Where have all the mosquitoes gone?

July 1, 1993

Where have all the mosquitoes gone?

By Robert A. Hedeen

By Robert A. Hedeen

Naturalist

During the cold months of the year, when the temperature is frequently below freezing and the wind blows briskly, few people give thought to the pestiferous mosquitoes which plague us during the spring and summer. Fewer yet wonder how these fragile, cold-blooded, vicious gnats manage to survive adverse climatic conditions at one time of the year and then produce prolific numbers of themselves at other times.

There are about 2,000 different species of mosquitoes worldwide with America north of Mexico playing host to approximately 125 of this number. As one goes north in this hemisphere, there is a steady decrease in the number of different kinds of mosquitoes liable to be encountered. There is, however, no corresponding decrease in the number of bites. The world’s greatest mosquito content per cubic foot of air probably occurs somewhere in Alaska or Siberia.

In temperate and cold climates, mosquitoes may be divided into two general classes based on in what stage of the life cycle they manage to survive the winter: those overwintering in the adult stage and those overwintering as eggs (in warm climates, insects usually do not require protection from the elements and breed continuously throughout the year). The most troublesome types of mosquitoes in the Rock River Valley are those overwintering in the egg stage.

In the fall, the female mosquitoes of this group sense the coming of winter and deposit their last batches of eggs of the year in dry depressions which, by instinct, they know will be flooded in the spring. The females, along with the males, then die. The egg deposition sites are usually in wooded areas. The thick-shelled eggs lay dormant and resistant to freezing during the winter. If, by chance, a thaw occurs during the winter, the eggs will not hatch, but remain dormant until spring, when the temperature of the water inundating them has reached a certain temperature. Then the tiny larva inside the egg uses a can opener sort of device on its head to cut its way out of the shell to continue development in the surrounding water. After about two weeks, the adult flying insect emerges from its aquatic incubator and immediately seeks a member of the opposite sex with which to mate. Females of this group may travel several miles from their breeding grounds in search of a blood meal.

In the far north, there is usually one generation of this type of mosquito per year. I know, from personal experience in Alaska, that after about the middle of August, the mosquito problem has self-abated itself. In our section of the United States, this floodwater type of mosquito produces several generations of offspring during a single year, generally from April to October

The other type of mosquito in our area overwinters in the adult stage. Late in the fall, the female of the so-called household or domestic mosquito seeks out a male and mates for the final time of the year. After performing his biological duty, the male dies, but the fertilized female lives on until spring. She stores the sperm deposited in her in two special sacs adjoining the reproductive tract and seeks out a safe place to hibernate and pass the winter. The hibernation site can be deep in a culvert, in a basement or crawl space of a house, a rot cavity in a tree, or any other place that provides protection from the elements.

When spring arrives, the impregnated female’s reproductive physiology is stirred into action, and she flies away from her winter home. Finding almost any location that contains standing water, she lays her some 200 eggs, releasing a stored sperm cell for each egg as it is laid. The eggs are glued together forming a raft-like structure that resembles a fleck of soot. This type of mosquito is commonly called the household mosquito because it frequently breeds in standing water near our homes and does not venture far from its point of origin. Bird baths, children’s wading pools, tin cans, or clogged roof gutters provide an ideal spot for egg laying. The household mosquito is the most common intruder into our bedrooms at night, disturbing our sleep while seeking a blood meal. Most female mosquitoes require nutrient-rich blood to nourish their developing eggs, while the males have not developed this annoying habit. Male mosquitoes acquire the nourishment they require by subsisting on nectar and other plant secretions.

Female mosquitoes developed the blood-sucking habit long before man entered the picture. Fossil mosquitoes from the Oligocene period of geological history, some 140 million years ago, appear hardly different from the species we recognize today, and, presumably, they had the same habits.

As the snow falls and the wintry winds blow, rest assured, the two types of mosquitoes that bother us the most during the spring, summer, and early fall are snug and safe as embryos in protective shells or as fertilized females safely hidden away in protected locations—waiting patiently for spring to arrive so they may resume their relentless and ruthless attacks on warm-blooded animals.

Dr. Robert Hedeen is a former resident of eastern Maryland, with degrees in zoology and botany. He is a former professor of biological science. He has had 30 scientific papers published, has written numerous magazine articles, and is the author of two books on marine biology of Maryland.

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