Exotic species can pose serious problems to ecosystems that are not 'accustomed' to them. In these new environments, there are few or no predators, parasites, or diseases to keep the populations of an invader in check. Sometimes exotic species multiply so quickly that individuals cover all available habitat, literally crowding out other species, as in the case of zebra mussels (Dreissena polymorpha), which settle on all available surfaces. Sometimes an exotic disease organism encounters a native species with no defenses, such as the Chestnut Blight (Cryphonectria parasitica), imported from Europe, which virtually wiped out the American Chestnut (Castanea dentate) in the early part of the 20th century. Sometimes an introduced species out-competes all related species, as in the case of the Red Imported Fire Ant (Solenopsis invicta), which in the Southeast USA has caused not only a decline in native ants species, but also population decreases of ground-dwelling birds, mammals, and reptiles.1
Two insect species whose recent introduction to the United States has generated a lot of concern are the Asian Tiger Mosquito or Forest Day Mosquito (Aedes albopictus), and the Asian Longhorned Beetle (Anoplophora glabripennis). Both these species have entered the U.S. with commercial shipments within the past twenty years.
The Asian Tiger Mosquito or Forest Day Mosquito (Aedes albopictus), native to southern and eastern Asia, is about a quarter inch long, with striped black and white legs and many white spots on its body. Its aggressive biting makes its presence known, especially in the early morning and late afternoon.
||This mosquito was first found in North America near Houston, Texas, in
1985, and is thought to have entered the country with a shipment of used tires.2 Since that time it has spread widely throughout the
United States, especially through the southeast of the country. By 1992 it was considered
to be established as far north as Delaware and Minnesota, and a 1999 survey showed that it
was established in 25 states.3
Photo Source: http://www.mda.state.md.us/mosquito/tigermos.html
Mosquitoes apparently can not transmit AIDS, as the HIV virus is digested in their bodies; however other viruses and protozoa replicate within mosquitoes. In its native range, the Asian Tiger Mosquito is an efficient vector of dengue fever, and laboratory work has shown that it is also an efficient vector of eastern equine encephalitis (EEE) virus.
Dengue is an acute febrile illness characterized by frontal headache, retro-ocular pain, often severe pain in muscles and joints, and rash. While the principal vector is the Aedes aegypti mosquito (which also transmits Yellow Fever), several related species of Aedes, including the Asian Tiger Mosquito, are also vectors.4 Transmission usually occurs in tropical or subtropical areas. Severe manifestations (e.g., dengue hemorrhagic fever and dengue shock syndrome) are rare but may be fatal. Public health specialists are concerned that if dengue is introduced to the USA from the Caribbean area, the presence of this mosquito will make it difficult to eradicate the dengue virus.
The Eastern Equine Encephalitis (EEE) Virus normally cycles between passerine birds (songbirds, which include American Robin and Northern Mockingbird) and the mosquito Culiseta melanura, in forested wetlands.5 This mosquito species feeds exclusively on birds and, thus, does not transmit the EEE virus to mammals such as horses or people. However, other species of mosquito, such as Aedes albopictus, that feed on both passerine birds and mammals, readily transmit this virus to mammals.
EEE is prevalent in many areas where the Asian Tiger Mosquito is establishing itself. In the USA, EEE causes periodic epidemic mortality in horses. Most such outbreaks are small and localized, but some have been large, involving as many as 14,000 horses and mules. Typically, 80%-90% of horses that develop clinical disease die of the infection. The disease has occurred at times in horses in eastern Canada. Captive birds raised for human consumption, hunting, or ranching, such as domestic ducks and turkeys, Ring-necked Pheasants, and Emus occasionally become infected.
Approximately a third of people who develop Eastern Equine Encephalitis die of the disease, and many survivors are permanently incapacitated. However, EEE is not a common human disease; there have been only 153 cases recorded in the United States in the past 35 years, and in Canada, there have been no recognized cases. EEE is not contagious from human to human or horse to horse; infected people and horses have too little virus in their blood streams to infect new mosquitoes.
Female mosquitoes deposit eggs in a water source. While marshes are typical places, the Asian Tiger Mosquito can use small areas of stagnant water in suburban areas including roof rain gutters needing to be cleaned, empty flowerpot saucers, and tin cans. The aquatic mosquito larvae feed on microorganisms, pupate in the water, and emerge as adults a few weeks later. During the warmest part of the summer, adult mosquitoes can emerge less than a month after eggs have been laid. Adults normally live for about two weeks, though some species can persist for up to three months under favorable conditions.6
Sugar is the chief source of energy for mosquitoes, and both males and females feed on sap, nectar, and juices from fruits. Female mosquitoes of most species also require blood to manufacture eggs. A female mosquito pierces the skin of an animal with her mouthparts, injects a small amount of saliva into the resulting wound, and draws blood. The saliva makes penetration easier and prevents the blood from clotting in the narrow channel of her food canal. The welts that appear after the mosquito has finished feeding are an allergic reaction to her saliva. Normally, itching and swelling subside within a few hours; however, some people are so sensitive that the symptoms persist for several days.
