Source: https://www.federalregister.gov/documents/2009/07/23/E9-17535/importation-of-plants-for-planting-establishing-a-category-of-plants-for-planting-not-authorized-for
Timestamp: 2018-09-23 18:04:50
Document Index: 162273140

Matched Legal Cases: ['art 319', '§\u2009319', 'art 360', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', 'art 319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', 'art 360', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', 'art 360', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', '§\u2009319', 'art.\n2']

Federal Register :: Importation of Plants for Planting; Establishing a Category of Plants for Planting Not Authorized for Importation Pending Pest Risk Analysis
A Proposed Rule by the Animal and Plant Health Inspection Service on 07/23/2009
36403-36414 (12 pages)
E9-17535
https://www.federalregister.gov/d/E9-17535 https://www.federalregister.gov/d/E9-17535
Start Preamble Start Printed Page 36403
Federal eRulemaking Portal: Go to http://www.regulations.gov/​fdmspublic/​component/​main?​main=​DocketDetail&​d=​APHIS-2006-0011 to submit or view comments and to view supporting and related materials available electronically.
The regulations in 7 CFR part 319 prohibit or restrict the importation of certain plants and plant products into the United States to prevent the introduction of plant pests that are not already established in the United States or plant pests that may be established but are under official control to eradicate or contain them within the United States.[1] The regulations in “Subpart—Nursery Stock, Plants, Roots, Bulbs, Seeds, and Other Plant Products,” §§ 319.37 through 319.37-14 (referred to below as the regulations, and often referred to colloquially as the “Quarantine 37” regulations), restrict, among other things, the importation of living plants, plant parts, seeds, and plant cuttings for planting or propagation. The regulations in 7 CFR part 360, “Noxious Weed Regulations,” contain restrictions on the movement of noxious weeds or plant products listed in that part into or through the United States and interstate; the importation of some plants is subject to both the nursery stock regulations and the noxious weed regulations.
To refer to the articles subject to the nursery stock regulations collectively in this document, we will use the term “plants for planting,” which the International Plant Protection Convention (IPPC) defines as: “Plants intended to remain planted, to be planted or replanted.” Planting is defined by the IPPC as: “Any operation for the placing of plants in a growing medium, or by grafting or similar operations, to ensure their subsequent growth, reproduction or propagation.”[2] In a final rule published in the Federal Register on August 6, 2007, and effective September 5, 2007 (72 FR 43503-43524, Docket No. 03-002-3), we added a definition of plant to the regulations that is drawn from the Plant Protection Act and includes any plant (including any plant part) for or capable Start Printed Page 36404of propagation, including a tree, a tissue culture, a plantlet culture, pollen, a shrub, a vine, a cutting, a graft, a scion, a bud, a bulb, a root, and a seed. We consider the term “plants for planting” to include all the articles subject to the nursery stock regulations, and thus to be equivalent to the term “nursery stock” as it is used in the current regulations.
Plants for planting that cannot be feasibly inspected, treated, or handled to prevent quarantine pests that may accompany them from being introduced into the United States are listed in § 319.37-2(a) or (b) of the regulations as prohibited articles. Prohibited articles may not be imported into the United States, unless imported by the U.S. Department of Agriculture (USDA) for experimental or scientific purposes under safeguards specified in the permit issued for the importation of the articles.
Plants for planting that can be inspected, treated, or handled to mitigate the risk of introduction of quarantine pests associated with the importation of the plants into the United States are referred to in the regulations as restricted articles. Restricted articles may be imported into the United States if they are imported in compliance with conditions that may include permit and phytosanitary certificate requirements, inspection, treatment, postentry quarantine, or combinations of these safeguards. Except for certain bulbs from the Netherlands, Canadian greenhouse-grown plants, small lots of seed, and certain seeds from Canada (as described in § 319.37-4(a)(4), (c), (d), and (e), respectively), the regulations require that a phytosanitary certificate issued by the exporting country's national plant protection organization (NPPO) accompany all restricted articles imported into the United States.
