Aedes albopictus

Aedes albopictus

The tiger mosquito (Aedes albopictus Skuse, 1894) is an insect belonging to the Culicidae family.

Systematics –
From a systematic point of view it belongs to:
Eukaryota Domain,
Kingdom Animalia,
Sub-kingdom Eumetazoa,
Bilateria branch,
Phylum Arthropoda,
Subphylum Tracheata,
Superclass Hexapoda,
Insecta class,
Subclass Pterygota,
Endopterygota cohort,
Superorder Oligoneoptera,
Panorpoidea section,
Diptera Order,
Suborder Nematocera,
Infraorder Culicomorpha,
Culicidae family,
Subfamily Culicinae,
Aedini tribe,
Genus Aedes,
Subgenus Stegomyia,
A. albopictus species.
The terms are synonymous:
– Culex albopictus Skuse, 1894;
– Stegomyia albopicta (Skuse, 1895);
– Stegomyia nigritia Ludlow, 1910;
– Stegomyia quasinigritia Ludlow, 1911;
– Stegomyia samarensis Ludlow, 1903.

Geographic Distribution and Habitat –
Although Aedes albopictus is an indigenous species in tropical and subtropical areas, it is rapidly adapting to colder regions as well.
This insect, in particular, is native to Southeast Asia but exploiting human commercial transport it has spread to many areas of the world: in the mid-twentieth century it spread to Africa and the Middle East and later to the South American continent, the United States. of America, in Oceania and lastly in Europe.
The first specimens reproduced in Europe were found in Laç, northern Albania in August 1979. The Albanian surveillance group hypothesized that the mosquito had been present at least since 1975 and identified the responsible means in a cargo ship from China docked. at the port of Durres.
In Italy it made its appearance in 1990 in Genoa, in a warehouse of used tires imported from the United States. From here it spread practically throughout the peninsula and is present in many other European countries.
Aedes albopictus is included in the list of the 100 most harmful invasive species in the world drawn up by the IUCN / ISSG.
In warm and humid tropical areas it is active all year round while in temperate regions it hibernates in the form of eggs, resuming the cycle in adult form during spring (in Italy it is active from March to October).
The eggs of varieties found in temperate zones tolerate cold (sub-zero temperatures, snow) better than those found in warmer regions. Furthermore, adult specimens can survive the winter in suitable micro habitats.

Morphology –
The tiger mosquito is an insect that measures from 5 to 10 millimeters in length with the males that are distinguished from the females as they are about 20% smaller even if they have a very similar morphology.
It has a showy black and white stripes.
Like all species of mosquitoes, the antennae of males are much denser than those of females.
The maxillary palps of males are longer than their trunks; instead the maxillary palps of the females are much shorter.

Attitude and Life Cycle –
Aedes albopictus is diurnal and carries out its activity mainly in the late afternoon and early morning.
It is also anthropophilic as it has a marked preference for mankind. It is the most represented species in urban environments, where it finds many places suitable for its reproduction and numerous guests on which to dine.
The females lay their eggs on the walls of containers of various kinds capable of containing water. They prefer small and dark places that simulate their natural environment, represented by small cavities present in the trees of the tropical forests where they come from. In urbanized environments, their choice falls on all those small containers present in gardens and terraces (small amphorae, saucers, rainwater collection pits, gutters, games for children, watering cans), but also along the streets, in parks , squares and streets.
The eggs resist drought and can survive over 6 months in these conditions, but to hatch and thus start the larval cycle they must necessarily be submerged by water. The life cycle consists of 4 distinct phases: egg, larva, pupa and adult. Larvae and pupae can develop only and exclusively in the presence of water. The tiger mosquito has a limited range of action (less than 200 meters), so its spawning sites are close to where its presence is observed.
The life cycle of Aedes albopictus is similar to that of other mosquitoes.
A few days after a single blood meal, necessary for the maturation of the eggs, the females look for a suitable site for oviposition, usually consisting of the internal walls of a container containing or that has contained stagnant water. Most of the eggs are laid a few millimeters higher than the existing surface of the water, where they remain waiting for a level rise. Each female can lay up to 100 eggs (on average about sixty) for each gonotrophic cycle, taking care not to lay them all in the same outbreak. In the most favorable periods from the climatic point of view, the cycle repeats itself every 3-5 days. Each specimen usually lives from 2 to 4 weeks, stinging and oviposing numerous times.
Hatching occurs when the eggs are submerged, for example during a rain. Eggs are able to survive long dry periods in a quiescent state. As for other mosquitoes, aquatic larvae are born from the eggs which, through 4 stages of growth, separated by moults, reach the pupal stage. The adult mosquito flickers after about 48 hours, leaving the pupa’s shell (exuvia) in the water. In our latitudes the entire development cycle can last from 10 days to several weeks, depending on the temperature.
About 48 hours after the flicker, males and females are able to mate. Immediately afterwards, the female can have her first blood meal, while the male, having exhausted his reproductive function, will survive only a few days during which he will never feed on blood. Both males and females derive the energy necessary for their life from sugary juices of vegetable origin.

