The following information comes from an article written by Deborah Weisberg for the environment section of The New York Times.
Exotic pests found in imported nursery stock poses an increasing threat to the environment. The hemlock wooly adelgid, which has turned woods from green to brown in many areas of the United States arrived several decades ago on a single hemlock imported for a Virginian’s Japanese-style garden. And as news accounts in Massachusetts have covered, emerging threats, like the Asian longhorned beetle, are presenting new headaches.
The federal Agriculture Department, which is responsible for inspecting imported plants, says that the sheer volume arriving from offshore makes inspection of each one impossible. The International Plant Protection Convention recently advised signatory countries to be more vigilant about the health of plants traded on the global market. However, a comprehensive, integrated approach would most likely take years, if not decades.