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Mosquitoes
Conservation status: Secure
Anopheles gambiae
Anopheles gambiae
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Diptera
Suborder: Eudiptera
Family: Culicidae
Genera
See text.
The mosquito is a member of the family Culicidae; these insects have a pair of scaled wings, a pair of halteres, a slender body, and long legs. The females of most mosquito species suck blood from other animals. Size varies but is rarely greater than 15 mm (0.6 inch). Mosquitoes weigh only about 2 to 2.5 mg (0.03 to 0.04 grain). They can fly at about 1.5 to 2.5 km/h (0.9 to 1.6 mph) and most species are nocturnal.The family Culicidae belongs to the order Diptera and contains about 3500 species in three subfamilies: Anophelinae (3 genera), the Culicinae (9 genera and >80% of all the species) and the Toxorhynchitinae (1 genus). The genera include Anopheles, Culex, Psorophora, Ochlerotatus, Aedes, Sabethes, Wyeomyia, Culiseta, and Haemagoggus. Within the family Anophelinae six subgenera are recognized: Stethomyia, Lophopodomyia, Kerteszia, Nyssorhynchus (all South American), Cellia (Old World only) and Anopheles (worldwide).Mosquitos are principally nectar feeders with only the females requiring a meal of blood. In contrast to this rule the Toxorhynchites never drinks blood. This family includes the largest of the extant mosquitoes (colloquially referred to as "mosquito eaters") and their larvae are predatory on the larvae of other mosquitoes. Attempts have been made in the past to use these as mosquito control agents but with variable success.Mosquito is a Spanish word meaning "little fly", with its use dates back to about 1583. Before then, they were called "biting flies" in English, but the term "mosquito" was adopted to prevent confusion with the house fly. The word derives from Sanskrit maksh (fly) via the Latin word musca (fly) and the Italian moschetta or Spanish mosquito (little fly). The French word is moustique.The female mosquito (in almost all species) sucks the blood of mammals, including humans. Mosquito bites often swell up hours after happening, causing a red ringed white bump about a centimeter in diameter. This bump can itch for days and over-scratching the bite can cause it to bleed. Mosquito bites can transmit diseases, such as malaria and West Nile Virus, so authorities in many areas take measures to reduce mosquito populations through pesticides or more organic means. An easy way to reduce mosquito populations in a residential area is the removal of standing water (where mosquitoes breed), and the use of repellents, such as DEET.


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Contents

Natural history
Mosquitoes and health
Mosquito control and integrated mosquito management
Mosquito repellants and personal mosquito control



Natural history - Contents

Aedes aegypti biting a human.
Aedes aegypti biting a human.
In most female mosquitoes, the mouth parts form a long proboscis for piercing the skin of mammals (or in some cases birds or even reptiles and amphibians) to suck their blood. As opposed to a syringe's typically smooth needle, the mosquito proboscis is highly serrated, which leaves a minimal number of points of contact with the skin being pierced — this reduces nerve stimulation to the point where the "bite" is not felt at all, which is generally the case (see the Mosquitoes and health section below for an explanation on the swelling.) The females require protein for egg development, and since the normal mosquito diet consists of nectar and fruit juice, which has no protein, most must drink blood to get the necessary protein. Males differ from females, with mouth parts not suitable for blood sucking.
Mosquito larvae, "blood worms" or "wrigglers," grow suspended in stagnant pools of water and breathe air through siphons in their tails.
Mosquito larvae, "blood worms" or "wrigglers," grow suspended in stagnant pools of water and breathe air through siphons in their tails.
The mosquito undergoes complete metamorphosis, going through four distinct stages in its life cycle: egg, larva, pupa, and adult — a process that was first described by the Greek philosopher Aristotle. The length of the first three stages is dependent on the species and temperature. Culex tarsalis may complete its life cycle in 14 days at 20 °C (68 °F) and only ten days at 25 °C (77 °F). Some species have a life cycle of as little as four days or up to one month. The larvae are the "wrigglers" or "wigglers" found in puddles or water-filled containers. These breathe atmospheric oxygen through a siphon at the tail end. The pupae are nearly as active as the larvae, but breathe through thoracic "horns" attached to the thoracic spiracles. Most larvae feed on microorganisms, but a few are predatory on other mosquito larvae. Some mosquito larvae, such as those of Wyeomyia live in unusual situations. These mosquito wigglers live either in the water collected in epiphytic bromeliads or inside water stored in carnivorous pitcher plants. Larvae of the genus Deinocerites live in crab holes along the edge of the ocean.Most mosquito species outside of the tropics overwinter as eggs, but a significant minority overwinter as larvae or adults. Mosquitoes of the genus Culex (a vector for St. Louis encephalitis) overwinter as mated adult females.
A mosquito larva.
A mosquito larva.
The females of blood sucking species locate their victims primarily through scent. They are extremely sensitive to the carbon dioxide in exhaled breath, as well as several substances found in sweat. Some people seem to attract mosquitoes more than others. Empirical studies of mosquito bites suggest that the risk of being bitten follows an approximately negative binomial distribution. Being male, being overweight, and having type 'O' blood may increase the risk of being bitten. Mosquitoes can detect heat, so they can find warm-blooded mammals and birds very easily once they get close enough.


