to know more about the plants being introduced into the United States—the origin of the plants, their ecology in their native environments, and any biological characteristics that may contribute to their invasive potential.

Trends That Influence the Unintentional Arrival of Nonindigenous Plant Pests

In the absence of comprehensive information on the location, identity and characteristics of all the plant pests in the world, predictions that plant pests will arrive are based largely on history and on the identification of trends that increase the risk of introductions in the future. For example, we know that international trade in agriculture is a major source of nonindigenous pathogens and arthropods. Of the food consumed in the United States, 10% is imported, and horticultural products, including fruits and vegetables, top the list (USDA/ERS 2001). In addition, human travelers contribute to the inflow of pathogens by arriving with contaminated plants and foods. In one week in May 1990, an inspection of 16,997 passengers arriving on international flights at the Los Angeles International Airport led to the interception of 1357 lots of fruits and vegetables and 325 lots of animal products, for a total of 2635 kg of contraband plant material (U.S. Congress 1993). As with world trade, tourism is increasingly important as a mode of introduction of nonindigenous organisms. During the 1980s and 1990s, tourism became an emerging sector of the economy, contributing $26 billion in 1986; it has since risen to more than $110 billion per year. Over 46.5 million international visitors entered the United States in 1996, and the projected annual growth rate is 3-4% (Doggett 1997).

The spread of plant pests has been facilitated as intercontinental air commerce has flourished. No longer must an immigrant species be conveyed from an interior area to a seaport for embarkation, survive the sea voyage, be unloaded, and then be conveyed to an interior locale. The speed of air traffic has meant that cargo that accidentally or deliberately contains eggs, seeds, spores, or any other living stage of an organism can be transported from almost any point on the globe to the United States within 24-36 hours— well within the survival time of a great array of species, compared with the weeks or months at sea that can be fatal for many species. Such inland cities as Denver, St. Louis, and Dallas are now major points of disembarkation for all manner of air cargo, and a multitude of much smaller cities and even rural areas also receive international flights. This traffic includes agricultural products and increases the possibility that invasive organisms will reach vulnerable locations.

The use of containers for seaborne cargo has seen a steady increase since its introduction in the middle 1960s. In 1992, 9% (by weight) of all U.S. agricultural exports moved in containers; in 1998, the fraction increased to 13% (USDA/ AMS 2001). The universal adoption of containers in shipping might also affect detection of known, invasive species, especially if containers are not opened until



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