invasive plant species - cast

COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY—1COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY

NUMBER 13 FEBRUARY 2000

INVASIVE PLANT SPECIES

AUTHORS: Barbra H. Mullin (Chair), Montana

Department of Agriculture, Helena, Montana;

Lars W. J. Anderson, USDA-ARS, Exotic and

Invasive Weed Research, University of Califor-

nia, Davis; Joseph M. DiTomaso, Department

of Vegetable Crops, Weed Science Program,

University of California, Davis; Robert E. Eplee,

USDA-APHIS, Whiteville, North Carolina; Kurt

D. Getsinger, U.S. Army Engineer Research and

Development Center, Vicksburg, Mississippi;

Reviewers: Douglas J. Doohan, Department of

Horticulture and Crop Science, Ohio State Uni-

versity, Wooster; William T. Haller , Center for

Invasive and Aquatic Plants, University of Florida,

Gainesville; Robert H. Hedberg, Director of

Science Policy, Weed Science Society of America,

Washington, D.C.; Rodney G. Lym, Depart-

ment of Plant Sciences, North Dakota State Uni-

versity, Fargo; John M. Randall, The Nature

Conservancy, Wildland Weeds, Management and

Research, Department of Vegetable Crops and

Weed Sciences, University of California, Davis

SUMMARY

Invasive plant speciesare one of the greatest threatsto croplands, rangelands,aquatic areas, and wildlands inthe United States. They de-grade the productivity andbiological diversity of all eco-systems. W. M. Lonsdale(1994) noted that “A mine thatmodified the landscape on asimilar scale (to invasiveplants) would be subjected tointense scrutiny by the envi-ronmental impact assessmentprocess and would probablynot be approved.” The prob-lem increases each year asnew species are introducedand established. We can over-come and reverse this trendonly if we are willing to com-mit and use national resourcesto combat the problem. Theformula for success must in-clude a coordinated effort atthe federal, state, institutional, and private sector levelsthat will involve long-term commitments of adequateplanning, funding, scientists, and facilities to produceresults based on sound science. Programs based on arbi-trary geopolitical boundaries must be replaced by ap-proaches based on ecosystem-scale realities.

Increased and consistent funding to support publicand industry educational programs, a centralized detec-tion and reporting system, and expanded mitigation and

research programs are criticalto reducing the continuedspread and current damagecaused by invasive weed spe-cies. In addition, greater uni-formity and consistencyamong state and federal regu-latory and enforcement agen-cies, coordinated through aneffective national plan, wouldreduce the potential for inva-sive weed introductions intothe United States and betweenstates into sensitive habitatsthroughout the nation.

I NTRODUCTION

Agriculture in theUnited States depends on avariety of native and non-native plant species; mostU.S. crops are species,strains, varieties, and culti-vars of non-native plants. Incontrast, the stability andecological function of natu-

ral wildlands depend on a diverse community of na-tive plants. Throughout the last century, however,large numbers of extremely damaging plants havebeen introduced into and established in the UnitedStates (Table 1). These introductions range from ac-cidental, such as weed seed contamination of com-modities, to purposeful, such as purple loosestrife andother plant species introduced as ornamentalplantings. Regardless of the mode of entry of these

COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY—2

Table 1. Partial list of major economicall y and ecologicall y impor tant in vasive weed species in the United States

Habitat Scientific name Common name Distribution

Riparian Ailanthus altissima tree-of-heaven Widespread throughout U.S.Albizia julibrissin mimosa Expanding range in tropical regions,

southeastern U.S.Arundo donax giant reed Expanding range in Pacific Coast states, ArizonaCasuarina equisetifolia Australian pine Expanding range in Hawaii and FloridaDelairea odorata Cape ivy Expanding range in CaliforniaElaeagnus angustifolia Russian olive Sporadic infestations throughout most of U.S.Lepidium latifolium perennial pepperweed Rapidly expanding range in WestPhragmites communis common reed Widespread in eastern U.S., native to WestSapiem sebiferum Chinese tallow Carolinas to Florida, Texas, and central CaliforniaTamarix spp. tamarisk, saltcedar Rapidly expanding range in West

Aquatic or Wetlands Alternanthera philoxeroides alligatorweed Widespread in southeastern U.S., someinfestations in California

Egeria densa Brazilian elodea West of the Mississippi River; some in California and southeastern U.S.

