|Initial damage appears as thinning and dieback in the upper canopy of the tree as larvae feed under the bark they damage the conductive tissue http://www.entomology.wisc.edu/emeraldashborer/ .|
Some plant species are introduced by human activity into new ecosystems at such a rate that they can overcome the odds and establish themselves in the landscape. A few of these plant species do more than just establish they multiply so fast and so aggressively as to crowd out any other plant species creating a biological desert bereft of diversity. Some of these species have become notorious in the last few decades such as kudzu (Pueraria montana (Lour.) Merr. var. lobata (Willd.) Maesen & S. Almeida) and the various salt cedars or, tamarisks (Tamarix spp.); some such as common barberry (Berberis vulgaris L.) in the 18th century or field bindweed (Convolvulus arvensis L.) in the 20th century began their rise to pest status even before we used the term invasive species. Interestingly enough the colony of Connecticut tried to ban barberry in 1726 way before there even was a United States. [Connecticut keeps trying to ban plants. 2010]
These plants are not indigenous to North America and so by extension we begin think all alien non native plants are suspect. The cognitive dissonance that arises when we find out wheat (Triticum spp.) is also not native is disturbing to a native only perspective. Then there is the little problem of impact, benefit and harm when it comes to judging a species place in the ecosystems in which we find ourselves. And just to layer on the complications, the non native exotic plant itself may be harmless but maybe a host for invasive hitchhikers (insects and pathogens) The native ash tree. for exmaple, can be a vector or a transport platform for the non native emerald ash borer (Agrilus planipennis Fairmaire) which destroys both native and non native ash species. [A Menacing Discovery Of the Emerald Ash Borer in Moscow. 2007]
Defining invasive species is the first step to finding solutions. A good resource is the Invasive Species Definition Clarification and Guidance White Paper from 2006 which defines an invasive species as:
“an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.” In the Executive Summary of the National Invasive Species Management Plan (NISMP) the term invasive species is further clarified and defined as “a species that is non-native to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health.” To provide guidance for the development and implementation of the NISMP, the National Invasive Species Council (NISC) and the Invasive Species Advisory Committee (ISAC) adopted a set of principles outlined in Appendix 6 of the NISMP. Guiding Principle #1 provides additional context for defining the term invasive species and states “many alien species are non-invasive and support human livelihoods or a preferred quality of life.” However, some alien species (non-native will be used in this white paper because it is more descriptive than alien), for example West Nile virus, are considered invasive and undesirable by virtually everyone. Other non-native species are not as easily characterized. For example, some non-native species are considered harmful, and therefore, invasive by some sectors of our society while others consider them beneficial. This discontinuity is reflective of the different value systems operating in our free society, and contributes to the complexity of defining the term invasive species.
 Invasive Species Defined in a Policy Context: Recommendations from the Federal Invasive
Species Advisory Committee. 2006. K. George Beck, Kenneth Zimmerman, Jeffrey D. Schardt, Jeffrey Stone, Ronald R. Lukens, Sarah Reichard, John Randall, Allegra A. Cangelosi, Diane Cooper, and John Peter Thompson*