Overwintering

Where do mosquitoes go in the winter?

How come I see mosquitoes active on warm winter days?

Mosquitoes can spend the winter in any life stage but rarely do they live as pupa all winter long. Most species survive as eggs in the soil in a dormant state called diapause. The diapause is a state in which the eggs do not hatch in response to flooding. Instead, they wait for the days to grow long. Mosquitoes also spend winter as larvae living in water beneath trees and soil in cypress swamps. Here they avoid freezing temperatures while the cold water slows their development and metabolism. Many species can survive the winter in protected places as adult females. The males almost never survive. The adult females that do overwinter are inseminated and live most of the winter in diapause. Rarely do they bite, but salt marsh species in the genus Culex can be a nuisance on warm winter days in the southern reaches of Maryland.

Spring, Summer and Fall

Surely, the first signs of summer arriving is in seeing the first two flies and an army of mosquitoes. - Cora Lea Mariner

Maryland's 59 species of mosquitoes have an uneven distribution in the variety of habitats where they occur. Species distribution is clumped, that is, species are locally abundant. The same mosquitoes breeding in Washington County would be difficult to find in Worcester County and where overlap exists we find a great difference in population size and seasonality. These differences are due to the size and type of habitat. Seeing physical differences between the Coastal Plain, Piedmont and Appalachian regions of the state is easy for any of us. Each of these regions is unique with respect to topography, water regimes, soil types and mean low temperature. These factors, in turn, determine what type of habitat will exist for mosquitoes, not to mention other plant and animal life.

In Maryland, mosquito breeding takes place from the vernal equinox and extends slightly past the autumnal equinox into October. Most springtime mosquito breeding occurs in woodlands and adjacent swamps. Floodwater species dominate the landscape, their eggs flooded by snow melt and spring rains. These mosquitoes make visits to low woodland areas and scrub uncomfortable at best. As trees grow and temperatures increase with longer day length, soil moisture declines and the water table drops, reducing flooded areas and the species composition changes to those best adapted to permanently wet areas. This includes species adapted to breeding in swamps, ponds, sediment control areas, sewage lagoons and abandoned swimming pools. Hot summer temperatures help these mosquitoes proliferate and the deluge of rain from evening thunderstorms allows floodwater mosquito breeding in roadside ditches, containers, tree holes and lowlands. This results in swarms of mosquitoes. Lunar tides contribute further by causing salt marsh mosquito eggs to hatch unleashing hoards of the most vicious biters that make control measures necessary. Autumn sees a decline in floodwater species breeding and cooler temperatures make mosquito activity in daylight more prevalent while evening activity declines. Salt marsh species cause the worst problems in October.

E-Mail Jeannine Dorothy, Maryland Department of Agriculture Mosquito Control​