Asian tiger mosquito
Aedes albopictus, the Asian tiger mosquito or the Forest day mosquito is a small black and white mosquito, about 1/4-inch long. Its white and black color pattern gives it the name “Tiger Mosquito”. A white stripe runs down the center of its head and back with white bands on the legs.
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Aedes albopictus is native to the tropical and subtropical regions, primarily inhabiting the Asian continent. Its range extends from New Guinea, islands in Indian Ocean to Madagascar. Population further extends northward through India, Pakistan and China to Seoul, Korea and northern Japan. Interestingly, after World War II, it has expanded its territories eastward toward the Hawaiian and South Pacific islands. By 1985, places like Houston and Texas were found to be colonized by the Asian tiger mosquito. Gradually their population established across the southeastern regions of the United States. In 1987, presence of the Asian tiger mosquito was recorded in Maryland and Delaware (Novak, 1992), and far west as Chicago, Minnesota and Nebraska (Crans, 1996). According to Centers for Disease Control (2001), its population had been established in 866 counties in 26 states in the United States. Their entry into the United States has been primarily through shipment containing scrap truck tires from Asia. Although native to the tropical zones, the species have rapidly adapted themselves to the cooler regions and formed colonies quickly. Unlike other mosquito species which use marshes and swamps for breeding, the Asian tiger mosquito prefers to breed in collections of contained waters. Quite often it is known as a “Container Breeder” because it deposits its eggs in containers with collected waters. According to Crans (1996), this behavior pattern is said to have originated as a forest species that deposited its eggs in water that collected in tree holes and leaf axils of epiphytic plants. Overtime, the mosquito turned into an “ecological generalist” by forming a relationship with humans, thus colonizing newer regions in short spell of time. This association has created a prosperous environment for the Asian tiger mosquito to become an invasive species in newly colonizing areas.
Environmental Impacts and Public Health Implications
When Aedes albopictus invades a new region it out competes and slowly eradicates the established species in the region with similar breeding habitats. A significant decline in yellow fever mosquito population (Aedes aegypti) was observed with increasing population of Aedes albopictus (O’Meara, 2005). This was a result of competitive exclusion probably due to sharing of same breeding habitat by both species and superiority of Aedes albopictus larvae in resource competition and new parasites introduction by Aedes. albopictus. Aedes albopictus is not only a serious pest but also vector of several diseases. It serves as a vector for quite a number of arboviruses. Aedes albopictus is known to be a competent laboratory vector of Alphaviruses such as Chikungunya, eastern equine encephalitis (EEE), Mayaro, Ross River, western equine encephalitis, Venezuelan equine encephalitis and Sindbis. Of these only eastern equine encephalitis (EEE), Cache Valley virus, dengue, St. Louis and LaCrosse encephalitis viruses are known to affect man (CDC, 2001). Reports reveal that outbreaks of chikungunya fever in 2006 in the Andaman and Nicobar Islands, Sri Lanka and Maldives were primarily due to the Aedes mosquitoes. Although Aedes aegypti remains the major vector, Aedes albopictus were found in high density in some of the affected areas. In 2005 – 2006, the Asian tiger mosquito was responsible for spread of Chikungunya in the French Island La Reunion (ProMED, 2006). Aedes albopictus is also a competent experimental vector of the Flaviviruses such as dengue (DEN) serotypes 1, 2, 3, and 4, Japanese encephalitis, West Nile, and Yellow fever viruses. DEN and Japanese encephalitis viruses have been isolated from specimens of Aedes albopictus (Shroyer, 1986). Aedes albopictus has also been involved in the transmission of DEN viruses in Southeast Asia, southern China, Japan, and the Seychelles (Hawley, 1988). According to WHO (1997) Aedes aegypti is the major vector of Dengue fever and Dengue haemorrhagic fever, while Aedes albopictus is an important secondary vector of dengue in the South-East Asia and Western Pacific. Aedes aegypti and Aedes albopictus were abundant and widespread in when an epidemic of dengue haemorrhagic fever occurred Samui Island of Thailand in 1966 and 1967 (Winter et al., 1968). DEN-1 virus has also been isolated from Aedes albopictus larvae in Brazil (Serufo et al, 1993) thus impling that the Asian tiger mosquito is a potential disease vector is the native tropical regions and in the newly infested regions as well.
Management of the Asian tiger mosquito Infestations
Any approach focused towards eliminating the larval population of the species will be a good strategy to check further prolific invasion of this species. Elimination of breeding habitat, biological control and chemical control methods are the most effective ways practiced so far for mosquito abatement. Extermination of breeding habitat, by way of eliminating all water holding devices will greatly bring down the potential of invasions. This includes disposing unwanted water holding containers or stagnant water collections. Water should not be allowed to collect for more that 5 to 7 days on ornamental ponds, bird baths, pet water dishes, over flow dishes for plants, tires, buckets, cemetery vases, rain barrels or clogged gutters. This will not give time for the mosquito larvae to complete its life cycle and develop into adults. Biological control or control employing predators and pathogens is the best strategy to combat this pest population. The guppy Poecilia reticulata and the mosquito fish Gambusia affinis have been effectively used as predators. Bacterial pathogenic compound, Bacillus thuringiensis israeliensis (Bti) has been found to effectively control mosquito larvae. When consumed, the bacteria release toxins. This acts a systemic poison causing paralysis of the midgut, followed by death. However, the only limitation to this control strategy is the cost involved. The pathogen species is non self-perpetuating hence further re-application is required (Pratt and Moore, 1993).
Chemical control of adult and larval forms also reduces the pest population to a considerable extent. Spraying chemical into the air to kill adult mosquitoes is termed adulticiding. Nevertheless, this method is very not economically feasible as it needs re-application. There is also a possibility of adult mosquitoes to migrate back into sprayed areas from unsprayed areas (Pratt and Moore, 1993). This strategy targets only the adult population, thus leaving the larval and pupal population to emerge into new adults in due course. Using a larvicide along with adulticiding can enhance the level of control. Nontoxic insect growth regulating hormone larvicide is widely used for this purpose. Dry-scratched seed kernels of Andiroba are a larvicide against Aedes albopictus larvae (Silva et al, 2004). Alternatively, like in other controlling other mosquito species, a monomolecular, alcohol-based, biodegradable surface film applied to waters containing larvae can eliminate larval population to a greater extent. This would create an impermeable layer on the surface of the water which would prevent the larvae and pupae from coming to the surface to breathe, resulting in suffocation and death.
Another approach to control of the Asian tiger mosquito is by personal protection. Using an insect repellent can keep away mosquitoes. Creating awareness among the public on mosquito breeding habitats and ways to eliminate them can be an additional approach to manage this serious pest.
It is quite clear that the Asian tiger mosquito, given a place can breed exuberantly and infest a region in no time. Hence a regular surveillance is required to combat this nuisance pest and keep eradicate the population. Right control strategies should be followed at the right times to abate this nuisance pest propagation. Economics of the control methods is also an influencing factor. Although chemical control is aimed at abating the larval population to a greater extent, greater cost is also involved. Similarly use of pathogens to combat this mosquito population is not economically feasible as the control agents are non self perpetuating. Re-introduction of species is also a hilarious process. Considering all these, the only economical alternative to handle this menace is by eliminating the breeding habitat. Getting the environment rid of all unwanted water collections materials seems to ideal. This would not only curtail further continuation of the life cycle of this pest and decreasing its population, but also terminate breeding of the other mosquito populations and other disease causing vectors.
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