Aedes albopictus (Skuse, 1895)

Aedes albopictus (Stegomyia albopicta), from the mosquito (Culicidae) family, also known as (Asian) tiger mosquito or 'forest mosquito', is a mosquito native to the tropical and subtropical areas of Southeast Asia; however, in the past couple of decades this species has invaded many countries throughout the world through the transport of goods and increasing international travel.^[1] It is characterized by its black and white striped legs, and small black and white striped body.

This mosquito has become a significant pest in many communities because it closely associates with humans (rather than living in wetlands), and typically flies and feeds in the daytime in addition to at dusk and dawn. The insect is called a tiger mosquito because its striped appearance is similar to that of a tiger. Aedes albopictus is an epidemiologically important vector for the transmission of many viral pathogens, including the Yellow fever virus, dengue fever and Chikungunya fever,^[2] as well as several filarial nematodes such as Dirofilaria immitis.^[3]

Description[edit]

Name and systematics[edit]

In 1894, a British-Australian entomologist, Frederick A. Askew Skuse, was the first to describe scientifically the Asian tiger mosquito, which he named Culex albopictus (lat. Culex "gnat", "midge" and albopictus "white painted").^[4][5] Later, the species was assigned to the genus Aedes (gr. άηδής, "unpleasant")^[6] and referred to as Aedes albopictus.^[7] Like the yellow fever mosquito, it belongs to the subgenus Stegomyia (Gr. στέγος, "covered, roofed", referring to the scales that completely cover the dorsal surface in this Subgenus, and μυία, "fly") within the Aedes genus.^[8] In 2004, scientists explored higher-level relationships and proposed a new classification within the Aedes genus and Stegomyia was elevated to the Genus level, making Aedes albopictus now Stegomyia albopicta. This is, however, a controversial matter, and the use of Stegomyia albopicta versus Aedes albopictus is continually debated.^[9][10][11]

Characteristics[edit]

Aedes albopictus

The Asian tiger mosquito is about 2 to 10 mm length with a striking white and black pattern.^[4][12][13] The variation of the body size in adult mosquitoes depends on the density of the larval population and food supply within the breeding water. Since these circumstances are seldom optimal, the average body size of adult mosquitoes is considerably smaller than 10 mm. For example, the average length of the abdomen was calculated to be 2.63 mm, the wings 2.7 mm, and the proboscis 1.88 mm through a study of 10 images from 1962 of both male and female mosquitoes.^[14]

The males are roughly 20% smaller than the females, but they are morphologically very similar. However, as in all mosquito species, the antennae of the males in comparison to the females are noticeably bushier and contain auditory receptors to detect the characteristic whine of the female. The maxillary palps of the males are also longer than their proboscises whereas the females’ maxillary palps are much shorter. (This is typical for the males of the Culicinae.) In addition, the tarsus of the hind legs of the males is more silvery. Tarsomere IV is roughly three-quarters silver in the males whereas the females’ is only about 60% silver.

The other characteristics do not differentiate between sexes. A single silvery-white line of tight scales begins between the eyes and continues down the dorsal side of the thorax. This characteristic marking is the easiest and surest way to identify the Asian tiger mosquito.

The proboscis is dark colored, the upper surface of the end segment of the palps is covered in silvery scales, and the labium does not feature a light line on its underside. The compound eyes are distinctly separated from one another. The scutum, the dorsal portion of an insect’s thoracic segment, is black alongside the characteristic white midline. On the side of the thorax, the scutellum, and the abdomen there are numerous spots covered in white-silvery scales.

Such white-silvery scales can also be found on the tarsus, particularly on the hind legs that are commonly suspended in the air. The base of tarsomere I through IV has a ring of white scales, creating the appearance of white and black rings. On the fore legs and middle legs, only the first three tarsomeres have the ring of white scales whereas tarsomere V on the hind legs is completely white. The femur of each leg is also black with white scales on the end of the “knee”. The femurs of the middle legs do not feature a silver line on the base of the upper side, whereas, the femurs on the hind legs have short white lines on base of the upper side. The tibias are black on the base and have no white scales.