Mosquitoes and health - Contents

In the United States, mosquito bites are mostly a nuisance. However, worldwide, mosquitoes are a major public health problem; they are estimated to transmit disease to more than 700 million annually, and will be responsible for the deaths of about one in 17 people currently alive. [1]The mosquito genus anopheles carries the malaria parasite (see Plasmodium falciparum). Worldwide, malaria is a leading cause of premature mortality, particularly in children under the age of five, with around 1.3 million deaths annually. Most species of mosquito can carry the filariasis worm: this parasite causes a disfiguring condition (often referred to as elephantiasis); worldwide, around 40 million people are living with a filariasis disability. Most species of mosquito can carry the viral diseases yellow fever, dengue, epidemic polyarthritis, encephalitis, Rift Valley fever and West Nile virus. Viruses carried by arthropods such as mosquitoes or ticks are known collectively as arboviruses. West Nile virus was accidentally introduced into the United States in 1999 and by 2003 had spread to almost every state.A mosquito's period of feeding is often undetected; the bite only becomes apparent because of the immune reaction it provokes. When a mosquito bites a human, she injects saliva and anti-coagulants. For any given individual, with the initial bite there is no reaction but with subsequent bites the body's immune system develops antibodies and a bite becomes inflamed and itchy within 24 hours. This is the usual reaction in young children. With more bites, the sensitivity of the human immune system increases, and an itchy red hive appears in minutes where the immune response has broken capillary blood vessels and fluid has collected under the skin. This type of reaction is common in older children and adults. Some adults can become desensitized to mosquitoes and have little or no reaction to their bites, while others can become hyper-sensitive with bites causing large and painful red welts.Mosquitoes in flight emit a distinctive high-pitched buzz, which can interrupt sleep.