Ecological Role –
As in other mosquito species, only Aedes albopictus females need to assimilate blood for egg production. They supplement their energy needs with nectar and other sweet vegetable juices, just like males do. In the localization of the host, carbon dioxide, organic substances produced by the host and visual signals play an important role.
This mosquito stings in the daytime. Depending on the region and the biotype, there are different moments of more intense activity. However, it is generally inactive during the central hours of the day and at night. It looks for its prey inside and outside the houses, but is particularly active in the open air.
In addition to humans, Aedes albopictus also stings other mammals and birds. When dealing with a blood meal, the location of the host is very persevering and cautious. The search for a guest takes place in two stages. In a first stage the mosquito exhibits indistinct search behavior. Then, when it perceives the guest, the phase of targeted approach begins.
Often stops the blood meal before having ingested the quantity necessary for the development of the eggs; due to this behavior they can therefore sting various hosts during the egg production cycle. Since they bite hosts of different species, they are particularly apt to transmit diseases from one species to another (for example heartworm from dog to man). The amount of blood sucked with each sting depends on the size of the mosquito, but is usually around 2 microliters.
Long-distance spread of Aedes albopictus occurs primarily via sea routes. Usually shiploads of used tires, lucky bamboo (Dracaena sanderiana) and flowers can carry the eggs, larvae and pupae of tiger mosquitoes. Private cars, trucks and trains are important means of transport at the regional level.
As for the close socialization with humans, the tiger mosquito in Europe is very similar to the common mosquito (Culex pipiens) but in addition to the differences with respect to their biology, those of the species Culex pipiens prefer larger bodies of water for the deposition of the eggs and tolerate cold better. In Europe, Aedes albopictus occupies a new rather large niche that does not overlap with that of native and / or naturalized species, which are thus unable to hinder its spread. A possible competition between the species that lay their eggs in tree trunks (Aedes cretinus, Aedes geniculatus and Anopheles plumbeus) has yet to be established. These species, however, do not seem to prefer environments populated by humans.
As regards the health hazards, it should be remembered that since Aedes albopictus is particularly aggressive towards humans, the fact of being active during the day and the ability to develop in urban environments, therefore in close contact with humans, can transmit nematodes such as heartworm and viruses responsible for diseases such as yellow fever, St. Louis encephalitis, dengue, chikungunya and zika.
This mosquito has been responsible for various epidemics over time.
Furthermore, the tiger mosquito is also important in veterinary medicine as it is a vector of parasitic worms of the genus Dirofilaria, agents of cardiovascular heartworm in dogs and cats.
Tiger mosquito containment methods are based on the use of carbon dioxide and a combination of chemicals naturally present on human skin (fatty acids, ammonia, lactic acid) which give the best results.
In addition, other mosquito larvae, flatworms, swimming beetles, fungi, ciliates, paramecia, parasitic protozoa, predatory copepods and spiders are natural enemies of the larval stage of Asian tiger mosquitoes.
In particular, the larvae of Toxorhynchites speciosus feed on the larvae of Aedes albopictus.
Toxorhynchites larvae, a genus of mosquito that does not suck blood, feed on other mosquito larvae and are often found with tiger mosquito larvae. Flatworms and small swimming beetles are considered natural predators.
Fungi of the genus Coelomomyces (order Blastocladiales) develop within the visceral cavity of mosquito larvae. The Coelomomyces stegomyiae species was first found on the Asian tiger mosquito.
Paramecia can also affect tiger mosquito larvae and the first species detected was Lambornella stegomyiae (Hymenostomatida: Tetrahymenidae). The virulence, mortality rate and subsequent chances of adopting Lambornella as a biological remedy for Aedes albopictus control, however, still holds conflicting views.
Among the other enemies it is recalled that the sporozoa of the genus Ascogregarina (Lecudinidae) infect the larval stage of mosquitoes.
Other predators have been found in other habitats and their actual uses in biological struggle are being studied.