The terga on segments II through VI of the abdomen are dark and have an almost triangular silvery-white marking on the base that is not aligned with the silvery bands of scales on the ventral side of the abdomen. The triangular marking and the silvery band are only aligned on abdominal segment VII. The transparent wings have white spots on the base of the Costas. With older mosquito specimens, the scales could be partially worn off making the previously mentioned characteristics not stand out as much.^[12][14]

The typical Aedes albopictus individual has a length of about 2 to 10mm.^[13] As with other members of the mosquito family, the female is equipped with an elongated proboscis that she uses to collect blood to feed her eggs. The Asian tiger mosquito has a rapid bite that allows it to escape most attempts by people to swat it. By contrast the male member of the species primarily feeds on nectar.

The female lays her eggs near water; not directly into it as other mosquitoes do, but typically near a stagnant pool. However, any open container containing water will suffice for larvae development, even with less than an ounce of water in. It can also breed in running water, so stagnant pools of water are not its only breeding sites. It has a short flight range (less than 200 m), so breeding sites are likely to be close to where this mosquito is found.^[15]

Identifying tiger mosquitoes can seem easy with the above description, but many people mistakenly identify it. The best way to be sure is to compare the specimen with several approved pictures of the tiger mosquito.^[16]

Similar species[edit]

Some mosquitoes in North America, such as Ochlerotatus canadensis, have a similar leg pattern.

In Europe, the mosquito Culiseta annulata, which is very common, but does not occur in high densities, can be mistaken for an Asian tiger mosquito because of its black and white ringed legs. However, this species is missing the distinctive white line that runs from the middle of its head and down the thorax. It is also considerably larger than Aedes albopictus, is not black and white, but rather beige and grey striped, and has wings with noticeable veins and four dark, indistinct spots.

In the eastern Mediterranean area, Aedes albopictus species can be mistaken with Aedes cretinus, which also belongs to the subgenus Stegomyia and uses similar breeding waters. Aedes cretinus also has a white stripe on the scutum, but it ends shortly before the abdomen, and also has two additional stripes to the left and right of the middle stripe. So far Aedes cretinus is only located in Cyprus, Greece, Macedonia, Georgia and Turkey.^[17]

In Asia, the Asian tiger mosquito can be mistaken for other members of the subgenus Stegomyia, particularly the yellow fever mosquito Aedes aegypti (the most prevalent species in the tropics and subtropics), because both species display a similar black and white pattern. It can be hard to distinguish Aedes albopictus from the closely related Aedes scutellaris (India, Indonesia, Papua New Guinea, and the Philippines), Aedes pseudoalbopictus (India, Indonesia, Malaysia, Myanmar, Nepal, Taiwan, Thailand, and Vietnam) and Aedes seatoi (Thailand).^[12][18]

Diet and host location[edit]

Bloated female at the end of a meal

Like other mosquito species, only the females require a blood meal to develop their eggs. Apart from that, they feed on nectar and other sweet plant juices just as the males do. In regards to host location, carbon dioxide and organic substances produced from the host, humidity, and optical recognition play important roles.

The search for a host takes place in two phases. First, the mosquito exhibits a nonspecific searching behavior until it perceives host stimulants, whereupon it secondly takes a targeted approach.^[19] For catching tiger mosquitoes with special traps, carbon dioxide and a combination of chemicals that naturally occur in human skin (fatty acids, ammonia, and lactic acid) are the most attractive.^[20]

The Asian tiger mosquito particularly bites in forests during the day and has been known as the forest day mosquito for this very reason. Depending upon region and biotype, there are differing active peaks, but for the most part they rest during the morning and night hours. They search for their hosts inside and outside of human dwellings, but are particularly active outside. The size of the blood meal depends upon the size of the mosquito, but it is usually around 2 microlitres. Their bites are not necessarily painful, but they are more noticeable than those from other kinds of mosquitoes. Tiger mosquitoes generally tend to bite a human host more than once if they are able to.^[19][21]

Aedes albopictus also bites other mammals besides humans; they also bite birds.^[19][21] The females are always on the search for a host and are persistent but cautious when it comes to their blood meal and host location. Their blood meal is often broken off short without enough blood ingested for the development of their eggs. This is why Asian tiger mosquitoes bite multiple hosts during their development cycle of the egg, making them particularly efficient at transmitting diseases. The mannerism of biting diverse host species enables the Asian tiger mosquito to be a potential bridge vector for certain pathogens that can jump species boundaries, for example the West Nile virus.