All plants for planting imported into the United States must be presented for inspection. Plants for planting that are required to be imported under a written permit under § 319.37-3(a)(1) through (a)(6) and that are not from Canada must be imported or offered for importation at a USDA plant inspection station. Such stations are listed in § 319.37-14. Plants for planting that are offered for inspection at a USDA plant inspection station are inspected by Plant Protection and Quarantine (PPQ) inspectors. Plants for planting that are not required to be inspected at a USDA plant inspection station may be presented for inspection either at one of the ports listed in § 319.37-14 or at a Customs designated port of entry indicated in 19 CFR 101.3(b)(1). Such plants are inspected by the Department of Homeland Security's Bureau of Customs and Border Protection.
Currently, the regulations categorize imported plants for planting as either prohibited articles or restricted articles. We are proposing to create a new category of plants for planting whose importation is not authorized pending the completion of a pest risk analysis. We will refer to the category in this document as the “not authorized pending pest risk analysis” (NAPPRA) category. The NAPPRA category would include two lists: A list of taxa that we have judged, on the basis of scientific evidence, to be potential quarantine pest plants, and therefore potential noxious weeds; and a list of taxa that we have judged, on the basis of scientific evidence, to be potential hosts of quarantine pests.[3]
We first notified the public that we were considering establishing a new category of imported plants for planting in an advance notice of proposed rulemaking (ANPR) that we published in the Federal Register on December 10, 2004 (69 FR 71736-71744, Docket No. 03-069-1).[4] This ANPR presented several possible changes that we were considering to revise the plants for planting regulations, one of which was implementing the NAPPRA category. (In the December 2004 ANPR, the NAPPRA category described above was called “Option 2” for establishing a category of plants excluded from importation pending risk evaluation and approval; we have changed the terminology we are using in this proposal in an effort to improve clarity. Option 1 in the December 2004 ANPR was to exclude from importation into the United States all taxa that were not currently being imported in significant amounts; we are not proposing to implement Option 1 or requesting comment on it in this document.)
As we discussed in the December 2004 ANPR, the only pest risk mitigation measures required for the importation of most taxa of plants for planting are that they be accompanied by a phytosanitary certificate and that they enter the United States through a USDA plant inspection station, at which the plants for planting are sampled and visually inspected. The Plant Protection Act provides APHIS with the authority to require individual shipments of plants for planting to be treated, destroyed, or reexported if inspectors find quarantine pests in the shipments. However, this inspection may not always provide an adequate level of Start Printed Page 36405protection against quarantine pests, particularly if a pest is rare, small in size, borne within the plant, or an asymptomatic plant pathogen.
The National Plant Board's 1999 “Safeguarding American Plant Resources” report [5] contrasted the approach of the regulations governing the importation of plants for planting with the approach of the regulations governing the importation of fruits and vegetables, which are found in “Subpart—Fruits and Vegetables” (§§ 319.56-1 through 319.56-49) within 7 CFR part 319.
By contrast, the importation of fruits and vegetables is generally prohibited under the regulations in “Subpart—Fruits and Vegetables,” and the completion of a PRA is generally required before a commodity can enter from a new area. The process of allowing the importation of a fruit or vegetable from a particular area or country begins when APHIS receives an import request from an exporting country or when there is a request to reconsider the entry status of a commodity previously denied entry. The request must be accompanied by information about the commodity proposed for importation into the United States, shipping information, a description of pests and diseases associated with the commodity, and current strategies for risk mitigation or management, as described in § 319.5.
The North American Plant Protection Organization (NAPPO), a regional plant protection organization recognized by the IPPC, coordinates efforts among the NPPOs of Canada, the United States, and Mexico to protect their plant resources from the entry, establishment Start Printed Page 36406and spread of regulated plant pests, while facilitating intra- and interregional trade. In its Regional Standard for Phytosanitary Measures No. 24, NAPPO examined the regulatory issues associated with international trade in plants for planting. The standard ultimately concluded that “current regulatory measures are insufficient to ensure adequate protection for NAPPO countries in today's trading environment.” [6] The standard called for regulatory officials, the horticulture industry, and the environmental community from Canada, the United States, and Mexico to adopt more effective regulations to mitigate the risk of pest introductions on plants for planting.
