Mosquitoes are active in Wisconsin from roughly May through September, with populations surging after warm, wet stretches in June and July. The state is home to dozens of species, and while most are just a nuisance, several can transmit serious illnesses. Here’s what you need to know about living with mosquitoes in Wisconsin, from the diseases they carry to the most effective ways to protect yourself.
When Mosquito Season Starts and Peaks
Wisconsin’s mosquito season typically runs from May to September, though mosquitoes can remain active into October if temperatures stay mild. The season doesn’t flip on like a switch. It builds gradually as spring temperatures rise and standing water from snowmelt and rain gives larvae a place to develop. By mid-June, populations are usually noticeable, and peak biting pressure hits in July and August when warmth, humidity, and rainfall all converge.
Activity also varies by time of day. Most Wisconsin mosquitoes are most aggressive at dawn and dusk, though some species will bite throughout the day, especially in shaded, wooded areas. If you’re planning outdoor time during summer evenings, that’s when you’ll want protection most.
Two Types of Mosquitoes, Two Types of Habitat
Wisconsin mosquitoes fall into two broad categories based on where they lay their eggs: standing water mosquitoes and floodwater mosquitoes. Standing water mosquitoes breed in permanent or semi-permanent pools, think birdbaths, rain barrels, clogged gutters, old tires, and retention ponds. Floodwater mosquitoes lay eggs in low-lying areas that periodically flood, like ditches, marshes, and river floodplains. After heavy rain, those eggs hatch almost simultaneously, producing the massive swarms that make certain weeks in summer feel unbearable.
Both types thrive across Wisconsin’s landscape of lakes, wetlands, and river corridors. Forests, marshes, and tall grasses all provide the sheltered, humid resting spots mosquitoes need during the heat of the day. Northern Wisconsin, with its dense forests and abundant water, tends to have heavier mosquito pressure than urban areas in the southern part of the state, though no region is immune.
Diseases Mosquitoes Carry in Wisconsin
Most mosquito bites in Wisconsin result in nothing worse than an itchy welt. But the state does see mosquito-borne illness every year, and a few viruses deserve your attention.
West Nile Virus
West Nile is the most commonly reported mosquito-borne disease in Wisconsin, averaging 19 cases per year. In 2025, the state recorded 27 confirmed and probable cases. Most people infected with West Nile never develop symptoms. About 1 in 5 gets a fever, headache, and body aches that resolve on their own. A small fraction, roughly 1 in 150 infections, develops serious neurological illness including inflammation of the brain or spinal cord. Older adults and people with weakened immune systems face the highest risk of severe disease.
West Nile circulates between mosquitoes and birds, with humans as incidental hosts. Wisconsin tracks the virus through surveillance of human cases, horses, wild birds, and mosquito pools collected from the environment. Cases peak in late summer when virus levels in mosquito populations are highest.
La Crosse Encephalitis
Named after the Wisconsin city where it was first identified, La Crosse virus primarily affects children. It’s transmitted by tree-hole breeding mosquitoes found in wooded areas. Most infections cause mild illness or no symptoms, but severe cases can lead to brain inflammation, seizures, and, rarely, long-term neurological problems. Wisconsin reports cases most years, particularly in the western and central parts of the state.
Jamestown Canyon Virus
Jamestown Canyon virus is less well known but increasingly recognized across the upper Midwest. Many different mosquito species can transmit it, which makes it harder to predict and control than diseases carried by a single vector. Symptoms range from a mild fever to meningitis or encephalitis in severe cases. Adults are more commonly affected than children.
Eastern Equine Encephalitis
Eastern equine encephalitis (EEE) is rare in Wisconsin but carries the highest fatality rate of any mosquito-borne virus in North America, killing roughly 30% of people who develop symptoms. Cases occasionally appear in the state, and even a single detection in mosquitoes or horses prompts public health alerts. The virus circulates in freshwater swamp habitats and is most active in late summer and early fall.
Protecting Yourself From Bites
Effective mosquito repellents use one of a handful of active ingredients, each with good evidence behind it. DEET at concentrations below 30% remains the most widely studied option and provides reliable protection for several hours per application. Picaridin at 20% concentration protects for 8 to 14 hours and feels less oily on skin than DEET. Oil of lemon eucalyptus (at 30 to 40% concentration) is a plant-derived alternative that performs well in testing, though it should not be used on children under 3.
A few practical guidelines make repellents safer and more effective:
- Infants under 6 months: No repellent at all. Use mosquito netting over strollers and carriers instead.
- Children 6 months to 12 years: Stick to DEET concentrations of 5 to 10%. Apply the repellent to your own hands first, then rub it on the child, avoiding eyes, mouth, and hands (since kids put their hands in their mouths).
- Sunscreen and repellent: Don’t use combination products. Sunscreen needs reapplication every two hours, which would lead to overexposure to the repellent chemical. Apply sunscreen first, then repellent on top.
- End of the day: Wash repellent-treated skin before bed.
Clothing matters too. Long sleeves and pants in light colors reduce exposed skin. Permethrin-treated clothing adds another layer of protection, especially useful for hikers, campers, and anyone spending extended time in northern Wisconsin’s woods.
Reducing Mosquitoes in Your Yard
The single most effective thing you can do at home is eliminate standing water. Mosquitoes can breed in as little as a bottle cap’s worth of water, and most backyard species don’t travel far from where they hatched. Walk your property weekly during mosquito season and dump, drain, or cover anything that collects water: plant saucers, buckets, wheelbarrows, tarps, clogged gutters, and kiddie pools left sitting. Change water in birdbaths at least twice a week.
For water features you can’t drain, like rain barrels or ornamental ponds, Bti (a naturally occurring soil bacterium sold as dunks or granules) kills mosquito larvae without harming fish, pets, or other wildlife. Drop a dunk in and it works for about 30 days.
Yard sprays and misting systems that kill adult mosquitoes offer temporary relief but come with trade-offs. They kill indiscriminately, wiping out pollinators and other beneficial insects along with mosquitoes. For most Wisconsin homeowners, removing breeding habitat and using personal repellent provides better, more sustainable control than broad pesticide application.
Heartworm Risk for Dogs
Mosquitoes don’t just bite people. They also transmit heartworm to dogs (and less commonly, cats). Heartworm disease has been reported in dogs in all 50 states, and Wisconsin’s warm, wet summers create prime transmission conditions. An infected mosquito deposits microscopic larvae during a bite, and over about six months those larvae grow into foot-long worms that live in the heart and pulmonary arteries. Left untreated, heartworm is fatal.
The American Heartworm Society recommends year-round prevention and annual testing, a protocol they call “Think 12”: 12 months of preventive medication and testing every 12 months. Even though Wisconsin mosquitoes die off in winter, year-round dosing eliminates any gaps from unpredictable warm spells or missed doses. Preventive medications are inexpensive relative to the cost and difficulty of treating an active heartworm infection.