Pathways of Exotic Invasives

Many invasive forest plants were intentionally introduced as ornamentals or forage crops (Examples of Intentionally Introduced Invasive Nonnative Weeds), often as a result of Government-sponsored plant introduction programs (Mack and Lonsdale 2001). Some of these plants are still being sold as nursery stock. Herbaceous weeds are more likely to have been introduced as seed contaminants or in soil used as ballast (Reichard and White 2001).

In contrast, most nonnative insects and pathogens were introduced unintentionally as contaminants on nursery stock (U.S. Congress Office of Technology Assessment 1993). The sudden oak death pathogen probably arrived on infected rhododendron (Rhododendron spp.) nursery stock. Its origin is unknown. The American strains of this pathogen cause only small leafspots and twig blight on rhododendron and many other hosts, but cause lethal cankers on oaks in coastal regions surrounding the San Francisco Bay (Rizzo and others 2002). Species killed by the pathogen include coast live oak (Q. agrifolia Nee), tanoak [Lithocarpus densiflorus (Hook. & Arn.) Rehd.], and California black oak (Q. kelloggii Newb.). Nursery sanitation practices and fungicide applications can sometimes mask infection, particularly in the case of Phytophthora species, and may allow infected material to pass inspection. Sometimes an import host is only slightly susceptible to a disease but may harbor the nonnative pathogen, as infected Chinese chestnut (Castanea mollissima Blume) probably harbored chestnut blight. The associated pathogen is unnoticed on the resistant host, but under particularly favorable conditions may sporulate and spread to more susceptible native species. Nurseries with overhead irrigation systems often provide this ideal environment.

Another common source of nonnative insects and pathogens has been the trade in wood and wood products (U.S. Congress Office of Technology Assessment 1993). In the United States, 35 percent of all softwood consumed is imported, and up to 70 percent of all international cargo arrives supported by solid wood packing material. The recent arrival of the Asian longhorned beetle [Anoplophora glabripennis (Motschulsky)] in solid wood packing material has focused attention on this previously loosely regulated pathway. In addition to established populations in New York and Chicago, the beetles have been intercepted in 26 warehouse locations in 12 other States. Solid wood packing material is usually constructed of poor-quality wood, often from trees damaged or killed by pests. Bark remnants increase the likelihood of pest association, and boards with bark attached can be hidden in middle layers of products such as wooden spools. One study found 2,500 live insects in 29 short log bolts used to brace granite blocks in metal containers (Allen 2001).

The particularly invasive nature of many nonnative forest pests first became apparent near the close of the 19^th century. Over the past 100 years, plant pathologists, entomologists, and weed scientists have developed a broadly applicable concept of Integrated Pest Management (IPM) in order to combat these and other pests.