The Asian Tiger Mosquito in Maryland

The Asian tiger mosquito (Aedes albopictus) is an exotic species introduced to North America from Asia and has become a major pest and threat to public health in Maryland. From an initial discovery in Baltimore City in 1987, the tiger mosquito has extended its known range to all Maryland counties except Allegany and Garrett.

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How did it get here? || Biology of the Tiger Mosquito || Tiger Mosquitoes as pests || Public Health importance of the Tiger Mosquito || Surveillance of the Tiger Mosquito || Control of the Tiger Mosquito in Maryland

How Did it Get Here?

The tiger mosquito is native to Asia. It is believed the species spread to the Western Hemisphere as a result of the international trade in used tires. The United States imports millions of tires from Asia due to the high rubber content of Asian tries, for remanufacturing purposes. Tiger mosquitoes are closely associated with used tires, which are used as sites for egg deposition and larval development. It is likely that tires imported to Houston, Texas from Japan in 1985 brought tiger mosquitoes to the United States. The trade of used tires within the U. S. has dispersed the mosquito throughout the eastern and midwestern states.

The tiger mosquito has thrived in the urban and suburban environment of Maryland. The initial discovery of tiger mosquitoes in Maryland occurred in 1987 in the City of Baltimore at a used tire processing plant. From there, it spread to nearby communities where buckets, cans, flower vases and many other artificial water holding containers were as suitable as tire casings for breeding sites. Many communities in Maryland which experienced very little mosquito annoyance in the past are now infested by tiger mosquitoes. The tiger mosquito does best in residential areas where shade and water-holding containers are common. It is found in all neighborhoods, from the poorest to the most affluent. Older residential areas with a good deal of shade are preferred sites. Areas near commercial establishments which store a large number of tire casings outside are often infested with the greatest number of tiger mosquitoes.

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Biology

Female tiger mosquitoes are the sex of most concern to humans because, as in the case for all mosquitoes, only females bite. The reason for the particular lust for blood by the female mosquitoes is the drive to reproduce. Blood is a rich source of protein which nourishes mosquito egg development and has since the age of the dinosaurs. Blood is not a food to sustain mosquito physiology aside from ovarian development. Carbohydrates from flower nectar fuel the daily activity of male and female mosquitoes.

Female tiger mosquitoes seek water-holding containers in which to lay their eggs. Any container from a tire casing to a tree hole is a possible breeding site, but this mosquito has preferences. Outdoor containers are greatly preferred over indoor containers and outdoor containers in the shade are preferred over those in full sunlight. Containers holding dark stained water high in organic content are preferred over containers holding clear, clean water.

Eggs are deposited along the sides of a container, just above the water surface. The rate of hatching success increases if the eggs remain unflooded for a few days after being laid and the eggs can remain viable for long periods before flooding, such as during prolonged droughts. The eggs are stimulated to hatch when the water level in the container rises and floods the eggs, provided the water temperature is above 60¤F. If colder water temperatures prevail, the eggs will not hatch, but can remain viable for long periods (overwinter) until warmer temperatures return. After hatching, mosquito larvae live in the water for one to several weeks, depending on water temperature and the amount of food present.

Immature mosquitoes go through four growth stages and molt their skins four times as their size increases. The last immature stage is known as the pupa. In the pupal stage, changes occur allowing the transformation from an aquatic larva to a terrestrial, free-flying adult mosquito. During the summer, the immature life stage typically lasts five to ten days.

Mating takes place shortly after adults emerge from breeding sites. Females mate only once in their lifetime. Sperm is stored in the females' bodies and they can lay fertile eggs several times during a life span. Two to three days after emergence, female mosquitoes take their first blood meal. Tiger mosquitoes rest, fly and bite close to the ground. They bite in the daytime, rarely at night. Early morning and late afternoon are peak biting times. Tiger mosquitoes are strongly attracted to bite humans, but will feed on cats, dogs and other mammals, as well as birds active on the ground. They will bite any exposed skin surface, but prefer to feed around the ankles and knees. They bite outdoors and indoors, but are usually found outside. On average, tiger mosquitoes ingest 2 - 6 milliliters of blood per bite.

