Why Are There Mosquitoes in My House?

The first is infiltrators — mosquitoes (mostly Aedes vexans, Ochlerotatus species, and outdoor Culex) that entered within the past few days from the yard through a gap, open door, or damaged screen. These mosquitoes are lost, disoriented, and will not breed indoors unless they find standing water. The second, more insidious problem is overwinterers and indoor breeders — Culex pipiens (the Northern house mosquito) females that entered your basement or crawlspace in fall and are now emerging from diapause, or an established indoor population that has created its own breeding cycle on standing water you have not yet found. The Rutgers Center for Vector Biology describes Cx. pipiens bluntly: it readily bites humans, enters houses at night, and does not disperse far from breeding sites — usually less than 500 meters. A persistent indoor mosquito problem almost always means the breeding site is inside your house or within a block of it.

Lab studies have documented females surviving up to 56 days under protected indoor conditions with blood and water access. Cx. pipiens females in reproductive diapause — the condition of overwintering females tucked into NH fieldstone basements — carry lipid reserves sufficient for two full overwintering periods. A single female that moved into your cellar in October can still be alive and biting the following April. This is why killing indoor mosquitoes one by one is ineffective: you must address the source.

Inside a typical NH home, the suspects in order of likelihood are: sump pits (the #1 indoor source — the pit almost always holds residual water below the float-switch threshold; cover with a tight lid and drop a Bti dunk monthly); floor drains and utility-sink drains that have not been run in weeks (traps dry out, larvae thrive in the organic film — pour a cup of water down every floor drain weekly); dehumidifier reservoirs, condensate pans under AC coils and high-efficiency furnaces, and refrigerator drip pans (classic nurseries that are never inspected); plant saucers, lucky-bamboo vases, and pet water bowls left more than a week; unused toilets in guest baths or basement half-baths (flush weekly); and basement water intrusion itself — a wet spot on concrete or a puddle behind the oil tank is enough. If you find mosquitoes indoors in February or March, you have either an overwintering adult population in the basement or an active indoor breeding site. Fix the water first.

The fall peak runs September through October, as Cx. pipiens females seek overwintering shelter in basements — they are actively entering homes through bulkheads, sill gaps, and fieldstone cracks during this period. The spring resurgence runs March through April, as those same females emerge from diapause during warm spells, often weeks before the outdoor season begins. If you are getting bitten in your bedroom in early April, the mosquito almost certainly came from inside the house. A single overwintering female found in February is confirmation: there are more in the basement.

NH led the nation with 7 human EEE cases and 2 deaths in 2005. The 2024 Hampstead fatality was NH's first EEE death in a decade. Jamestown Canyon Virus has emerged as the most frequently diagnosed mosquito-borne infection in the state, with 13+ confirmed cases since 2018. West Nile is transmitted primarily by Cx. pipiens — the same species that overwinters in NH basements. Rockingham, Strafford, southern Merrimack, and eastern Hillsborough counties are the documented hotspots. Indoor mosquitoes during elevated arbovirus risk periods are not a nuisance problem — they are a public health concern.