Eichhornia crassipes water hyacinth Widespread throughout southeastern U.S.and California

Hydrilla verticillata hydrilla Widespread in Southeast and mid-Atlanticcoast to Connecticut, threatens western states

Lythrum salicaria purple loosestrife Widespread in northern and central states,expanding range in West

Melaleuca quinquenervia melaleuca Widespread in FloridaMyriophyllum aquaticum parrotfeather Widespread throughout U.S.Myriophyllum spicatum Eurasian watermilfoil Widespread throughout U.S.Salvinia molesta giant salvinia Well established in Texas, new infestations in

California and other western and southeastern statesSpartina alterniflora smooth cordgrass Native in estuaries of eastern U.S., spreading

along coast of Pacific NorthwestTrapa natans water chestnut Expanding range in northeastern U.S.

Rangeland and Acacia auriculiformis earleaf acacia Expanding range in SoutheastWildland Acroptilon repens Russian knapweed Widespread throughout U.S., particularly western

statesAegilops spp. goatgrasses Widespread in western U.S.Ammophila arenaria European beachgrass Isolated infestations along sand dunes of CaliforniaAndropogon virginianum broomsedge Hawaii (native to southeastern U.S.)Bromus madritensis ssp. rubens red brome Widespread in western states, especially Mojave

and Sonoran desertsBromus tectorum downy brome Widespread throughout U.S., particularly western

statesCardaria draba hoary cress Widespread in western U.S.Carduus nutans musk thistle Widespread throughout U.S.Carpobrotus edulis iceplant, sea fig Spreading in coastal areas of WestCentaurea calcitrapa purple starthistle Expanding range in CaliforniaCentaurea diffusa diffuse knapweed Widespread in western U.S.Centaurea maculosa spotted knapweed Widespread throughout U.S., particularly western

statesCentaurea solstitialis yellow starthistle Western states, particularly California, Idaho,

OregonCentaurea squarrosa squarrose knapweed Expanding range in western U.S.Chondrilla juncea rush skeletonweed Expanding range in western U.S.Cirsium arvense Canada thistle Widespread throughout U.S.Cirsium vulgare bull thistle Widespread throughout U.S.Conium maculatum poison hemlock Widespread throughout U.S.Convolvulus arvensis field bindweed Widespread throughout U.S.Cortaderia jubata jubatagrass Widespread along California and Oregon coastCortaderia selloana pampasgrass Widespread along California and Oregon coastCrupina vulgaris common crupina Expanding range in California and

northwestern statesCynara cardunculus artichoke thistle Expanding range in California


Table 1. (contin ued)

Habitat Scientific name Common name Distribution

Cynoglossum officinale houndstongue Expanding range in many regions of U.S.Cytisus scoparius Scotch broom Widespread throughout Pacific Coast statesEhrharta spp. Veldtgrass Expanding range in coastal areas of CaliforniaEuphorbia esula leafy spurge Widespread in northern states, particularly

western U.S.Foeniculum vulgare fennel Widespread throughout Pacific Coast states,

especially southern CaliforniaGenista monspessulana French broom Widespread in western U.S.Hedychium gardnerianum Kahili ginger HawaiiHieracium aurantiacum orange hawkweed Expanding range in NorthwestHieracium pratense meadow hawkweed Expanding range in NorthwestHypericum perforatum St. Johnswort Widespread in western U.S.Imperata cylindrica cogon grass Expanding range in tropical and sub-tropical

areas of U.S., southeastern U.S. to Texasand southern California

Isatis tinctoria Dyer’s woad Spreading in Utah, California, and other westernstates

Lantana camara lantana Expanding range in Florida and HawaiiLepidium latifolium perennial pepperweed Rapidly expanding range in WestLeucanthemum vulgare oxeye daisy Widespread throughout U.S.Linaria dalmatica Dalmatian toadflax Expanding range in WestLinaria vulgaris yellow toadflax Expanding range in WestLonicera japonica Japanese honeysuckle Eastern and central U.S. and HawaiiMelia azedarach Chinaberry tree Spreading in SoutheastMiconia calvescens Miconia HawaiiMyrica faya firebush HawaiiOnopordum acanthium Scotch thistle Widespread throughout WestPassiflora mollissima banana poka HawaiiPolygonum perfoliatum mile-a-minute Expanding range in EastPotentilla recta sulfur cinquefoil Widespread in northern statesPsidium callleianum strawberry guava HawaiiPueraria lobata kudzu Widespread in Southeast to Pennsylvania and