Based on the increased diversity and volume of plants currently being imported, we have determined that the current regulations need to be enhanced to provide a level of phytosanitary protection commensurate with the risks posed by the importation of plants for planting. For this proposal, APHIS has prepared a risk document, “Foundation Document Demonstrating the Risk Basis for Establishing the Regulatory Category ‘Not Authorized Pending Pest Risk Analysis’ (NAPPRA) Associated with the Importation of Plants for Planting,” which analyzes current trends in the importation of plants for planting and the general risks associated with plants for planting.[7] It concludes, “The risk associated with imported plants is considered, by APHIS, to be higher than other pathways, e.g., imported fruits and vegetables. Because they are normally placed in conditions that encourage growth, plants serve as long-term hosts to the pests that they carry and therefore increase the probability that these pests will establish, and spread. In addition, the importation of plants that develop invasive or other harmful characteristics is particularly dangerous because the original intent of importation was to introduce and spread the plant. * * * Evidence indicates that while the original assumptions and designs for Quarantine 37 and the noxious weed regulations may have been valid when the challenges to the system were less intense, the contemporary situation is orders of magnitude more challenging.”
While a comprehensive PRA is necessary to determine all the quarantine pests that may be associated with a taxon and, if appropriate, offer means to mitigate the risk associated with these pests, the scientific evidence we would use to add a taxon to the NAPPRA category would be sufficient to establish that the taxon is a quarantine pest or is a host of a quarantine pest. This proposal would provide the public with the ability to request that a PRA be Start Printed Page 36407conducted for any taxon that we add to the NAPPRA category.
The regulations currently do not contain definitions of the terms noxious weed, official control, planting, plants for planting, quarantine pest, and taxon. (The concept of official control is part of the IPPC definition of quarantine pest.) Therefore, we are proposing to add definitions for these terms to the “Definitions” section in § 319.37-1.
We would add a definition of noxious weed based on the definition of that term in the Plant Protection Act. The definition of “noxious weed” in the Plant Protection Act refers to nursery stock rather than plants for planting; the definition we would add in § 319.37-1 would refer to plants for planting, to be consistent with the other changes we are making to the regulations. Thus, the definition of noxious weed would read as follows:
The definition of quarantine pest that we are proposing refers to “a plant pest or noxious weed” rather than “a pest.” Such an approach is consistent with our authority under the Plant Protection Act, which specifically refers Start Printed Page 36408to plant pests and noxious weeds. It is also consistent with the IPPC definition, since the IPPC definition of “pest” includes plants as well as animals and pathogenic agents. (The Plant Protection Act definition of noxious weeds includes references to the weed's impact on agriculture, natural resources, public health, and the environment, among other things, while the IPPC definition of quarantine pest itself refers only to economic importance. However, Appendix 2 to the Glossary of Phytosanitary Terms explains that the term “economic importance” is to be understood as having a broad meaning encompassing potential damage to the natural environment as well.)
In addition to adding a definition of quarantine pest to the regulations, we are proposing to remove the term plant pest and add the term quarantine pest in its place in the regulations. We would also remove the definition of plant pest in § 319.37-1. APHIS takes action on plant pests based on whether they qualify as quarantine pests, in keeping with our commitments under international trade agreements. For example, APHIS typically would not restrict the importation of a taxon of plants for planting because it could introduce a plant pest if that plant pest is already present in the United States and not under official control; such a restriction could be inconsistent with the national treatment principle of the WTO. Therefore, we believe it is appropriate to refer specifically to quarantine pests rather than to plant pests in the plants for planting regulations.
The nursery stock regulations in §§ 319.37 through 319.37-14 currently address only plants for planting that have been determined to be hosts of quarantine pests. Plants for planting that are themselves quarantine pests have been regulated under 7 CFR part 360, “Noxious Weed Regulations.” However, the new definition of quarantine pest that we are proposing includes a specific reference to noxious weeds, and the definition of noxious weed from the Plant Protection Act would be added to the regulations as well, meaning that the definition of quarantine pest would allow us to address both plants for planting that are potential hosts of quarantine pests and plants for planting that are potential noxious weeds (i.e., quarantine pest plants).
We are proposing to add a new § 319.37-2a, “Taxa whose importation is not authorized pending pest risk analysis,” to the regulations to describe the process by which taxa of plants for planting would be added to the lists of taxa whose importation is not authorized pending pest risk analysis (the “NAPPRA lists”), to describe the criteria we would use when determining whether to add a taxon to the NAPPRA lists, and to provide instructions to persons who wish to request that taxa be removed from the NAPPRA lists.