Mosquito control and integrated mosquito management - Contents

There are two kinds of mosquito control: large, organized programs to reduce mosquito populations over a wide area, and actions an invidual can take to control mosquitoes with respect to themselves and their own property.Organized mosquito control programs today draw on the principles of integrated pest management. An integrated mosquito control program typically includes the following measures, all guided by surveillance of mosquito populations and knowledge of the mosquito life cycle: [2]
  • source reduction - the removal of mosquito breeding habitats
  • habitat modification - manipulating habitats to reduce breeding
  • biocontrol - introducing natural predators of mosquitoes
  • larvicide - using pesticides to reduce larval populations
  • adulticide - using pesticides to reduce adult populations
Since many mosquitoes breed in standing water, source reduction can be as simple as overturning an old tin can, or can be as complex as permanently draining marshes. Much source reduction is a matter of education. For example, homeowners can eliminate mosquito breeding grounds by removing unused plastic pools, old tires, or buckets; by clearing clogged gutters and repairing leaks around faucets; by regularly changing water in bird baths; and by filling or draining puddles, swampy areas, and tree stumps. Eliminating such mosquito breeding areas can be an extremely effective and permanent way to reduce mosquito populations without resorting to insecticides.Habitat modification, such as ditching or diking marshes or manipulating daily water flows can be effective at reducing mosquito populations by disrupting the mosquito life cycle, but experience has shown that such large-scale programs can be harmful to the ecosystem if not undertaken carefully. One example of a successful approach to habitat modification, open marsh water management, involves the use of shallow ditches to connect the shallow waters where mosquitoes breed to deeper waters where natural predators live. Simply giving the predators access to the mosquito larvae can result in long-term mosquito control.Biocontrol is the direct introduction of predators to target mosquitoes. Effective biocontrol agents include predatory fish that feed on mosquito larvae such as Gambusia affinis and other minnows and killifish. Some other biocontrol agents that have had lesser degrees of success include the predator mosquito Toxorhynchites and predator crustaceans, nematodes, and fungi. Some public agencies also employ other predators such as birds, bats, dragonflies, and frogs, but evidence of effectiveness of these agents is only anecdotal. In particular, there is no documented study that establishes that bats or purple martins consume enough mosquitoes to significantly control mosquito populations.Also used as biological control agent are the dead spores of varieties of the natural soil bacteria Bacillus thuringiensis, especially Bt israelensis (BTI). BTI is used to interfere in the digestion systems of larvae. It can be dispersed by hand or dropped by helicopter in large areas. BTI is no longer effective after the larvae turn into pupae, because they stop eating.At this point larviciding oils, such as Golden Bear, can be used which increase the water tension until the pupae and larvae cannot break the surface to obtain air and therefore drown. A chemical commonly used in the United States is methoprene, considered slightly toxic to larger animals, which mimics and interferes with natural growth hormones in mosquito larvae, preventing development. Methoprene is frequently distributed in time-release briquette form in breeding areas.Adulticide, the ground or aerial application of chemical pesticides, is less effective than the other methods of mosquito control and is generally considered a method of last resort. Nevertheless, careful application of adulticide is considered a critical part of integrated mosquito management. For example, ultra low volume (ULV) spraying of Malathion has been used in metropolitan areas like New York City to decrease the mosquito population and prevent the spread of West Nile Virus.The most effective solutions for malaria control efforts in the third world are: mosquito nets ( klamboe), insecticide-laced mosquito nets, and DDT. [3] Plain mosquito nets are cheap, they are completely effective in protecting humans within the net, they do not adversely affect the health of natural predators such as dragonflies, and do not require sophisticated public health capacity on the part of the government.The role of DDT in combating mosquitoes has been the subject of considerable controversy. While some argue that DDT deeply damages biodiversity, others argue that DDT is the most effective weapon in combating mosquitoes and hence malaria. While some of this disagreement is based on differences in the extent to which disease control is valued as opposed to the value of biodiversity, there is also genuine disagreement amongst experts about the costs and benefits of using DDT. Moreover, DDT-resistant mosquitoes have started to increase in numbers, especially in tropics due to mutations, reducing the effectiveness of this chemical.


Mosquito repellants and personal mosquito control - Contents

Mosquito repellants generally contain one of the following active ingredients: DEET, Catnip oil extract, nepetalactone, citronella, or eucalyptus oil extract. Often the best "repellant" is a fan or gentle breeze as mosquitoes do not like moving air.Other popular methods of household mosquito control include use of small electrical mats, mosquito repellent vapour, and mosquito coil, all containing a form of the chemical allethrin. Mosquito repellant candles containing Citronella oil is another method to keep mosquitoes at bay. Some more lesser known methods use the cultivation of plants like wormwood or sagewort, lemon balm, lemon grass, lemon thyme and the mosquito plant (Pelargonium) which act against mosquitoes. However scientists have determined that these plants are effective only when the leaves are crushed and used.There are several widespread theories about mosquito control such as the assertion that Vitamin B, garlic, ultrasonic devices, incense, bats, purple martins and bug zappers can be used to repel or control moquitoes [4]. Whether these methods are effective at deterring mosquitos or significantly reducing mosquito populations remains disputed. [5] Moreover, some manufacturers of "mosquito repelling" ultrasonic devices have been found to be fraudulent [6].Although bats can be prodigious consumers of insects, many of which are pests, less than 1% of a bat's diet will consist of mosquitoes; bats mostly feed on larger insects such as beetles. Bats are known carriers of rabies, so care must be taken if attempting to use bats to control insects. Similarly, bug zappers kill a wide range of flying insects including many beneficial insects as well as mosquitoes; bug zappers are not effective at controlling mosquito populations.Some newer mosquito traps emit a plume of carbon dioxide together with other mosquito attractants such as chemical scents, warmth, water vapor and sounds. By mimicking a mammal, these factors draw female mosquitoes toward the trap, where they are typically sucked into a net or holder where they collect. According to the American Mosquito Control Association [7], "these devices will, indeed, trap and kill measurable numbers of mosquitoes," but their effectiveness in any particular case will depend on a number of factors such as the size and species of the mosquito population and the type and location of the breeding habitat. The relative effectiveness of these newer mosquito traps is still being studied.
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