The Asian Tiger Mosquito is a serious vector for many human diseases. (Photo: U.S. Department of Agriculture)

Invasive species have arrived in the United States through a multitude of means, including introductions by early human settlers who seek reminders of their homelands, to importation of ornamental plants, to introductions by government agencies to combat some other problem (often an agricultural one), to an expanding global trade enterprise that inadvertently allows the rapid spread of species. Modern trade has greatly increased the spread of a number of species. Asian tiger mosquitoes hitchhike into new areas in rainwater pools in discarded tires and even aboard water-filled depressions on ship structures. This mosquito is associated with the transmission of many human diseases, including dengue virus, West Nile virus, and Japanese encephalitis (Global Invasive Species Database 2006).

Ship ballast, typically water pumped into a ship's tanks at one port and pumped out at another, is used to balance the weight and control the steerage of freight vessels and is a well-documented vector. The most noted species introduced by ballast is the zebra mussel. Zebra mussels (Dreissena polymorpha) are native to the Caspian Sea, but long ago began spreading throughout much of Europe. In 1988, they were detected in the Great Lakes where they had caused serious problems by out-competing native species for food and damaging harbors, boats, and power generation plants.

In some cases, the military itself unintentionally may have been responsible for the spread of invasive species. While it is difficult to pinpoint the precise time, location, and cause of introduction, there is speculation that the military introduced the brown tree snake to Guam, African iceplant to the San Francisco Bay area, black rats to the Midway Islands, and sakosia shrubs (Timonius timon) to Palau. The military has taken a leadership role to reduce future unintentional introductions. The Armed Forces Ballast Water Management Program, which requires DoD vessels to twice flush ballast water at least twelve nautical miles from shore, should be used as an example to commercial vessels. Transportation policy and procedures rules already require the washing of vehicles after field operations. The primary purpose is to extend the life of field equipment, but it also has a secondary purpose of reducing hitchhiking foreign pests from entering U.S. borders.²

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