lay their eggs in small containers of water.

In August 2019, Aedes aegypti was first detected in south Placer County east of Auburn Boulevard at Interstate 80. Additional invasives Aedes were detected in July 2022. We have not detected Aedes albopictus in the County.

Download fact sheet on Invasive Aedes.

Download fact sheet about our approach and Integrated Vector Management for invasives.

Aedes aegypti Fast Facts

  • Small and dark with white lyre-shaped marking on their bodies.
  • Life cycle, from egg to adult, can occur in as little as 7-9 days.
  • Life span for adult mosquitoes is around three weeks.
  • Short flight range so mosquitoes are usually found in the same general area as the water source where they developed.
  • Eggs are laid over a period of several days, are resistant to drying out and can survive for periods of six or more months.
  • Remain alive through winter in egg stage, in warm weather when eggs are covered with water, the larvae hatch.
  • Use artificial and/or natural habitats to lay their eggs.

Aedes albopictus Fast Facts

  • Commonly called the Asian tiger mosquito.
  • Can transmit dengue and chikungunya viruses.
  • Daytime biting mosquito.
  • Small with black and white stripes across its body and legs.

West Nile Virus

Symptoms of WNV

People typically develop symptoms of West Nile virus between three and 14 days after they are bitten by an infected mosquito.

No Symptoms

About 80 percent of people, four out of five, who are infected with WNV will not show any symptoms.

Mild to Moderate Symptoms

Up to 20 percent, about one in five, of people who become infected will display symptoms which can include fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms generally last for just a few days, although even previously healthy people can become seriously ill.

Severe Symptoms

About one in 150 people infected with WNV will develop severe illness. Severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks and neurological effects may be permanent. WNV can be fatal.

WNV Risk

People over 50 are at higher risk for severe illness and serious symptoms of WNV. The more time you’re outdoors, the greater potential for mosquito bites and increased West Nile virus risk. 

WNV Treatment

There is no specific treatment for WNV. In cases with mild symptoms, people can usually recover at home. In more severe cases, people may need hospitalization and supportive treatment including intravenous fluids to help with breathing and nursing care.

If you develop symptoms of WNV illness, such as unusually severe headaches or confusion, seek appropriate medical attention.