Natural enemies[edit]

Primarily, other mosquito larvae, flatworms, swimming beetles, fungi, ciliates, paramecia, protozoans which act as parasites, predatory copepods and spiders are natural enemies of the larval stage of Asian tiger mosquitoes.

Toxorhynchites speciosus larvae (an adult is shown here) feed on the larvae of Aedes albopictus.

Toxorhynchites larvae, a mosquito genus that does not suck blood, feeds upon other mosquito larvae and are often found with tiger mosquito larvae. Flatworms and also small swimming beetles are considered natural predators.^[21]

Fungi from the genus Coelomomyces (Order Blastocladiales) develop inside the visceral cavity of mosquito larvae. The species Coelomomyces stegomyiae was first found on the Asian tiger mosquito.^[21]

Paramecia, or ciliates, can also affect Aedes albopictus larvae, and the first detected species was Lambornella stegomyiae (Hymenostomatida: Tetrahymenidae).^[21] The virulence, mortality rate, and subsequent possibilities of Lambornella being implemented as a biological remedy to control Aedes albopictus, however, has conflicting views.^[22][23]

Sporozoans of the genus Ascogregarina (Lecudinidae) infect the larval stage of mosquitoes. The species Ascogregarina taiwanensis was found in Asian tiger mosquitoes.^[21] When the adult mosquitoes emerge from their pupal case, they leave the infectious intermediary stage of parasites in the water and close off the infection cycle. Infected adults are generally smaller than non-infected adults and have an insignificantly higher mortality rate; therefore, food supply and larval density apparently play a role. In competitive situations, an infection with sporozoans can also reduce the biological fitness of other non-infected mosquitoes. However, the use of the parasites as an effective biological remedy to control mosquito populations is implausible because it is essential that the host reaches the adult stage for the transmission of the parasites.^[24]

Though they do not commonly occur in the natural habitats of Asian tiger mosquitoes, predatory copepods from the Cyclopidae family seem to willingly feed on them given the opportunity.^[21] Relatives of different genera could therefore present a possibility in the control of tiger mosquitoes.^[25]

Predators of adult Aedes albopictus in Malaysia include various spider species. Up to 90% of the gathered spiders from rubber plantations and a cemetery fed upon Asian tiger mosquitoes. Whether the spiders would have an effect on the mosquito population is still unclear. Tiger mosquitoes were abundantly present despite the existence of the spiders.^[26]

Distribution[edit]

Climatic adaptations[edit]

Aedes albopictus eggs

Although Aedes albopictus is native to tropical and subtropical regions, they are successfully adapting themselves to cooler regions. In the warm and humid tropical regions, they are active the entire year long; however, in temperate regions they hibernate over winter. Eggs from strains in the temperate zones are more tolerant to the cold than ones from warmer regions.^[27][28] They can even tolerate snow and temperatures under freezing. In addition, adult tiger mosquitoes can survive throughout winter in suitable microhabitats.^[29]

Invasive species[edit]

The Asian tiger mosquito originally, came from Southeast Asia. In 1966, parts of Asia and the island worlds of India and the Pacific Ocean were denoted as the area of circulation for the Asian tiger mosquito.^[30] Since then, it has spread to Europe, the Americas, the Caribbean, Africa and the Middle East. Aedes albopictus is one of the 100 world's worst invasive species according to the Global Invasive Species Database.^[31]

The Aedes albopictus mosquito is not native to Australia and New Zealand.^[32][33] In fact, the species was introduced there multiple times, but has yet to establish itself. This is due to the well organized entomological surveillance programs in the harbors and airports of these countries. Nevertheless, on the islands in the Torres Strait between Queensland, Australia and New Guinea Aedes albopictus has become domestic.^[34]

In Europe the Asian tiger mosquito first emerged in Albania in 1979, where they were evidently introduced through a shipment of goods from China. In 1990–1991, they were most likely brought to Italy in used tires from Georgia (USA), and since then have spread throughout the entire mainland of Italy as well as parts of Sicily and Sardinia. Since 1999, they have established themselves on the mainland of France, primarily southern France. In 2002, they were also discovered in a vacation town on the island of Corsica, but did not completely establish themselves there until 2005. In Belgium, they were detected in 2000 and 2013,^[35] 2001 in Montenegro, 2003 in Canton Ticino in southern Switzerland and Greece, 2004 in Spain and Croatia, 2005 in the Netherlands and Slovenia, and 2006 in Bosnia and Herzegovina.^[1]

In the fall of 2007, the first tiger mosquito eggs were discovered in Rastatt (Baden-Wuerttemberg, Germany).^[36] Shortly before, they were found in the northern Alps of Switzerland in Canton Aargau.^[37] It is also being sighted increasingly in Malta during summer since 2010.