IllinoisRubus argotus Florida prickly blackberry Hawaii (native to southeastern U.S.)Salsola tragus (= S. kali) Russian thistle Widespread in West

(tumbleweed)Salvia aethiopis Mediterranean sage Expanding range in western U.S.Schinus terebinthifolius Brazilian pepper Expanding range in southwestern U.S.Senecio jacobaea tansy ragwort Widespread in Pacific NorthwestSolanum viarum tropical soda apple Spreading in southeastern U.S.Spartium junceum Spanish broom Spreading in western statesTaeniatherum caput-medusae medusahead Widespread in WestUlex europaeus gorse Isolated infestations on Pacific Coast

Cropland1 Abutilon theophrasti velvetleaf Widespread throughout much of U.S.Amaranthus retroflexus redroot pigweed Widespread throughout U.S.Aegilops cylindrica jointed goatgrass Widespread throughout U.S.Chenopodium album common lambsquarters Widespread throughout U.S.Cirsium arvense Canada thistle Widespread throughout U.S.Convolvulus arvensis field bindweed Widespread throughout U.S.Cyperus esculentus yellow nutsedge Widespread throughout U.S.Cyperus rotundus purple nutsedge Widespread throughout U.S.Echinochloa crus-galli barnyardgrass Widespread throughout U.S.Elytrigia repens quackgrass Widespread throughout U.S.Kochia scoparia kochia Primarily invasive in western U.S.Setaria spp. foxtails Widespread throughout U.S.Sorghum halapense Johnsongrass Widespread throughout U.S.Striga asiatica witchweed Eradicated or close to eradication in North and

South Carolina

1Adapted from Bridges, 1992


introductions, the public has suffered economic lossesto food and fiber production, deterioration of naturalresources (rangeland and wildlands, aquatic and ripar-ian sites, and forests), loss of native species and theirhabitat, and further economic losses due to restrictionson the export of pest-contaminated commodities.

The risk of injurious introductions of undesir-able plants has greatly accelerated in the last threedecades due to exponential increases in air-travel,increased ports of entry, expanded export/import tointernational markets, and increased access to foreignecosystems. Through commercial air-cargo, ship bal-last water, and private travel, hundreds to thousandsof non-native plant species are brought into the UnitedStates each year (Cohen and Carlton, 1998). Thoughmost of these species have not survived or establishedin the United States, enough have to cause a contin-ued threat to an already pest-burdened crop-produc-tion system and to our sensitive and dwindling natu-ral resources. Examples include millions of acres ofleafy spurge and spotted knapweed on western range-lands; the parasitic plant witchweed, which threatensimportant agronomic crops across the southern UnitedStates; and the recent invasion of giant salvinia, whichposes a severe ecological and economic threat to thelower Colorado River system.

The President’s Executive Order on InvasiveSpecies (Clinton, 1999) defines an invasive species as“an alien species whose introduction does or is likelyto cause economic or environmental harm or harm tohuman health.” Although there are some federal regu-latory constraints on movement, sale, and possessionof exotic organisms, the overall resources directed atprevention, intervention, quarantine, removal, publicawareness, and the enforcement of existing federalstatues are seriously inadequate. Many states alsohave regulations governing invasive plant species butthese regulations are inconsistent from state to state,may be confusing or contradictory, and do not ad-equately address the scope of the problem.

The economic impact of many weeds is poorlydocumented because of the difficulty in assessing im-pacts to habitats, such as forests, hay and pasturelands,rangelands, wildlands, aquatic, wetland, and ripariansites. Most documented impacts are from croplands,where a conservative estimate of the impact of weedsto agriculture is $20 billion each year (Bridges, 1994).

This paper was developed to give a clear over-view of some of the most economically damaging in-

vasive plants in rangeland, wildland, aquatic, riparian,and cropland ecosystems. Strengths and weaknessesof current weed programs are presented with sug-gested recommendations that will enhance our abil-ity to slow new plant invasions and mitigate expan-sion and damage caused by established invasive plantspecies.

