predicting the species that could arrive and their means of arrival in the future. Historical accounts of introductions of nonindigenous species are invaluable in identifying the forces that facilitate the arrival of plants and plant pests, and they provide a basis for predicting the entry of other detrimental organisms. The implications of what we understand about pathways and modes depend on whether we are considering nonindigenous plants, arthropods, or plant pathogens. A chief distinction is that plants are usually introduced deliberately; arthropods and plant pathogens, except those introduced as agents of biological control or for confined research or public display (for example, in insectaria), are almost always introduced by accident. That difference has important implications for developing comprehensive predictors of the modes and pathways of entry.
The little we know about the arrival of plants and plant pests in North America and elsewhere by natural means is largely anecdotal (Ridley 1930). Clues to the likelihood and frequency of natural arrivals in the United States from distant places can be deduced from Smith’s (1999) accounts of seeds that have drifted onto Australian beaches; such landfalls can generally be predicted from tides and currents. The numbers of nonindigenous species and their seeds that arrived in Australia were small, especially in comparison with the native species that drifted onto shore.
Natural dispersal of arthropods and plant pathogens over long distances does occur. Facilitated by migrating birds, mammals, and insects and by air and water currents, natural transport is best documented for plant pathogens. For example, Pectobacterium (=Erwinia) carotovora, which causes soft rot of fruits and vegetables, has been found in surface water, rain, clouds, and snow (Hirano and Upper 1990). Many fungal pathogens of plants produce enormous numbers of spores that are passively carried by wind over long distances (Nagarajan and Singh 1990). Yellow stripe rust of barley, caused by the fungus Puccinia striiformis f. sp. hordei, might have spread as windborne spores from South America to Texas via Mexico. Other examples of long-distance transport of rust fungi within Europe and between Africa and both Australia and South America have been documented. Spores of Peronospora tabacina, the cause of blue mold of tobacco, have been trapped over ocean and polar latitudes far from their source (Main et al., 1998). Whiteflies are vectors for several types of plant viruses; if carried by air currents, they might be a means of arrival of a virus (Polston and Anderson 1997, Polston et al. 1999). For example, Blair et al. (1995) speculate that bean golden mosaic virus might have been introduced via its whitefly vector from the Caribbean into the Homestead area of Florida by Hurricane Andrew. Similarly, the pink hibiscus mealybug (Maconellicoccus hirsutus), an Asian pest, was recently found in the Caribbean and is expected to be carried eventually to Florida by hurricanes (NAPIS 2000).