They don’t evoke fear and loathing like snakes or spiders, but they can be far more dangerous. Mosquitoes are considered by many public health specialists to be among the deadliest creatures in the world because of the many diseases they help spread: malaria, dengue fever, eastern equine encephalitis, and Zika, to name a few. Of the more than 3,000 species of mosquitoes on earth (176 in the United States), only a small number carry diseases.
Mosquitoes are found on every continent except Antarctica. While mosquitoes reach their greatest diversity in warm, humid climates, there are quite a few species that inhabit northern environments, such as the tundra and the high altitudes of the Himalayan Mountains. In addition to natural distribution, many species have been introduced around the world by humans. The yellow fever mosquito (Aedes aegypti), which also carries the Zika virus, originated in Africa, but was brought to the Western Hemisphere by European explorers hundreds of years ago. It has spread throughout southern North America, the Caribbean, and South America.
The name mosquito is Spanish, meaning “little fly,” and they are indeed in the same family as other flies. All mosquitoes have just one pair of wings. Both male and female mosquitoes feed mostly on flower nectar, but when the female is preparing to lay eggs, she feeds on blood from an animal. Blood is very high in protein, which is critical for the development of the eggs. After a female mosquito has her “blood meal,” she will rest and let her eggs fully develop before laying.
Some mosquitoes lay a single egg, while others may lay 200 to 300 at a time, and she may lay eggs two or three times before she dies. Depending on the species, female mosquitoes may live two to three weeks, or anywhere from two to six months.
Mosquito eggs are laid in a variety of areas that include but are not limited to standing water. Some mosquitoes lay their eggs at the edge of wet areas where they are conditioned by drying out and don’t hatch until the site is flooded. These eggs can survive droughts and remain viable for up to three years. These are referred to as “floodwater mosquitoes.” Other mosquitoes lay their eggs in permanent water sources, such as lakes, swamps, and marshes. Some mosquitoes prefer to lay their eggs in containers that have collected water, like tires, birdbaths, buckets, knot holes in trees, and so forth.
After hatching, mosquitoes will go through a larval stage, a pupa stage, and finally develop into an adult. The larval mosquitoes feed on organic matter, such as rotten vegetation, algae, bacteria, and more. During this stage, they are called “wigglers” and can be seen near the surface of the water. Most larval mosquitoes breathe air from the surface through a tube at the back of the body called a siphon.
Mosquitoes remain in the larval stage for about one to two weeks, depending on the species and water temperature. During this time, they go through a series of four molts, called “instars,” as they continue to grow. At the end of the fourth instar, the mosquito goes from being a larva to a pupa, and looks like a comma. During the pupa stage, which only lasts about two days, mosquitoes do not feed. At the end of the pupa stage, an adult mosquito emerges.
Most mosquitoes are weak fliers, only traveling a few hundred feet from where they hatch. Some species of salt marsh mosquitoes, however, can travel up to a mile. Mosquitoes typically fly less than 25 feet above the ground and only fly about 1.5 miles per hour.
Mosquito activity varies from species to species. Some of the permanent water species are day-time biters, while other species are most active around sunset. Once the air temperature drops below 50 degrees Fahrenheit, most mosquito species become inactive. During cold weather, some species will find protected areas to overwinter. Others die off, leaving their eggs to hatch when temperatures warm up. Mosquitoes also are not active on windy days. Due to their small size, they cannot fly very well when the winds are more than about 10 miles per hour.
Most mosquitoes are harmless to humans, with just a handful of the world’s species acting as vectors for diseases and parasites. These diseases are malaria, yellow fever, dengue fever, West Nile virus, Zika virus, and eastern equine encephalitis, to name a few. Also included in this list are dog heartworms and tapeworms. Without a doubt, the worst of these is malaria, which even with today’s medical advancements kills about one million people a year, mostly in Africa.
Malaria is carried by members of the mosquito genus Anopheles . It is not caused by bacteria or virus, but rather a single-celled parasite that lives in the gut of Anopheles mosquitoes. It is transmitted through the saliva of these mosquitoes when they bite a human victim. Once infected with malaria, the victim carries the parasite for the rest of their life. It encysts in the bloodstream and can lay dormant for years. Once active, malaria results in high fever, chills, diarrhea, vomiting, and body aches. Severe cases can cause liver and spleen damage, as well as death.
Malaria in the United States is extremely rare today. Aggressive mosquito control programs, which include the treatment of breeding sites, spraying for adults, and public education, have been successful in practically eliminating malaria from most of North America. The same is true for yellow fever, which had devastating effects on large areas in the 19th and early 20th centuries.
Other diseases, such as West Nile, Zika, and several types of encephalitis, persist, but remain relatively rare in most temperate climates. Any of these diseases can be serious under the right circumstances, although they usually show no symptoms in most people.
The best way to prevent mosquito-borne diseases is to eliminate breeding areas, especially for the so-called “container mosquitoes.” Birdbaths, old tires, buckets, and other containers that hold water should be monitored for signs of mosquito larvae. Birdbaths and outside pet water dishes should be changed at least once per week. Windows should be kept shut, and avoid being outside during peak mosquito times — summer and early autumn evenings. When outside in mosquito-infested areas, wear long sleeves and light colors, as mosquitoes are attracted to dark colors. Likewise, mosquito repellents containing deet can be used.
Mosquitoes have been around since the Jurassic Period, and it looks like they will be around for a long time to come. They present many challenges to public health, as well as environmental protection (insecticides and wetlands conservation). Knowledge of their behavior and biology are the best weapons in helping humans cope with these diverse and persistent little bloodsuckers.
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