STRENGTHS AND WEAKNESSES OF

CURRENT INVASIVE PLANT PROGRAMS

Prevention ProgramsA step-by-step approach to protecting ecosys-

tems from invasive plant species should include (1)predicting which species are likely to enter the UnitedStates and successfully erecting barriers (regulation,inspection, quarantine, destruction) to prevent entryof those species; (2) implementing specific site man-agement and control measures to prevent establish-ment and spread from sites of initial introduction; and(3) developing management zones in sites of establish-ment to initiate control and prevent spread to newareas. Prevention of the introduction of known pestsand the establishment of newly introduced invasiveplant species are economical and rational approachesto stopping the spread of invaders before they becomeeconomically damaging to the environment.

Preventing introduction of invasive plant pestsinto the United States depends on awareness and ac-tion along a chain of transport that includes (1) a per-mitting system to bring plants into the United States;(2) point of entry inspections at airports, state borders,and post offices; (3) point-of-sale permitting and edu-cation for retailers; and (4) point-of-use education forhome, industrial, aquatic, and terrestrial landscapemanagers and hobbyists. Because many invasiveplants are not restricted under the Federal NoxiousWeed Act, the current permitting system is inadequateto stop entry of species that have been identified asdetrimental and are currently sold and distributed inthe United States. Inspections have been most effec-tive in stopping a few highly regulated species suchas hydrilla, but increased volume of air traffic andshipments has overwhelmed present border inspectioncapacities.

Wholesalers and retailers often are not knowl-edgeable enough to distinguish between native andexotic species. There is no adequate educational pro-gram to ensure that retailers are fully aware of import


restrictions, that allow for proper identification ofplants, nor provide information on where to reportsuspect species. The user-end knowledge base andawareness levels of invasive plants are limited. Manyrestoration managers and landscape architects areaware of problem species; however, the average prop-erty owner is poorly informed. The historical recordclearly demonstrates that the widespread introduction,distribution, and, in some cases, promotion of manyenvironmentally costly exotic plants were made bystate and federal agencies or by well-intentioned com-mercial growers and suppliers. Prevention programsfor invasive plant species must include enforcementand controls coupled with adequate educationthroughout the entire sequence of importation.

Public Education and Awareness ProgramsAwareness of invasive plant species and the

problems they cause will help the general public,policy and lawmakers, administrators, and land man-agers understand why the development of long-termweed management programs is critical to protect theenvironment. Communicating the impacts of invasiveplant species, their correct identification, and themanagement tools available to land managers will aidin development of management plans at all levels ofownership—local, state, and federal.

Educational efforts are being made throughacademia, industry, commodity groups, and farm andnonprofit organizations to enhance awareness of thethreats associated with invasive weeds as biologicalpollutants and the methods of dispersal of pest plantsas contaminants of agricultural and horticultural com-modities. These educational efforts can be accom-plished by a number of methods: through brochures,posters, internet websites, calendars, scientific papers,and other written media; with educational programsfor landowners, land managers, or the general publicthat include public seminars, professional symposia,school programs, and volunteer field workshops; andby using the media to educate the public through ra-dio or television news stories, public service an-nouncements, and newspaper. Educational activitieswill facilitate greater cooperation between private,federal, state, and county agencies, industries, land-owners, and the general public and increase the like-lihood of early detection and rapid response to newinfestations.

Recent educational efforts that have greatly im-

proved public awareness of noxious and invasiveweeds can be found across the United States. Forexample, excellent printed materials and roadside bill-boards educating the public about invasives such asspotted knapweed in Montana, hydrilla in California,and Eurasian watermilfoil in Minnesota have beendeveloped. Unfortunately, for most exotic species, thelevel of awareness by the general public is low. Edu-cational efforts for invasive rangeland, wildland, andcropland weeds have been directed mainly at ruralpopulations. Impacts of invasive weeds on the envi-ronment need to be the focus of educational programsin urban areas and within the nursery industry. Shortand effective public service announcements directedat larger urban centers and targeted to those groupsand individuals that provide plants or promote theiruse in ornamental and aquatic settings should be de-veloped.

Early Detection, Monitoring, and ReportingPrograms

Early detection means locating, identifying, anddocumenting newly introduced species. This is essen-tial to reduce the long-term economic impact of newinvading species. It is critical that early identificationand monitoring be accurate and timely. Detected in-vaders must be identified and reported to an appropri-ate organization to allow for initiation of assessmentand response. Centralized monitoring programs couldpool all available information on an invasive plantspecies and help in evaluating the success or failureof weed management programs. Reporting require-ments allow all available information on invasiveplant species to be documented and shared with landmanagers.

