Short answer: Mosquito season in California runs March through October statewide, but in Riverside and the Inland Empire it runs March into November, with the worst of it June through September. Culex mosquitoes peak in early summer. The day-biting Aedes mosquitoes peak in August and September and keep going until nights drop below 50 degrees, which in Riverside is usually mid-November.
Inland Empire summers run hotter and later, the irrigated yards never dry out, and the invasive Aedes mosquitoes that arrived here in the last decade bite in daylight and breed in a bottle cap. The result is a season that starts on the state calendar and ends about a month after it. Southland Pest Control has treated Riverside yards since 2012 from offices in Riverside and Moreno Valley. This is the calendar we work from and what to do at each point in it.
When Does Mosquito Season Start in California?
Mosquito season starts in March across most of California, when daytime highs hold above 60 degrees and winter rain leaves water sitting in gutters, saucers, and low spots. In Riverside the first calls come in late March or early April, almost always about mosquitoes at dusk. Those are Culex mosquitoes, the ones that have always been here.
The Northwest Mosquito and Vector Control District, which covers the City of Riverside, Corona, Norco, Jurupa Valley, and Eastvale, notes that a mosquito can go from egg to adult in as little as seven days in warm weather. April in Riverside is warm weather. One rain-filled wheelbarrow in early April is a few hundred adults by the middle of the month.
What Month Are Mosquitoes the Worst in Riverside?
August and September are the worst months in Riverside, and that is a change from ten years ago. Culex mosquitoes, the dusk biters that carry West Nile virus, peak in June. The invasive Aedes mosquitoes, the ones with striped legs that bite ankles in the middle of the afternoon, build all summer and peak in late August through September, right as the coast is starting to cool off.
| Month | What is flying | What to do |
|---|---|---|
| March to April | Culex at dusk | Empty every container now, before the first generation lays eggs. |
| May to June | Culex peak, first Aedes | West Nile surveillance ramps up. Start barrier treatment if last year was bad. |
| July to September | Aedes peak | Daytime bites, ankle bites. The district runs wide-area treatments in Riverside during this window. |
| October | Aedes, still active inland | Coast is winding down. Riverside afternoons are still warm enough to bite. |
| November | Tapering | Activity ends when nights hold below about 50 degrees, usually mid-month here. |
Our mosquito control in Riverside page explains why treatment runs monthly through this window and why it takes about two months to see full effect.
Why Are the Mosquitoes So Bad in California This Year?
Mosquitoes are worse in California this year for the same reason they were worse last year: Aedes aegypti and Aedes albopictus are now established in every Southern California county, and each summer they spread into new neighborhoods. These mosquitoes did not exist in Riverside before roughly 2015. They breed in tiny amounts of water, they lay eggs that survive dry spells and hatch when the saucer refills, and they bite during the day, which is when people are outside.
The other reason is local. Riverside yards are irrigated. A lawn watered three times a week keeps every container on it wet enough to breed through a drought year, when the natural creeks and channels have gone dry. The mosquito biting you in Jurupa Valley or Woodcrest almost certainly hatched within a football field of where you are standing, because that is about the total flight range of an Aedes mosquito.
Do Riverside Mosquitoes Carry West Nile Virus?
Yes. Mosquito samples in western Riverside County tested positive for West Nile virus this summer, and the district reported a dead bird in Eastvale that tested positive as well. West Nile moves between wild birds and Culex mosquitoes, and people and horses get it from a bite. Most infections cause no symptoms, but the county tracks it every season for a reason.
The Aedes species can carry dengue, chikungunya, and Zika. Local transmission of those in Riverside County is rare, but it is the reason the district treats Aedes as a public health problem and not a nuisance. The district’s job is the block: public channels, storm drains, surveillance, and the wide-area treatments it announces for the City of Riverside. Your yard is your job, or ours.
What Time Do Mosquitoes Stop Bothering You?
It depends on which mosquito you have. Culex mosquitoes bite at dusk and into the night and rest during the day. Aedes mosquitoes bite in daylight, heaviest in the late afternoon, and rest at night. If you are getting bitten at 4 p.m. by the pool, that is Aedes, and no citronella candle at dinner was ever going to help.