Paragraph (a) of proposed § 319.37-2a would state that we have determined that certain taxa of plants for planting potentially pose a risk of introducing quarantine pests into the United States and that the importation of these taxa is not authorized pending the completion of a pest risk analysis.
There would be two lists of taxa whose importation is not authorized pending pest risk analysis: A list of taxa of plants for planting that are potential quarantine pests, and a list of taxa of plants for planting that are potential hosts of quarantine pests. These lists would be established on the PPQ Web site at http://www.aphis.usda.gov/​import_​export/​plants/​plant_​imports/​Q37.shtml.
This proposed process for adding taxa of plants for planting to the NAPPRA lists would streamline the process of taking action based on sound scientific evidence while providing the public with the opportunity to participate. We Start Printed Page 36409invite public comment on the process we have described.
Paragraphs (c) and (d) of proposed § 319.37-2a would describe the criteria that we would use in determining whether to add a taxon of plants for planting to the NAPPRA category.
Proposed paragraph (c) would state that a taxon will be added to the list of taxa whose importation is not authorized pending pest risk analysis if scientific evidence causes APHIS to determine that the taxon is a potential quarantine pest, as we are proposing to define that term in § 319.37-1.
APHIS specifically requested that WSSA develop the prioritization model to screen taxa of plants for planting that could be quarantine pests and to rank the taxa based on how much potential risk they pose. WSSA has also provided detailed fact sheets on the taxa deemed to pose the greatest risk. We plan to use the information generated by the WSSA to add taxa to the NAPPRA category.[8]
We would also consider using other work that is being done in this area. Several such systems besides the WSSA prioritization model already exist, including models developed by Australia, Western Australia, and the Hawaiian Ecosystems at Risk project. Several university scientists are also studying invasiveness prediction, and some have published articles on various models.[9]
For those sources of scientific evidence for which we have provided examples, it is important to note that the examples are not intended to be exhaustive. For example, we would consider evidence from all peer-reviewed scientific journals in determining whether to add a taxon of plants for planting to the NAPPRA category, not just those we have listed for the purposes of illustration. Similarly, we would consider information from scientific screening systems other than the WSSA's system, provided that we judged those screening Start Printed Page 36410systems to be as rigorous and useful as the WSSA's system.
Articles from peer-reviewed scientific journals. Examples of journals that we might consult are Phytopathology, Plant Disease, Mycologia, Plant Pathology, Journal of Economic Entomology, and Annals of Applied Biology.
Any information available from other APHIS PRAs, particularly PRAs prepared to allow the importation of plants in growing media under § 319.37-8(e) and APHIS fruit and vegetable commodity PRAs. Besides containing a weediness screening component, as discussed earlier, APHIS fruit and vegetable commodity PRAs typically examine the scientific evidence and establishes a list of quarantine pests associated with all parts of the taxon of plants in question, even if not all of the plant would be imported for consumption. For example, while a pest associated with the stem of a plant may not affect importation of the fruit of that plant, it would be useful information in determining how to regulate that plant when it is imported for planting.
Paragraph (e) of proposed § 319.37-2a would state that any person may request that APHIS remove a taxon from the list of taxa whose importation is not authorized pending pest risk analysis. We would encourage persons who submit such a request to provide as much information as possible regarding the taxon and, if the taxon is a potential host of a quarantine pest, any quarantine pests that may be associated with it. It is likely that providing such information would allow us to complete a PRA more promptly than we would otherwise be able to.
The regulations in § 319.37-2(c) provide that articles listed as prohibited articles in paragraphs (a) and (b) of § 319.37-2 may nevertheless be imported if they are imported under a permit for prohibited articles, referred to in the regulations as a Departmental permit. Such articles must be imported by the USDA for experimental or scientific purposes and imported at the Plant Germplasm Quarantine Center or at a plant inspection station and must be labeled with the permit number. The permit must specify conditions for importation that are adequate to prevent the introduction of plant pests into the United States. These provisions exist because scientific and experimental research must be done on plants for planting in order to understand their biology and develop effective mitigation strategies for any risks their importation may pose.