In general, for the containment and suppression of Aedes albopictus due to its remarkable ability to adapt to different environments, its close contact with humans and its reproductive biology, it has proved very difficult to implement.
However, the surveillance of areas not yet colonized and the monitoring of areas already infested are useful tools in managing the problem. In addition to the surveillance of ports, transport loads of tires, refreshment points on motorways, shops with imported plants and railway stations should be monitored by appropriate methods.
In controlling the tiger mosquito, the elimination or inactivation of the spawning sites, which are never far from where people are bitten, plays an important role. The foci of larval development such as road wells, blocked gutters, old tires containing water, bird troughs, sewer siphons, drainage systems with stagnant water and any other accumulation of water must be identified and inactivated. To prevent mosquitoes from laying their eggs, it is advisable to fill vases, knot holes in trees and other crevices capable of collecting water with sand or fine gravel. Waste can also contain rainwater and must therefore be removed. (Think for example of a can of tuna thrown on the ground or more simply of a lid of a jar. The water it can contain is sufficient for the reproduction of the mosquito)
The places where it is impossible to avoid the presence of water (road drainage, etc.) can be periodically treated with chemical or biological larvicides including Bacillus thuringiensis israelensis (Bti). Bti is a bacterium that produces toxins capable of killing mosquito larvae and certain other dipters, without however being harmful to other organisms. In the face of a lower environmental impact, the BTI is less effective (about 20% less) against Aedes albopictus than that recorded against Culex pipiens.
It should be remembered that the application of insecticides against adult mosquitoes has only a limited effect and instead have contraindications of an ecological nature and on human health that are greater than the remedy itself (ISPRA report 2015). The sprays used at night against mosquitoes have no effect if you do not also treat the resting places of tiger mosquitoes (using the spray during the day can violate the instructions for correct use described on the package when, for example, they are present in the bees in search of pollen). The larvae and eggs are a good reserve for the mosquito population, which usually recover quickly.
The surveillance against these mosquitoes can be carried out through ovitraps which are usually used for the monitoring of Aedes albopictus; they are black containers with water and polystyrene blocks, masonite sticks or small floating pallets. Female tiger mosquitoes lay their eggs on these rough surfaces, just above the surface of the water. By identifying the eggs and larvae that are born in the laboratory from these eggs, the presence and quantity of these species can be estimated. Versions of such traps with a sticky trap, which catch mosquitoes when they are about to lay their eggs, greatly facilitate the analysis and make it faster. However, traps of this type are complicated in terms of use. The results of ovitrap often vary and depend on the possible proximity of other places for laying eggs; in order to compare different samples, the sampling method and the data analysis methodology must then be standardized, taking into account the number of ovitraps found actually active and the number of days of exposure in the field.
Finally, it should be noted that there are few effective traps against the adult tiger mosquito; traps that capture other species do not trap it satisfactorily. However, a new type of trap manages to capture significant quantities of Aedes albopictus; this medium produces, with the help of a fan, an air current impregnated with a mixture of substances (ammonia, fatty acids, lactic acid) which is directed upwards to simulate the shape and odor of the human body . With the addition of carbon dioxide, the yield of the trap can be further improved. This trap is an adequate means of catching adult tiger mosquitoes and also, for example, of examining the existence of viruses in trapped mosquitoes. Before, to study mosquitoes, they had to be collected on the bodies of volunteers. This is ethically questionable, especially during epidemics.

Guido Bissanti

Sources
– Wikipedia, the free encyclopedia.
– Russo G., 1976. Agricultural Entomology. Special Part. Liguori Editore, Naples.
– Pollini A., 2002. Manual of applied entomology. Edagricole, Bologna.
– Tremblay E., 1997. Applied entomology. Liguori Editore, Naples.
Photo source:
https://inaturalist-open-data.s3.amazonaws.com/photos/132243000/original.jpg



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