Female tiger mosquitoes lay 40 to 150 eggs after obtaining a blood meal. The cycle of blood feeding and egg laying will continue throughout the mosquito's life span. Egg laying occurs about once per week. The maximum number of eggs laid per lifetime by female tiger mosquitoes is about 300.

Adult tiger mosquitoes live from a few days to several weeks, largely depending on weather conditions. Hot, dry weather reduces life expectancy. Regardless of life span, adult tiger mosquitoes seldom move far from the containers in which they were born. Most adults will be found within a few hundred yards of the breeding container.

In Maryland, tiger mosquito eggs are present year round. Larvae are present from April through October. Adult tiger mosquitoes are found May through October. The period of peak population is June through September.

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Tiger Mosquitoes As Pests

The tiger mosquito has become the primary pest species in many areas of Maryland where, before the arrival of the tiger, mosquito problems were slight. The intensely developed areas of Baltimore City, northern Anne Arundel County, Prince George's County, etc., provide little wetland habitat essential for most mosquito species to thrive. However, these urban areas are highly suited for the container breeding tiger mosquito. Breeding sites range from industrial yards with outdoor storage of tires and steel drums to affluent residential areas where outdoor statuaries and the pans beneath outdoor plants are common. If a Marylander is bothered by tiger mosquitoes, it is likely that the mosquitoes were produced in his/her yard or the properties adjacent to it.

Adult tiger mosquitoes are medium sized, black in color with distinctive white stripes. This color pattern is the basis for the name "tiger." Tiger mosquitoes are persistent, moderately aggressive biters. They prefer to feed on the lower legs. The mosquito is very agile and can be difficult to kill with a casual slap. Young children playing outdoors during the early morning or late afternoon in shaded areas are particularly vulnerable to being bitten as they are often sitting or crawling on the ground and, being distracted by play, do not notice the mosquitoes. The bite of the tiger mosquito is not painful and often goes unnoticed. Interrupted feeding is common and a female mosquito may bite the same person several times or move from person to person before the urge to bloodfeed is satiated.

The itch and swelling of mosquito bites is caused by an anticoagulating substance from the mosquito's salivary glands injected into the bite wound to maintain a free flow of blood while she feeds. The itching sensation occurs soon after the mosquito bite and may persist for a few days. Scratching the bite can lead to secondary infection, especially on young children.

People's tolerance to mosquito annoyance is variable. For some, any mosquito annoyance is too much, but others tolerate large numbers without complaints. In 1932, Dr. Thomas Headlee established "by experience" that most people in New Jersey complain if mosquito bites exceed one in fifteen minutes. In Florida, an extensive public opinion survey found that a bite rate of one per twelve minutes caused most people to rate the problem as "moderate" and one mosquito bite in one minute was a "bad" outbreak. In Minnesota, an annoyance threshold of five mosquitoes in five minutes is used for landing rate counts. The mosquito annoyance threshold in Maryland is one mosquito bite in one minute.

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Public Health Importance of Tiger Mosquitoes

Tiger mosquitoes are known to transmit the causative agent of dog heartworm disease. In New Orleans, the tiger mosquito is a principal vector of dog heartworm. In Polk County, Florida, field populations of tiger mosquitoes were found to carry eastern equine encephalitis virus in 1991. In Asia, this species is a vector of dengue fever and Japanese encephalitis. Laboratory studies have found the tiger mosquito to be an efficient vector of many viral disease agents including yellow fever, West Nile virus, St. Louis encephalitis and LaCrosse encephalitis.

In 2001, tiger mosquitoes collected in Maryland, New Jersey and Pennsylvania tested positive for West Nile virus. Aedes albopictus is considered to be a vector of West Nile virus in Maryland.