Asian tiger mosquitoes were first found in North America in a shipment of used tires at the port of Houston in 1985. Since then they have spread across southern USA, and as far up the East Coast as Maine.^[38] This species is an introduced species in Hawaii as well, but has been there since before 1986.

In 1986, the Asian tiger mosquito was discovered in Brazil^[39] and in 1988 in Argentina and Mexico,^[40] as well. Other parts of Latin America where the Asian tiger mosquito was discovered are the Dominican Republic in 1993, Bolivia, Cuba, Honduras, and Guatemala in 1995, El Salvador in 1996, Paraguay in 1999, Panama in 2002, and Uruguay and Nicaragua in 2003.^[41]

In South Africa, the species was detected in 1990.^[42] In Nigeria it has been domestic since at least 1991.^[43] It spread to Cameroon in 1999/2000,^[44] to the Bioko Island of Equatorial Guinea in 2001,^[45] and to Gabon in 2006.^[46]

In the Middle East, the species was detected in the Lebanon in 2003 in Syria in 2005; the first record in Israel was published in 2003.^[47]

Competition with established species[edit]

Aedes albopictus

Aedes albopictus can outcompete and even eradicate other species with similar breeding habitats from the very start of its dispersal to other regions and biotopes.^[48] In Kolkata, for example, it was observed in the 1960s that egg depositing containers were being settled by the Asian tiger mosquito in city districts where the malaria mosquito (genus Anopheles) and yellow fever mosquito (Aedes aegypti) had both been eliminated by the application of DDT.^[49] The reason why, in this case, may be due to the fact that primarily the inner walls of the houses were treated with DDT to kill the mosquitoes resting there and fight the malaria mosquito. The yellow fever mosquito also lingers particularly in the inside of buildings and would have been also affected. The Asian tiger mosquito rests in the vicinity of human dwellings would therefore have an advantage over the other two species. In other cases, where the yellow fever mosquito was repressed by the Asian tiger mosquito, for instance in Florida, this explanation does not fit.^[50][51] Other hypotheses include competition in the larval breeding waters, differences in metabolism and reproductive biology, or a major susceptibility to sporozoans (Apicomplexa).^[52]

Another species, which was suppressed by the migrating Aedes albopictus was Aedes guamensis in Guam.^[53]

The Asian tiger mosquito is similar, in terms of their close socialization with humans, to the common house mosquito (Culex pipiens). Among other differences in their biology, Culex pipiens prefers larger breeding waters and is more tolerant to cold. In this respect, there is probably not any significant competition or suppression between the two species.^[52]

A possible competition among mosquito species that all lay their eggs in knotholes and other similar places (Aedes cretinus, Aedes geniculatus and Anopheles plumbeus) has yet to be observed.

In Europe, the Asian tiger mosquito apparently covers an extensive new niche. This means that there are no native, long-established species that conflict with the dispersal of Aedes albopictus.

Role as disease vectors[edit]

It is known that Aedes albopictus can transmit pathogens and viruses, such as the Yellow fever virus, Dengue fever and Chikungunya fever.^[54]

The Asian tiger mosquito was responsible for the Chikungunya epidemic on the French Island La Réunion in 2005–2006. By September 2006, there were an estimated 266,000 people infected with the virus, and 248 fatalities on the island.^[55] The Asian tiger mosquito was also the transmitter of the virus in the first and only outbreak of Chikungunya fever on the European continent. This outbreak occurred in the Italian province of Ravenna in the summer of 2007, and infected over 200 people.^[56][57] Evidently, mutated strains of the Chikungunya virus are being directly transmitted through Aedes albopictus particularly well and in such a way that another dispersal of the disease in regions with the Asian tiger mosquito is feared.^[58]

The Department of Epidemiology, School of Public Health, University of Alabama at Birmingham, USA, state that on the basis of experimental evidence and probability estimates, it has been concluded that the likelihood of mechanical or biological transmission of HIV by insects is virtually nonexistent.^[59]

The tiger mosquito is also relevant to veterinary medicine. For example, tiger mosquitoes are transmitters of Dirofilaria immitis, a parasitic round worm that causes heartworm in dogs and cats.^[60]

Control and suppression[edit]

Litter in roadside ditches makes an ideal breeding ground for the Asian tiger mosquito

Aedes albopictus has proven to be very difficult to suppress or to control due to their remarkable ability to adapt to various environments, their close contact with humans, and their reproductive biology.