The Asian Longhorned Beetle (Anoplophora glabripennis) is another recently naturalized insect from East Asia that is spreading through North America. It is extremely destructive and attacks a wide variety of urban shade trees.
The adult's body length ranges from 20 to 35 millimeters (about 0.75 to
inches). The antennae (feelers) of females are about 1.3 times longer than the body, while those of the males are about 2.5 times longer than the body. The bullet-shaped adult is shining coal black with white spots; the antennae are black with white rings. Most other pest longhorned beetles (family Cerambycidae) are a different size, have a different appearance, prefer different species, and/or have a different mode of attack (i.e., roots rather than trunks).
Photo Source: http://www.aphis.usda.gov/oa/alb/albphotos.html
The Asian Longhorned Beetle apparently entered the United States in solid wood packing material from China; infestations were noted in New York City in 1996 and Chicago in 1998. The beetle has been detected in many other parts of the country, but so far no further established populations are known.7 It has the potential to show up anywhere, making interception and control a problem.
The white larvae bore into tree trunks and branches, provoking heavy flow of sap from wounds, and producing noticeable piles of sawdust at the bases of trees. The adults exit trees via round holes that are about one centimeter (3/8 inch) in diameter or larger. Other signs of the presence of this beetle are yellowing or dropping leaves. Control programs in the Chicago and New York City areas involve locating infested trees, cutting them down, and replanting. Over 5000 trees in five locations in New York State have been cut, while in Illinois about 1450 trees in six locations have been removed.8
In China, the species occupies a wide range of latitudes and environment types, and causes severe damage to trees. It prefers trees that are already stressed (by disease, pollution, drought, etc.). In the USA, the beetle has no natural predators and has been found to attack healthy trees as well as those under stress.
The beetles favorite trees are maples (Acer) and Horsechestnuts (Aesculus), but it has also been found on poplar (Populus), willow (Salix), elm (Ulmus), mulberry (Morus), and black locust (Robinia pseudoacacia), as well as other hardwoods. The maple species include the introduced Norway (Acer platanoides) as well as the native Sugar (A. saccharum), Silver (A. saccharinum), and Red Maples (A. rubrum). The Asian Longhorned Beetle is predicted to severely impact U.S. forest resources and related industries, including the timber, nursery, tourist, and maple syrup industries. According to the USDA, if the beetle becomes established in U.S. forests, it could change the composition of tree species enough to cause significant ecological impacts.
Chances of eradicating these two exotic species from the USA are slim. The Asian Tiger Mosquito looks like it has become a permanent fixture of the USA entomofauna. It is likely to continue its spread in the eastern two thirds of the country, and possibly establish itself along the west coast from introductions via western ports. Its potential as a disease vector is of great concern, for both currently established as well as unestablished diseases for humans and animals. The Asian Longhorned Beetle could be eradicated where it is currently established in the Chicago and New York City areas, though fighting the populations on a tree-by-tree basis is a complicated process. More likely, it will be found in other cities in both the USA and Canada, and may become established elsewhere.
- Some sites for further information on exotic species and their effects:
Invasive Plants of Canada http://infoweb.magi.com/~ehaber/ipcan.html
Overviews of exotic species in North America
- Reiter, P. 1998. Aedes albopictus and the world trade in used tires, 1988-1995: The shape of things to come? Journal of the American Mosquito Control Association. 14(1):83-94.
- Moore, CG, 1999. Aedes albopictus in the United States: Current status and prospects for further spread. Journal of the American Mosquito Control Association. 15(2): 221-227.
- Gleeson, F; McBride, J; Norton, R. 1999. Culture-amplified detection of dengue virus from serum in an outbreak of dengue fever. Journal of Medical Virology. 57(2): 212-215.
More information on dengue: http://www.cdc.gov/epo/mmwr/other/case_def/dengue.html
- More information about EEEV: http://wildlife.usask.ca/bookhtml/arbovirus/arboeee.htm
- The biology of mosquitoes: http://www.rci.rutgers.edu/~insects/mosbiol.htm
- See map: http://willow.ncfes.umn.edu/beetlemaps/alhb_detections.htm