Both species rest in shade during the hot part of the day: under leaves, along fence lines, under decks, in dense shrubs. That is where a barrier treatment goes, because that is where the mosquitoes are.
When Do Mosquitoes Go Away in California?
Mosquitoes go away in California when nighttime temperatures drop below about 50 degrees for a stretch, which is late October on the coast and mid-November in Riverside. They do not die off so much as stop. Adult mosquitoes can live several months over winter in sheltered spots like garages and crawl spaces, and Aedes eggs survive dry and cold, waiting for water in March.
That is why the yard that was bad last September is bad again next June. Nothing reset. The eggs are in the saucers.
What to Do This Week in Riverside
Start with the water. All mosquitoes need standing water to complete their life cycle, so the cheapest control on earth is a weekly walk around the yard.
Flip the containers
Plant saucers, buckets, toys, and the tray under the AC unit. A bottle cap of water is enough.
Clear the gutters
Gutters that hold water after irrigation runs are the breeding site nobody checks.
Change standing water weekly
Birdbaths, pet bowls, animal troughs. Seven days is the egg-to-adult clock.
Fix the leak
The drip line that keeps one corner of the yard wet all summer is a nursery.
Stock the pond
The district provides mosquito fish for ponds and out-of-service pools at no charge.
If you are still getting bitten after that, the mosquitoes are resting in shade you cannot empty, and that is where a barrier treatment earns its keep. Southland’s mosquito yard treatment is $375 to start and $135 a month through the season, monthly only, because the barrier breaks down in Inland Empire heat before a second month is up. Expect about two months before full effect, because the generations already breeding in the yard have to die off first. The same treatment covers fleas, ticks, and mites.
We use EPA-registered products applied per label by state-licensed technicians, and every visit ends with clear re-entry times for kids and pets. Southland holds license PR 7461, Branch 2 and 3.
Get the yard back before October
September is peak Aedes month in Riverside, which makes it the right time to start rather than the time to wait it out.
Schedule a mosquito treatment or call (951) 653-7964. Riverside and Moreno Valley offices, family owned since 2012, license PR 7461, Branch 2 and 3. Pricing effective 2026, confirmed at inspection.
Mosquito Questions From Riverside Homeowners
Are mosquitoes a problem in California?
A bigger problem now than at any point in the last 30 years, because the invasive Aedes species changed the rules. The old season was dusk, June through September, and mostly a nuisance. The new one is daytime, March into November in the inland valleys, and carries diseases the state did not have to think about before. Riverside sits in the warmest part of that map.
What city in California has the most mosquitoes?
The lists that circulate online rank Los Angeles first because of population, but the inland valleys east of it run the longest season. Riverside, Corona, Jurupa Valley, and Moreno Valley combine irrigated yards, hot afternoons into November, and established Aedes populations, which is the worst combination in the state for daytime bites.
Are mosquitoes in California active in December?
Rarely biting, but not gone. Adults shelter in garages, crawl spaces, and dense shrubs and can live several months over winter, and Aedes eggs sit dry in containers until spring water hits them. A warm December week in Riverside can produce a few bites. The population does not rebuild until March.
What smell do mosquitoes hate?
Nothing you can grow or burn keeps Aedes off a patio for an afternoon. The repellents that work on skin are the EPA-registered ones: DEET, picaridin, IR3535, and oil of lemon eucalyptus. For the yard itself, removing water and treating the shade where adults rest does more than any candle or plant.
How long do mosquitoes live?
About three weeks in summer, according to the Northwest Mosquito and Vector Control District, and several months over winter in sheltered spots. That three-week adult lifespan is why yard treatment runs every 21 to 30 days. Treat on that clock and the population never gets a full cycle to rebuild.
Does the county spray for mosquitoes in Riverside?
The Northwest Mosquito and Vector Control District treats public channels, storm drains, and flood basins, and announces wide-area adult treatments in the City of Riverside during peak season. It does not treat private backyards. For standing water on public land or an abandoned pool, call the district at (951) 340-9792. For your own yard, that is what we do.
