Understanding Mosquito Biology and Behavior
Only female mosquitoes bite—they need blood protein to produce eggs. Males feed on nectar. Females are attracted to carbon dioxide (from breath), body heat, and certain skin chemicals. They can detect CO2 from up to 50 feet away. After feeding, a female can lay 100–300 eggs on or near water. Eggs can remain dormant for months in dry conditions, hatching when water returns. This is why removing standing water is critical: you break the life cycle before new adults emerge. In San Francisco, the mild climate means eggs can survive through winter, leading to early spring outbreaks.
Health Risks: More Than Just Itchy Bites
Mosquitoes are the deadliest animal on Earth, killing more people than sharks, snakes, and crocodiles combined. In the US, West Nile virus is the most common mosquito-borne disease, with thousands of human cases reported annually. Symptoms range from mild (fever, headache) to severe (neurological damage, death). California consistently ranks among the top states for West Nile cases. Other risks include St. Louis encephalitis and, in rare cases, imported dengue or Zika. Pet owners should know that mosquitoes transmit heartworm—a potentially fatal parasite that infects dogs and cats. Prevention is far safer and cheaper than treatment.
San Francisco's Unique Mosquito Landscape
San Francisco's Mediterranean climate, with average humidity around 75% and moderate temperatures year-round, means mosquito season never truly ends. Unlike inland areas that freeze or dry out, our city's coastal fog and frequent drizzle keep water sources wet even in winter. The most common species is the house mosquito (Culex pipiens), a known West Nile vector. Since the early 2000s, the invasive Aedes aegypti has also established populations in the Bay Area. This daytime biter is aggressive and can transmit dengue, chikungunya, and Zika. Its small, dark body and white leg bands make it easy to misidentify. Control efforts must target both species, which have different habits: Culex prefers dusk, while Aedes bites during the day.
The research on mosquitoes in U.S. homes, and what it means in San Francisco
Most mosquito advice online repeats the same unsourced claims, so for San Francisco homeowners the statements below are tied to specific surveys, studies and published cost guides, and each one says what it does and does not prove for a San Francisco property. About 32% of homeowners worried that a family member could contract West Nile virus from mosquitoes in their yard, according to NaturaLawn of America's March 2024 SurveyMonkey poll of 1,163 U.S. homeowners, a provider-sponsored, self-reported survey that captures concern rather than actual disease risk. Sixteen percent of U.S. homeowners in Mosquito Squad's 2016 Fight the Bite survey said Zika news had led them to plan limits on outdoor time, according to the provider-sponsored TNS Global poll, which measured intentions and did not disclose its sample size. Worth knowing before you call anyone in San Francisco: in Pest Management Professional (PMP) magazine's February 2025 mosquito management survey, which asked about 2024 operations, the most common answer on callbacks was a rate of 1% to 4%, given by 56% of responding providers, a professional-side figure from an undisclosed sample. Another data point for San Francisco readers: in the 2016 survey TNS Global ran for Mosquito Squad, 17% of U.S. homeowners called themselves somewhat likely to hire mosquito professionals, on top of 7% very likely; the company-sponsored poll measured intentions, not bookings, and did not report its sample size. One figure San Francisco homeowners tend to underestimate: some 17% of respondents reported taking no action at all against household pests in ConsumerAffairs' February 2024 survey of 2,000 U.S. respondents, a self-reported poll that covered pests and mold together and describes broad consumer behavior rather than a rate for any single pest. None of these sources surveyed San Francisco specifically, so read them as national and regional context; the local facts that matter most are the ones an inspector documents at your San Francisco property, meaning where activity is, what is attracting it and how it is getting in.