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Surveillance of Tiger Mosquitoes

Adult tiger mosquitoes are not readily attracted to standard light traps which are used for determining the population level of most Maryland mosquito species. Traps using carbon dioxide as an attractant are useful for monitoring population trends of adult tigers. The most efficient and widely used surveillance technique in Maryland is the landing rate count. Landing rate counts are taken by inspectors using themselves as "bait" to attract female tiger mosquitoes. As they land on the inspector to bite, mosquitoes are identified, killed and tallied. Counts are taken for two to five minutes, during which the inspector tallies the total number of mosquitoes landing. Inspectors wear dark colored clothing and, of course, are not allowed to use mosquito repellent. Trap collections and landing rate counts are taken between the hours of sunrise to sunset when tiger mosquitoes are most active.

Larval surveillance is carried out by visual inspection of containers and by dipping. The larvae are easily disturbed by vibration or shadows passing over their surface, and either event will send the larvae to the bottom of the container where they are difficult to find.

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Control of Tiger Mosquitoes in Maryland

Control of tiger mosquitoes by conventional methods in the United States has proven to be difficult. The impact of several predators and parasites as biological control agents of larvae has been investigated. In general, these agents have been found to play a small role in regulating the number of mosquitoes but not a significant impact.

The most promising predators of tiger mosquito larvae are mosquitofish (Gambusia spp.) and cannibal mosquitoes (Toxorhynchitus spp.). Fish are very effective when stocked in cisterns, water barrels and ornamental ponds, but many of the breeding sites of tiger mosquitoes are so small and cryptic as to make the use of fish of limited value.

Cannibal mosquitoes are predaceous as larvae on a wide range of aquatic organisms, including mosquito larvae. These mosquitoes are also container breeders and would seem to be an ideal candidate species as a biocontrol agent of tiger mosquitoes. A large scale "cannibal mosquito" project was initiated in New Orleans, but had minimal success in controlling tiger mosquitoes. A smaller scale project was tried in Maryland with the same result. In New Orleans and Maryland, Toxorhynchites mosquitoes were raised in the laboratory and adults were released in areas infested with tiger mosquitoes. In theory, these mosquitoes would find containers, lay eggs, and the cannibal mosquito larvae would eat tiger mosquito larvae. A fine idea, but it did not work. In Maryland, no reproduction of Toxorhynchites in the release area could be documented. The project has been discontinued.

Tiger mosquito larvae are susceptible to the toxic spores produced by the bacteria Bacillus thuringiensis israelensis (Bti). The insect juvenile hormone mimic methoprene does not kill tiger mosquito larvae, but prevents maturation to adult mosquitoes. The problem of controlling tiger mosquitoes with Bti and methoprene is how to deliver the products to the breeding sites. Due to the large number and cryptic location of breeding sites, application of larvicides is labor intensive and beyond the resources of public agency mosquito control programs.

Control of adult tiger mosquitoes by various insecticides can be effective, providing temporary relief from biting annoyance and can reduce the risk of disease transmission. Spraying is most effective when done during early evening (one hour before to two hours after sunset) and early morning (two hours before to one hour after sunrise). Those mosquitoes killed by spraying can be replaced by newly emerged adults because of the rapid breeding cycle of the tiger mosquito. In communities infested by moderate to high populations of tiger mosquitoes, adult mosquito control spraying may be necessary once per week, or more frequently, from June through September.

The most effective method of controlling tiger mosquitoes is reducing or eliminating the containers which are the source of the problem. Draining or removal of water holding containers, even on a localized basis, will produce remarkable long-term reductions in mosquito annoyance. The list of breeding sites is extensive and includes any water holding containers, but the primary sites in residential areas include clogged rain gutters, tires, buckets, cans, bottles, boats, flower pots, bird baths, outdoor statuary, ornamental pools, plastic or canvas tarpaulins, children's toys, rain barrels, and pet food and water dishes.

It is estimated that over 100,000 residential properties in Maryland provide breeding sites for tiger mosquitoes. Public mosquito control agencies do not have the resources or the legal authority to remove and drain mosquito breeding containers over such a large area.

The elimination of the breeding containers for tiger mosquitoes is largely the responsibility of the individual to conduct thorough and repeated efforts to remove or drain all such containers on his/her property. On an individual basis, this is not a large task. The original cleanup of containers on a residential area should take no more than a few hours and periodic maintenance to keep each yard free of breeding containers will require a minimal time investment by individual residents.