Why Are Mosquitoes Biting Me During the Day?

If mosquitoes are biting you during the day in the Bay Area, you’re not imagining it. The culprit is the invasive Aedes mosquito, which has been spreading through California since 2013 and is now established across most of the Bay Area. Unlike the mosquitoes you grew up with, Aedes feed in the middle of the day.

How daytime biting is new for the Bay Area

Until about a decade ago, virtually all mosquitoes in California were Culex (house mosquitoes) — the gray-brown species active at dusk and dawn. If you got bitten, it was usually around a backyard barbecue at sunset or while watering plants in the early morning. Daytime hours were safe.

Then in 2013, invasive Aedes aegypti (the yellow fever mosquito) was detected in California. Aedes albopictus (the Asian tiger mosquito) followed. These are aggressive daytime biters — most active in late morning and afternoon — and they’ve spread rapidly across the state, including throughout the Bay Area.

So if your bite pattern has changed in the last few years, this is almost certainly why.

How to identify an Aedes mosquito

Aedes look noticeably different from traditional Culex mosquitoes:

• Smaller — about half the size of a Culex mosquito
• Black body with bright white stripes on the legs and a distinctive white pattern on the thorax (the “tiger” in tiger mosquito)
• Daytime active — peak biting between roughly 10am and 4pm
• Bites low — typically around the ankles and calves, often when you’re sitting still on a patio

If you see those features, you’re dealing with Aedes, and standard “spray at dusk” mosquito advice doesn’t apply.

Why Aedes are harder to control than Culex

Two reasons:

1. They breed in shockingly small amounts of water. A Culex mosquito needs a stagnant pond, gutter, or birdbath. An invasive Aedes can complete its full life cycle in as little as a bottle cap of water. That means a plant saucer, a wrinkled tarp holding a few drops, the inside of a corrugated drain pipe, or a kid’s toy left outside is enough.

2. They live close to humans. Aedes are urban mosquitoes by nature — they thrive in dense neighborhoods, breed in artificial containers, and rarely travel far from where they hatched. If your neighbor has a hidden water source, you’re sharing their mosquito population.

Source reduction (finding and eliminating breeding spots) becomes much more important — and much harder — than it was with Culex.

Are Aedes mosquito bites dangerous?

In the Bay Area, the realistic risk from any mosquito is West Nile virus, which is primarily transmitted by Culex (not Aedes). West Nile cases are reported every summer in Santa Clara and Contra Costa counties.

Invasive Aedes are capable of transmitting Zika, dengue, and chikungunya, which is why public health agencies track them. But local transmission of those diseases in the Bay Area has been very rare so far — most cases in California involve travelers returning from other countries. The bites themselves are itchy and unpleasant, but for now the disease risk locally is low.

What you can do

Start with source reduction: walk your yard and dump out any water you find — plant saucers, kids’ toys, sagging tarps, clogged gutters, birdbaths, hose ends, anything that holds even a tablespoon of water. Refresh standing water (like ornamental ponds) at least weekly. Trim back dense vegetation where mosquitoes rest during the day.

For sustained relief, professional treatment combines source reduction, a long-residual barrier spray on the underside of leaves and other resting spots, and larvicide stations in breeding sources that can’t be eliminated. Our Bay Area mosquito control program is built specifically around the invasive Aedes problem, not just generic mosquito spraying.