An Ovitrap, a tool for the detection of Asian tiger mosquitoes. Their presence is confirmed through the eggs they lay on the wooden paddle. The brown granules in the water are a Bti preparation that will kill hatching mosquito larvae.

Efficient monitoring or surveillance is essential to prevent the spread and establishment of the species. In addition to the monitoring of ports, warehouses with imported plants, and stockpiles of tires, rest areas on highways and train stations should be monitored with appropriate methods.^[61]

The control of the Asian tiger mosquitoes begins with destroying the places where they lay their eggs, which are never far from where people are being bitten, since they are weak fliers, with only about a 180-metres (200-yard) lifetime flying radius. Locate puddles that last more than three days, sagging or plugged roof gutters, old tyres holding water, litter, bird baths, inlets to sewers and drainage systems holding stagnant water and any other possible containers or pools of standing water. Flower pots, standing flower vases, knotholes and other crevices that can collect water should be filled with sand or fine gravel to prevent mosquitoes from laying their eggs in them. Litter can also hold rain water and should be removed.

Any standing water in pools, catchment basins, etc., that cannot be drained, or dumped, can be periodically treated with properly labeled insecticides or Bacillus thuringiensis israelensis (Bti), often formed into doughnut shaped "mosquito dunks". Bti is a bacterium that produces toxins which are effective in killing larvae of mosquitoes and certain other Dipterans, while having almost no effect on other organisms. Bti preparations are readily available at farm, garden, and pool suppliers.

Flowing water will not be a breeding spot, and water that contains minnows is not usually a problem, because the fish eat the mosquito larvae. Dragonflies are also an excellent method of imposing control. Dragonfly larvae eat mosquito larvae in the water, and adults will snatch adult mosquitoes as they fly.

In any case, an efficient surveillance is essential to monitor the presence of tiger mosquitoes and the effect of control programs. Ovitraps are normally used for the monitoring of Aedes albopictus. They are black water containers with floating Styrofoam blocks or small wooden paddles that are in contact with the surface of the water. Female tiger mosquitoes lay their eggs on these surfaces. Through the identification of these eggs or of the larvae that hatch from these eggs in the laboratory, the presence and abundance of mosquito species can be estimated. Versions of these traps with an adhesive film (sticky traps) that catch the egg depositing mosquitoes make the analysis much easier and quicker, but are more complicated in terms of handling.^[62][63] The results of ovitraps are often variable and depend on the availability of alternative egg depositing waters. Due to this, it is best to use them in large numbers and in conjunction with other monitoring methods.

To date, there are few effective traps for the adult Asian tiger mosquito. Those traps that catch other species of mosquitoes do not catch tiger mosquitoes efficiently. A form of an ovitrap called a lethal ovitrap mimics the breeding site for Aedes albopictus just like the monitoring tool, but it has the added benefit of contained chemicals that are toxic to the mosquitoes when they enter, but do not harm humans. These traps have had success in some countries to control Aedes mosquito populations.^[64] A new trap type has now been shown to catch significant numbers of Aedes albopictus.^[65][66] This device, with the help of a ventilator, produces an upward air current of ammonia, fatty acids, and lactic acids that takes a similar form and smell of a human body. With the addition of carbon dioxide, the efficacy of the trap is increased. This means there is a suitable tool available for trapping adult tiger mosquitoes and, for example, examining the existence of viruses in the trapped mosquitoes. Previously, the mosquitoes had to be collected from volunteers to be studied, which is ethically questionable, especially during epidemics. Recent research also indicates this trap type may also have a use as a control tool; in a study in Cesena, Italy, the amount of biting tiger mosquitoes was reduced in places where traps were installed.^[67]

References[edit]

Further reading[edit]