No-see-ums are present throughout California, especially in coastal areas, wetlands, and river valleys where moisture supports their breeding cycle. These tiny biting flies (genus Culicoides) are only 1 to 3 millimeters long, small enough to pass through standard window screens, and they deliver a sharp, burning bite that’s disproportionate to their size. If you’re dealing with clouds of invisible biters near your California home or campsite, here’s what you need to know.
Where They Live in California
No-see-ums thrive wherever there’s standing or slow-moving water and organic material for their larvae. In California, that means coastal marshes, estuaries, irrigated agricultural land, and properties near lakes, ponds, or poorly drained soil. The Central Valley, Bay Area wetlands, and Southern California coastal zones all support populations. They’re most active at dawn and dusk, and they tend to stay close to their breeding sites since they’re weak fliers. Wind above about 5 mph grounds them.
California’s dry summers can reduce their numbers inland, but any consistent water source (a birdbath, clogged gutter, or over-irrigated lawn) can sustain a local population. Coastal fog and marine moisture keep them active year-round in some areas, with peak activity in late spring through early fall.
What Their Bites Feel and Look Like
The bite itself is immediately noticeable: a sharp, burning sting caused by four tiny cutting blades in the midge’s mouthparts that lacerate the skin. If you look closely, you may spot a tiny red dot on your skin, which is the midge itself filling with blood. After the bite, reactions vary from a small reddish welt to a more intense local allergic response with significant itching and swelling. Some people develop clusters of bites because no-see-ums feed in groups.
Compared to mosquito bites, no-see-um bites tend to burn more initially and itch more intensely over the following days. The welts can persist for a week or longer in people with stronger reactions. Scratching increases the risk of secondary skin infection, so managing the itch early matters.
Treating the Itch
Wash the bites gently with soap and water as soon as you notice them. Apply a cold, damp cloth or ice pack for 10 to 20 minutes to reduce swelling and pain. For ongoing itch relief, calamine lotion, a baking soda paste, or a low-strength hydrocortisone cream (0.5% or 1%) applied several times a day works well. An over-the-counter antihistamine like cetirizine or loratadine can help if itching is widespread or keeping you awake at night.
Watch for signs of infection: increasing redness, warmth, pus, or spreading swelling around the bite site. This is more common in bites that have been heavily scratched.
Repellents That Actually Work
Not all insect repellents perform equally against no-see-ums. The most reliable options, based on what’s been tested and labeled for biting midges specifically:
- DEET is the best-documented choice. Products with up to 30% concentration (the recommended ceiling) work well against no-see-ums, mosquitoes, and black flies.
- Picaridin is effective against biting midges and is a good alternative if you dislike the feel or smell of DEET. It’s less likely to damage plastics and synthetic fabrics.
- Oil of lemon eucalyptus (PMD) is a plant-derived option that lists no-see-ums on the label, typically in 8% to 10% concentrations. It offers shorter protection time than DEET or picaridin.
- IR3535 is found in some Avon products and is registered for use against biting midges, though it generally contains lower concentrations of active ingredient.
Apply repellent to all exposed skin, not just arms and legs. No-see-ums will bite ankles, the back of the neck, ears, and hairline. Reapply after swimming or heavy sweating.
Keeping Them Out of Your Home
Standard window and door screens do not stop no-see-ums. The insects are small enough to fly right through the mesh. You need fine-mesh “no-see-um screen,” typically a 20×20 mesh with a 0.008-inch wire diameter. This screen has openings small enough to block the midges, but there’s a tradeoff: the finer mesh reduces airflow noticeably, so you may feel less of a breeze. It also isn’t commonly stocked at hardware stores, though it’s available online and through specialty suppliers.
Beyond screening, seal any gaps around windows, doors, and utility entry points with caulk. No-see-ums will exploit any opening. If you keep doors open frequently, consider a screen door fitted with the finer mesh.
Reducing Populations Around Your Property
The most effective long-term strategy is eliminating breeding habitat. No-see-um larvae develop in moist soil and shallow standing water rich in organic matter. Start by surveying your property for sources: clogged rain gutters, plant saucers, low spots in the yard that hold water after irrigation, compost piles that stay wet, and edges of ornamental ponds.
Drain what you can, and fix grading issues that create puddles. For small ponds or water features you want to keep, stocking with goldfish or other small fish can reduce larvae since the fish feed on them. (Don’t release these fish into natural waterways, where they can harm native ecosystems.)
For water sources you can’t drain or stock with fish, a biological larvicide containing Bti (a naturally occurring bacterium toxic to fly larvae) is effective. It’s available in granule and dunk form at garden centers. Bti targets only fly and mosquito larvae and is safe for pets, wildlife, and plants.
Outdoor Lighting Changes
No-see-ums are strongly attracted to lights at night, particularly bright white and mercury vapor bulbs. Switching outdoor fixtures to sodium-vapor or halogen bulbs with yellow, orange, or pink tints makes your property less appealing. Motion-activated lights that shut off automatically also help by reducing the total hours of illumination. If you have a large property, placing a decoy light away from your living areas can draw the swarms elsewhere.
Health Risks in California
In the United States, no-see-ums are primarily a nuisance pest. They do not transmit significant diseases to humans in California. In tropical regions outside the U.S., certain Culicoides species can transmit parasites and viruses to people, but that’s not a concern here. They can, however, transmit certain animal viruses to livestock, which is relevant for California ranchers and horse owners dealing with conditions like bluetongue in sheep and cattle.
For most Californians, the real impact is quality of life. A bad no-see-um population can make evenings outdoors unbearable, disrupt sleep if they get indoors, and leave you covered in intensely itchy welts. The combination of fine-mesh screening, targeted repellent use, and breeding site reduction is the most reliable way to reclaim your outdoor space.