The best management of invasive weeds is torecognize potential weed problems early, control thembefore they reproduce and spread, and monitor the siteregularly to maintain adequate follow-up control.Early detection depends on proper training of landmanagers, pest management professionals, and prop-erty owners. Understanding the potential threats thatmay exist on surrounding property and in aquatic sys-tems can provide motivation for the public to providean early warning system for weed invasion. Amonginvasive weed species, the hydrilla program is themost visible and organized. In California, the hydrillaprogram consists of interdiction at state borders andports of entry and field-monitoring to detect new sites


and to assess the effectiveness of eradication effortsin previously identified hydrilla infestations. Thesuccess of this approach relies on coordination be-tween the California Department of Food and Agri-culture (CDFA) and regulatory agencies. The CountyExtension offices serve as early detection centers forthe public and water managers. More recently, alertsregarding giant salvinia have been useful in discov-ering new or existing infestations. Unfortunately,staffing, training, and funding limitations typicallyprevent adequate surveillance for most invasiveweeds.

The cost of eradication or management yearslater is considerably more expensive than the initialcost of eliminating a population following early de-tection (Figure 1). Leafy spurge was detected in

new infestations. A much broader and coordinatedcommunication network is needed to ensure interstatecommunications to enable early and effective re-sponse. One possibility is to establish a national, ac-cessible reporting system such as a “Pest Alert” num-ber.

Mitigation and Control ProgramsMitigation and control of invasive plant species

is accomplished by the development and implemen-tation of a long-term management plan that integratesknowledge of plant biology and site requirements.Integrated weed management (IWM) programs arebuilt on an understanding of the biology of the weedspecies and the infested ecosystem and on the use ofthe most effective control techniques available for theweed species and site. Best management techniquesavailable for the target weed are employed in aplanned, coordinated program to limit the impact andspread of the invasive plant. Control methods aredetermined by the use objectives for the land, the ef-fectiveness of the control method on the target plant,environmental factors, economics, policy and legal re-strictions, and the extent and nature of the infestation.Common components of IWM programs include her-bicides; cultural control methods, including grazingmanagement, fertility management, prescribedburnings, and revegetation programs; physical andmechanical methods, including mowing, tillage, andpulling; and biological control, including the use ofhost-specific insects and plant pathogens.

Mitigation programs, especially in rangeland,wildland, aquatic, and riparian sites, are expensive andrequire a considerable knowledge of the ecosystemand environmental conditions. Most mitigation pro-grams are directly associated with a particular weedspecies and a specific management program. MostIWM programs are local in scale, rather than regionalor area-wide. For some sites, this small-scale ap-proach is acceptable due to the unique nature of thespecific location and habitat. However, area-widecooperative management programs are much moreeffective, in both management and cost, than indi-vidual programs with no planned coordination. Largerscale programs on squarrose knapweed in Utah; leafyspurge in Montana, Wyoming, North Dakota andSouth Dakota; and the Eurasian watermilfoil projectin the Panhandle of Idaho illustrate the success of re-gional invasive plant programs. At a minimum, re-

DisplacementEstablishment0

200

400

Phase

Cos

ts (

$M)

Intro

500

100

300

Figure 1. Management costs in million dollar s and in vasionphase relationship sho w that pre vention is theleast costl y phase , with e xponentiall y rising costsonce the in vading weed has become estab lishedand most costl y if it is displacing native speciesand/or disrupting native habitats (Ander son, 2000).

Montana in the 1930s and early extension reports in-dicated the seriousness of this plant and the need tocontrol infestations. Adequate controls were notavailable and awareness of the potential impacts werenot clearly understood. A cohesive program was notdesigned at that time and Montana now combats morethan 600,000 acres of leafy spurge and suffers an an-nual $2.2 million loss of grazing capacity (Bangsundand Leistritz, 1991).

Rapid response to new infestations is also ham-pered by the lack of a central, well-known, and easilyaccessible center for reporting. Thus, there are oftensignificant delays between detection and response to


gional programs based on similar ecosystem manage-ment goals should be encouraged across the UnitedStates. These programs could form the basis for sus-tainable wildlife habitats and, in the long run, helpreduce susceptibility to exotic invasive plants. Littlework has been conducted with control programs fornoxious weeds in wildland areas. Lack of funding forresearch has hampered these efforts.

