Some Species Hibernate, Others Leave Eggs Until Spring
Mosquitoes usually disappear when cold weather comes, but they don't go away for good. They have several strategies for surviving cold weather.
Many species of mosquitoes die off when the weather turns cold, leaving only eggs which lie on the ground like seeds, waiting for warmth and spring rains to hatch and produce a new generation. Except in the warmest part of their range, these adult mosquitoes actually do only live in the summer and disappear in winter.
Other species survive cold weather by hibernating. Mosquitoes that belong to the genera Anopheles, Culex and Culiseta hibernate, and so do some other less common types in the United States.
It's hard to believe that these fragile-looking creatures can survive freezing temperatures. J. Turner Brakeley, who owned a cranberry plantation in New Jersey, became fascinated around the turn of the 20th century with studying mosquitoes in the cold. He "tramped the bogs in sleet and rain storms in midwinter," according to entomologist John B. Smith, collecting specimens and making observations and sending them to entomologists who cataloged and published his findings.
He discovered a new species with a unique cold-weather survival strategy, the pitcher-plant mosquito, Wyeomyia smithii, but he also found and described hundreds of ordinary hibernating adult mosquitoes, alive and well in the midst of a New Jersey winter. Smith described Brakeley's work in the 1904 Report of the New Jersey State Agricultural Experiment Station, among other publications. The dedication of Brakeley and his contemporaries in studying mosquitoes is preserved in weird old reports still available like this.
Brakeley found Anopheles and Culex mosquitoes hiding outdoors in the sheltered areas of banks that overhung creeks, amid exposed tree roots, and in holes in the ground left by moles, mice and other animals, as well as in hollow trees.
Around human habitation, they hid in cellars of both barns and houses, where the underground temperature stayed warmer than the outside area. He also noticed them in vacant houses or buildings, or among boxes and other stored junk.
If they could find a partially heated space, such as a cellar in a heated home, they would fly away if he disturbed them. If they needed to survive where the temperature was truly cold, they went into full hibernation and he could pick them up or knock them loose so they fell to the ground.
The Hibernation Squat
To survive the cold, they took a position Brakeley called the "hibernation squat," bending their legs and tucking their body close to the surface they sat on. They may have been trying to absorb some heat from the surface, since they're cold blooded and can't make their own body heat. He found thousands of mosquitoes hibernating in homes and outbuildings around the cranberry bogs, some in the cellars, others squatting on the walls or ceilings of empty buildings.
Since they don't eat during the winter, they add weight in the fall by switching from blood to food with more sugar, such as rotting fruit or nectar, and can double their weight. Scientists have recently discovered what signals them to change their diet.
Some mosquitoes can stand the cold better than others and don't need to hibernate where winters are mild, or will come out of hibernation when the weather warms even a little in winter, or will hatch in the spring while snow is still on the ground. They're all usually nicknamed "snow mosquitoes," though they belong to several different species.
Aedes communis, the snow or snowpool mosquito, lives in the northern United States at high elevations, where it uses early spring pools in otherwise snow-covered forests to raise its larvae. The adult mosquitoes don't live over the winter, but the eggs that they lay in the fall hatch and develop into larvae so early that the new adults will be flying while snow is on the ground, even though other species may still be hibernating.
Another mosquito, Culiseta inornata, also goes by the name "snow mosquito" or "winter mosquito," though its life cycle is different. It hibernates as an adult in the northern United States, but will stay active all winter in the south where the temperature is not as cold. Other species of mosquitoes that hibernate, like Anopheles freeborni, also come out on warm winter days.
Winter Eggs and Frozen Larvae
Most Anopheles mosquitoes hibernate, but a few, like Anopheles walkeri, lay "summer eggs" which hatch in the summer, and larger, insulated, "winter eggs" which can survive the cold. The winter eggs, which they lay only in the fall, survive freezing temperatures and lie dormant until spring, when they hatch at the coming of warm weather.
A few mosquitoes hatch in the fall and hibernate as larvae, seeking the muddy bottoms of pools to protect themselves from freezing.
The most unusual cold-weather survival strategy belongs to the pitcher plant mosquito, Wyeomyia smithii, which spends the winter frozen in the ice trapped in the leaves of the pitcher plant. These mosquitoes are also one of the few who don't bite either humans or animals.
J. Turner Brakeley, who first discovered them, noticed the larvae frozen in solid ice inside the leaves of the pitcher plants, after the temperature had been two degrees below zero. He brought them indoors and let the water melt. Soon, they begin to move. He reported: "When the ice had all melted and the debris had settled, the larvae became busily engaged in feeding."
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Photos: Kurt Stuber via Wickimedia Commons, Homestead's image bank and the CDC.