Similar impetus would exist to import articles of taxa on the NAPPRA lists, Start Printed Page 36411and we believe the conditions under which prohibited articles have been allowed to be imported would be effective at mitigating risks associated with importation of taxa on the NAPPRA lists as well. Therefore, we are proposing to amend § 319.37-2(c) to indicate that it would also apply to articles whose importation is not authorized pending pest risk analysis, as listed in accordance with proposed § 319.37-2a.
A similar matter arises in the regulations in § 319.37-12. This section indicates that a restricted article for importation into the United States may not be packed in the same container as a prohibited article. We would amend this requirement to indicate that a restricted article also may not be packed in the same container as an article whose importation is not authorized pending pest risk analysis.
The definition of regulated plant in § 319.37-1 reads: “Any gymnosperm, angiosperm, fern, or fern ally. Gymnosperms include cycads, conifers, and gingko. Angiosperms include any flowering plant. Fern allies include club mosses, horsetails, whisk ferns, spike mosses, and quillworts.” We include a definition of regulated plant in the regulations because the definition of plant is drawn from the Plant Protection Act and does not specify the scope of plants that APHIS regulates in the nursery stock subpart.
Establishing the NAPPRA category would better protect U.S. agriculture and the environment from the introduction of plant pests and noxious weeds into the United States by allowing APHIS to take timely action to prevent their importation. Strengthening our safeguards against these invasive pests is expected to result in far-reaching economic and environmental benefits. In 1999, the National Plant Board reported that introduced invasive plant pests cost about $41 billion annually in lost production and in prevention and control expenses. One study estimates that in U.S. agriculture, noxious weeds cause an overall reduction in crop yield of 12 percent, which translates into a $23.4 billion loss annually.[10] It is important to note that invasive plant pests cause significant control expenses in addition to lost production. As a result of nonindigenous weeds, approximately $3 billion is spent each year on herbicides that are applied to U.S. crops. Pimentel et al. (2000) further estimate that nonindigenous plant pathogens cause $21 billion in U.S. crop losses each year, and that growers spend approximately $500 million annually on fungicides to combat these pathogens. Crop losses to invasive pests and weeds and related control costs contribute to lower levels of domestic production and, in general, higher prices for consumers. Given the current rate of inflation, it is estimated that the introduction of invasive plant pests could cost between $26.0 and 52.5 billion annually in lost production, prevention, and control costs depending on the value of the host crop. Furthermore, reduced crop yields could result in $29 billion in damages annually.[11]
Recent introductions of pests of plants demonstrate the need for proactively addressing the risks of invasive pests and the possible impacts we would avoid or lessen as a result of this proposed rule. For instance, in 2001 a plant pest called the citrus longhorned beetle (CLHB) was imported in a shipment of bonsai maple trees and detected in a Washington State nursery. The resulting response involved quarantining an area having a 1/2-mile radius around the infestation site, destroying about 1,000 trees, injecting surrounding trees with an insecticide to prevent the infestation's spread, and surveying of more than 20,000 trees in the quarantined area. As a result of these efforts, no new CLHB cases have been reported.[12]
Washington State officials responded aggressively to the CLHB introduction in light of the devastation caused on the East Coast by a similar introduced pest, the Asian longhorned beetle (ALB), which is believed to have been introduced via the importation of untreated wood packaging material. The fight to eradicate ALB has persisted for more than 11 years, and has involved the destruction of thousands of beetle-infested trees, and over 230 square miles Start Printed Page 36412have been quarantined, at a total cost of more than $350 million in public funds including APHIS and State obligations. APHIS obligations from 1997 to 2008 total $282 million, including more than $113 million in Commodity Credit Corporation transfers. State obligations for New York, New Jersey, and Illinois during the same time period amounted to nearly $68 million, and an additional $11.6 million was made available for the 2009 fiscal year. APHIS has treated approximately 72,000 trees susceptible to ALB with an insecticide in New York and New Jersey in 2009.
As another example of a pest introduced via the importation of plants for planting, in February 2003 the plant pathogen Ralstonia solanacearum race 3 biovar 2 was detected in geraniums in four greenhouses in Illinois, Indiana, Michigan, and Wisconsin. This plant pathogen was traced back to infected geraniums imported from Kenya. The resulting response cost growers and regulators an estimated $7 million and involved the destruction of over 2 million plants.[13] These are just two examples of the costs incurred due to the introduction of invasive pests that this proposed rule would help to prevent.
