Biting midges, commonly called no-see-ums, are active across much of California, with peak biting season running through summer and fall. These tiny flies measure just 1 to 3 millimeters long, small enough to pass through standard window screens, and their bites cause a sharp burning sensation followed by itchy red welts. They’re a genuine nuisance in several California regions, particularly the San Francisco Bay area, the southwestern deserts, and agricultural zones in the southern part of the state.
Where They Breed in California
The most significant biting midge species in California is Culicoides sonorensis, which thrives in the state’s agricultural landscape. Larvae develop in massive numbers in wastewater evaporation ponds associated with livestock operations and in mud contaminated by leaking watering troughs. The Chino dairy region of southern California is one of the densest hotspots, where year-round cattle operations provide ideal breeding conditions.
Beyond dairy country, biting midges breed in any moist, organic-rich soil or standing water: marshes, pond edges, irrigated fields, and even poorly drained yards. Coastal areas can also see populations develop in decaying seaweed and organic debris trapped in intertidal zones. If your property has consistently damp soil, ornamental ponds, or clogged drainage, you may be creating habitat without realizing it.
When They’re Most Active
Summer and fall are peak season. That’s when biting midge populations explode and biting pressure is heaviest across California. Activity drops during winter and early spring, but it doesn’t disappear entirely. Research conducted over three years in the Chino dairy region found that adult midges continued seeking hosts throughout the winter months in southern California, at least on warmer days. Trapping data showed continuous emergence of new adults from breeding sites even during the cooler season, meaning southern California never gets a true break from these insects.
Midges are most active at dawn and dusk, and they tend to avoid windy conditions. A light breeze of 5 to 8 miles per hour is often enough to ground them, which is why ceiling fans and portable fans can offer surprising relief on a patio.
What Their Bites Feel and Look Like
A biting midge bite starts with an immediate burning sting, disproportionately painful for such a tiny insect. Within minutes, a small reddish welt appears at the bite site. For most people, it stays small and fades within a day or two. Others develop a local allergic reaction with significant swelling and intense itching that can persist for a week or more. People who are new to an area with midges tend to react more strongly than long-term residents.
The biggest risk from bites is secondary bacterial infection caused by scratching. Breaking the skin with repeated scratching opens the door to infection, which can turn a minor annoyance into something that needs medical attention. If you notice increasing redness, warmth, or pus around a bite site, that’s a sign of infection rather than a normal reaction.
How to Treat Midge Bites
The itch is the main problem, and time is the main cure. To speed things along, apply an ice cube directly to the bite for about 30 seconds. This numbs the area and reduces swelling quickly. Over-the-counter hydrocortisone cream or calamine lotion applied up to three times a day helps keep itching manageable. A paste of baking soda and water works as a low-cost alternative.
For stronger reactions with widespread itching or multiple bites, a non-drowsy oral antihistamine like cetirizine or loratadine can take the edge off. The goal with all of these approaches is the same: reduce itching enough that you don’t scratch, because scratching is what turns bites into a real problem.
They Don’t Spread Disease to Humans
Despite how miserable the bites can be, biting midges in the United States are not known to transmit any disease to humans. This sets them apart from mosquitoes and ticks. The primary public health concern with California’s biting midges is simply the impact on quality of life, as heavy populations can make outdoor activities unbearable during peak season.
Livestock are a different story. Bluetongue virus, transmitted by C. sonorensis, is a major disease of cattle and sheep across the western U.S. In southern California’s dairy country, the virus is endemic, circulating through herds each summer and fall. If you keep livestock in California, bluetongue is a real veterinary concern worth discussing with a vet, particularly for sheep, which tend to show more severe symptoms than cattle.
Keeping Midges Out of Your Home
Standard window and door screens won’t stop biting midges. Most household screens have 16 to 18 holes per inch, and midges at 1 to 3 millimeters can slip right through. You need fine mesh with at least 50 holes per inch to physically exclude them. This type of mesh is sometimes sold as “no-see-um netting” and can be installed over existing screens or used as standalone panels. The tradeoff is reduced airflow, so pairing fine mesh with a fan helps maintain ventilation.
Air conditioning is the simplest barrier. Keeping windows and doors closed during dawn and dusk, when midges are most active, eliminates most indoor encounters without any screen upgrades.
Repellents That Actually Work
Four EPA-registered active ingredients have demonstrated effectiveness against biting midges:
- DEET is the most widely tested option and works well against midges, black flies, and mosquitoes. Products with higher concentrations last longer, but concentrations above 30% don’t add meaningful benefit.
- Picaridin is effective against midges and chiggers in addition to mosquitoes, and many people prefer it because it doesn’t have the greasy feel or plastic-damaging properties of DEET.
- Oil of lemon eucalyptus (PMD) is the strongest plant-derived option and is registered for use against no-see-ums, gnats, and mosquitoes. Products typically contain 8 to 10% PMD.
- IR3535 is found in several Avon products registered for biting midges, black flies, and mosquitoes, though these formulations tend to have lower concentrations and shorter protection times.
Apply repellent to exposed skin before heading outdoors during peak midge hours. Long sleeves and long pants reduce the amount of skin you need to cover and provide a physical barrier that midges can’t bite through.
Property-Level Control Is Limited
Controlling biting midges on your property is difficult compared to mosquito management. Chemical sprays, foggers, and mists kill adults on contact but provide only temporary relief, because new midges fly into the treated area as soon as the spray dissipates. Larvicide treatments are generally impractical for the types of wet, organic-rich habitats where midges breed.
The most effective property-level strategy is habitat reduction. Eliminate standing water where possible, fix leaking irrigation and hose bibs, improve drainage in low-lying areas, and remove decaying organic material. In coastal areas, clearing trapped seaweed and storm debris from the yard can reduce nearby breeding sites. None of these steps will eliminate midges entirely if you live near a major breeding source like a dairy, marsh, or large irrigation canal, but they can noticeably reduce the population on your property.
For large-scale midge problems affecting a neighborhood or community, wide-area control programs managed by professional vector control districts are the appropriate approach. Many California counties have active mosquito and vector control districts that also address midge complaints, and contacting your local district is a reasonable first step if the problem is severe.