Summer in Walnut Creek means warm evenings on the patio, hikes on Mount Diablo, and — whether you like it or not — mosquitoes. The Contra Costa Mosquito & Vector Control District has begun its seasonal surveillance and treatment operations, and early indicators suggest 2026 will be an active year for the insects that public health officials often call the deadliest animals on Earth.
While Walnut Creek residents are more likely to associate summer hazards with wildfire smoke or triple-digit heat, mosquitoes represent a genuine public health concern in Contra Costa County. West Nile virus has been detected in the county every year since its arrival in 2003, and a changing climate is extending the mosquito season earlier into spring and later into fall. Understanding what the district does — and what you can do at home — is the best defense against a nuisance that occasionally becomes something more serious.
Why Mosquitoes Matter in Contra Costa County
Contra Costa County is home to 23 species of mosquitoes, but two in particular demand attention: Culex pipiens, the common house mosquito that can carry West Nile virus, and Aedes aegypti, an invasive daytime-biting species that has been spreading through California and can transmit Zika, dengue, and chikungunya. Neither species respects city boundaries, which is why the county takes a regional approach to control.
West Nile virus remains the primary concern. In 2025, the California Department of Public Health reported over 380 human cases statewide with 19 deaths. Contra Costa County logged 14 positive mosquito pools and 3 dead birds that tested positive for the virus in 2025, along with 2 confirmed human cases. While those numbers are modest compared to the Central Valley, where agricultural irrigation creates ideal breeding habitat, the virus is endemic here — meaning it is not going away.
Most people infected with West Nile virus (about 80%) show no symptoms. About 20% develop fever, headache, body aches, and fatigue. Fewer than 1% develop neuroinvasive disease — encephalitis or meningitis — which can cause permanent brain damage or death. People over 60 and those with compromised immune systems face the highest risk. There is no vaccine or specific treatment for West Nile virus.
How the Vector Control District Protects Walnut Creek
The Contra Costa Mosquito & Vector Control District, based in Concord, operates year-round surveillance and treatment programs covering all 19 cities in the county, including Walnut Creek. The district monitors roughly 1,200 mosquito traps, tests dead birds for West Nile virus, and applies larvicide to known breeding sites — primarily neglected swimming pools, storm drains, catch basins, and agricultural areas.
When mosquito populations spike or a disease-positive sample is detected, the district may conduct adult mosquito control using truck-mounted foggers. These operations use ultra-low-volume (ULV) applications of pyrethrin-based insecticides at rates of less than one ounce per acre — a dosage that district officials say is not harmful to humans, pets, or beneficial insects when applied according to label instructions.
Residents who notice standing water on public property, neglected swimming pools, or unusual mosquito activity can submit a service request through the district website or by calling (925) 685-9301. Technicians typically respond within 48 hours to inspect the site and, if necessary, treat it. There is no charge for this service.
What Walnut Creek Residents Can Do
The single most effective mosquito prevention strategy is eliminating standing water — and it does not take much. A mosquito can complete its entire life cycle from egg to adult in as little as one teaspoon of water standing for five to seven days. That means common backyard items like plant saucers, clogged gutters, children’s toys, tarps, and bird baths can all become mosquito nurseries.
The district recommends a weekly walk-around of your property to dump and drain any container holding water. For bird baths and pet water bowls, scrub and replace the water every two to three days. For rain barrels, ensure they have tight-fitting screens. For ornamental ponds, consider stocking them with mosquitofish — the district provides them free to county residents.
DEET: Use EPA-registered insect repellent containing DEET, picaridin, IR3535, or oil of lemon eucalyptus.
DUSK & DAWN: Avoid outdoor activity during peak mosquito hours, especially for Culex species.
DRESS: Wear long sleeves, long pants, and light colors when outdoors in mosquito-heavy areas.
DRAIN: Empty standing water from containers around your property weekly.
DOORS: Ensure window and door screens are in good repair and fit tightly.
A Changing Climate, A Longer Season
One of the less-discussed consequences of California’s warming climate is the extension of mosquito season. Historically, West Nile virus transmission in Contra Costa County peaked in August and September. But in recent years, positive mosquito pools have been detected as early as May and as late as November — a shift that public health officials attribute to warmer winters that fail to reset mosquito populations.
This trend also increases the odds of new invasive species establishing themselves in the region. Aedes aegypti, the yellow fever mosquito, has been found as far north as Fresno and has been expanding its range steadily. Unlike native Culex mosquitoes, Aedes aegypti is an aggressive daytime biter, prefers to breed in small containers around homes, and is capable of transmitting multiple tropical diseases. While those diseases are not currently circulating in California, the presence of the vector increases the risk if an infected traveler introduces the pathogen.
| Year | Positive Mosquito Pools (CCC) | Dead Birds (CCC) | Human Cases (CCC) | CA Human Cases | CA Deaths |
|---|---|---|---|---|---|
| 2023 | 11 | 5 | 3 | 423 | 19 |
| 2024 | 17 | 6 | 4 | 341 | 12 |
| 2025 | 14 | 3 | 2 | 387 | 19 |
| 2026 (YTD) | 2 | 0 | 0 | TBD | TBD |
What to Do If You Find a Problem
If you notice standing water on a neighboring property that appears neglected — a green swimming pool, for example — the Contra Costa Mosquito & Vector Control District can inspect and treat it. One neglected pool can produce millions of mosquitoes that affect an entire neighborhood for blocks in every direction.
Residents should also report dead birds, particularly crows, ravens, jays, and magpies, which are especially susceptible to West Nile virus. The district tests dead birds as part of its surveillance program and uses the data to target treatment areas. To report a dead bird, call the California West Nile Virus Hotline at 1-877-WNV-BIRD or submit a report online through the state West Nile Virus website.
For non-emergency mosquito concerns, the Contra Costa Mosquito & Vector Control District accepts service requests at contracostamosquito.com or by phone at (925) 685-9301. The district also offers free mosquitofish for ornamental ponds and water features, available for pickup at its Concord office by appointment.
The Bottom Line for Walnut Creek
Mosquito-borne disease in Contra Costa County is a low-probability, high-consequence risk — the kind that is easy to ignore until it affects someone you know. The vector control district, a special district funded by a small portion of property taxes, does the heavy lifting on surveillance and treatment. But the single most important variable in mosquito control is what happens on private property.
As temperatures climb and the summer season gets underway, a few minutes spent walking your yard each week — dumping standing water, checking screens, refreshing the bird bath — will do more to protect your family and your neighborhood than any government program. And if you do spot something concerning, the professionals are a phone call away.