2. The heading of the subpart consisting of §§ 319.37 through 319.37-14 is revised to read as set forth above.
3. In § 319.37, paragraph (b) is amended by removing the words “plant pests” and adding the words “quarantine pests” in their place; and by removing the words “plant pest” and adding the words “quarantine pest” in their place.
a. By adding, in alphabetical order, new definitions of noxious weed, official control, planting, plants for planting, quarantine pest, and taxon (taxa).
b. By removing the definitions of nursery stock and plant pest. Start Printed Page 36413
Restricted article. Any plant for planting, excluding any prohibited articles listed in § 319.37-2(a) or (b) of this subpart, any articles whose importation is not authorized pending pest risk analysis under § 319.37-2a of this subpart, and excluding any articles regulated in §§ 319.8 through 319.24 or 319.41 through 319.74-4 and any articles regulated in part 360 of this chapter.
b. In paragraph (c) introductory text, by adding the words “, and any article listed in accordance with § 319.37-2a of this subpart as an article whose importation is not authorized pending pest risk analysis,” after the word “section”.
6. A new § 319.37-2a is added to read as follows:
§ 319.37-2a
Taxa of regulated plants for planting whose importation is not authorized pending pest risk analysis.
(a) Determination by the Administrator. The importation of certain taxa of plants for planting potentially poses a risk of introducing quarantine pests into the United States. Therefore, the importation of these taxa is not authorized pending the completion of a pest risk analysis. Lists of these taxa may be found on the Internet at http://www.aphis.usda.gov/​import_​export/​plants/​plant_​imports/​Q37.shtml. There are two lists of taxa whose importation is not authorized pending pest risk analysis: A list of taxa of plants for planting that are potential quarantine pests, and a list of taxa of plants for planting that are potential hosts of quarantine pests. For taxa of plants for planting that have been determined to be potential quarantine pests, the list includes the names of the taxa. For taxa of plants for planting that are potential hosts of quarantine pests, the list includes the names of the taxa, the foreign places from which the taxa's importation is not authorized, and the quarantine pests of concern.
(d) Criteria for listing a taxon of plants for planting as a potential host of a quarantine pest. A taxon will be added to the list of taxa whose importation is not authorized pending pest risk analysis if scientific evidence causes APHIS to determine that the taxon is a potential host of a quarantine pest. The following criteria must be Start Printed Page 36414fulfilled in order to make this determination:
7. In § 319.37-5, paragraph (i) introductory text is amended by removing the words “plant diseases” and adding the words “quarantine pests” in their place.
§ 319.37-12 Prohibited articles and articles whose importation is not authorized pending pest risk analysis accompanying restricted articles.
A restricted article for importation into the United States may not be packed in the same container as an article whose importation into the United States is prohibited by this subpart or in the same container as an article whose importation is not authorized pending pest risk analysis under § 319.37-2a of this subpart.
2. See the Glossary of Phytosanitary Terms (2007), which is International Standard for Phytosanitary Measures (ISPM) Number 5. To view this and other ISPMs on the Internet, go to http://www.ippc.int/​IPP/​En/​default.jsp and click on the “Adopted ISPMs” link under the “Standards (ISPMs)” heading.
4. The ANPR, as well as the comments we received on the ANPR, can be viewed on Regulations.gov at http://www.regulations.gov/​fdmspublic/​component/​main?​main=​DocketDetail&​d=​APHIS-2004-0024. The ANPR contains a detailed discussion of the history of the nursery stock regulations that is helpful for understanding their original intent and current state.
6. The standard (“Integrated Pest Risk Management Measures for the Importation of Plants for Planting into NAPPO Member Countries”) can be viewed on the Internet at http://www.nappo.org/​Standards/​NEW/​RSPMNo.24-e.pdf.
8. Parker, C., B.P. Caton and L. Fowler. 2007. “Ranking non-indigenous weed species by their potential to invade the United States: `The Parker model.' ” Weed Science 55:386-397.
10. Pimentel, D., L. Lach, R. Zuniga, and D. Morrison. “Environmental and Economic Costs of Nonindigenous Species in the United States.” BioScience 50.1 (2000): 53-65.