Managing invasive plant species requires a clearunderstanding of the land use objectives and benefi-cial vegetation needed to accomplish established man-agement goals. Programs that concentrate on the con-trol of the weed, with no plan for development ofbeneficial species in the area, are often unsuccessful.Much more effort must be directed at establishmentand encouragement of native, beneficial plants or highquality perennial grasses. In many cases, residentnative vegetation is adequate to replace the invasivespecies. In Florida, melaluca control programs allownatural re-establishment with sawgrass and other veg-etation. Revegetation may not be practical or neededin many aquatic sites. However, some projects needa specific revegetation plan and effort to restore thenative or beneficial vegetation in infested areas. Of-ten, funding limitations prohibit these integrated ef-forts. There are some revegetation efforts underwayin areas of northern California along riparian and ir-rigation systems and in wildlife refuges and in Mon-tana rangelands heavily infested with spotted knap-weed. However, there are currently no consistentprograms or sources for native and other beneficialplants used in these revegetation efforts, nor fundingfor actual planting.

Judicious and selective use of herbicides is animportant tool in any IWM program. Herbicide usemay be limited in aquatic, riparian, rangeland, andwildland sites due to site-use restrictions, lack of ef-ficacy on the target species, lack of research on thetarget species, cost factors, or public opposition to theuse of herbicides in many of these areas. The cost ofdevelopment for herbicides limits the target weeds andcontrol situations where agricultural chemical compa-nies are willing to invest research funding. Additionalpublic research into new herbicide uses, herbicide userates, and targeted application techniques would ben-efit the IWM approach, especially when economics donot attract private sector research. The USDA-Coop-erative State Research, Education, and Extension Ser-vice IR-4 program funds herbicide research on minor

use crops. This process should be expanded to includeherbicide use in rangelands and wildlands as well.

Site restrictions often limit the use of manyphysical and mechanical methods of control, but,again, funds are limited for development of appropri-ate and creative integrated approaches for the manage-ment of invasive species.

Biological control agents, if available, can be ef-fective components of an IWM approach. All inva-sive species are candidates for the development of abiological control program. Some species are soclosely related to economically important crops, how-ever, that classical biological control agents may notbe available. Selective biological control agents havenot been developed for all weed species and may notbe appropriate for all weed species. For example,tropical soda apple is closely related to tomatoes, pep-pers, eggplant, and other valuable crops and may notbe a good candidate for a biological control researchprogram. Careful assessment of the risks and benefitsfor biological control for a weed species should betaken prior to the initiation of a research program.

The most successful research, development, andimplementation programs for biocontrol agents havebeen through cooperative efforts of federal and stateresearchers and agencies with support from countyand local entities. Biological control agents must gothrough rigorous host specific testing prior to intro-duction into the country to prevent the accidental in-troduction of a serious crop pest. Currently, the de-velopment of effective biological control agents islimited by federal regulations that classify biocontrolagents as plant pests, the lack of a coordinated pro-gram at the national level, and the lack of consistentfunding. Several promising biocontrol agents recom-mended for approval by the USDA’s Animal and PlantHealth Inspection Service Technical Advisory Group(APHIS-TAG) based on scientific data have beenstopped in the final stages of the approval process.Objections for release are often from U.S. Fish andWildlife Service (FWS) representatives who have notbeen involved in the review process early and do notclearly understand the research and review on whichTAG bases its recommendation. TAG includes rep-resentation from FWS and the research and approvalprocess is clearly outlined in APHIS policy. The FWSand other agency representatives on TAG must in-volve their concerned personnel early in the approvalprocess.


Research ProgramsResources and scientific expertise dedicated to

economically important elements of plant scienceshave been historically funded at levels that have al-lowed the United States to achieve great advances inthe field of weed science in cropland settings, lead-ing to unprecedented production of food and fiber.However, funding and the number of experienced re-searchers focused on understanding the biology, ecol-ogy, and sustainable management strategies of inva-sive exotic weeds and their inherent impact areextremely limited. Rangeland and wildland researchprograms are limited to the western and southernstates. Funding for these programs has declined byas much as 50% during the past decade, in spite ofincreased demands for solutions to problems createdby invasive plants. Current research programs in ri-parian and aquatic areas include the few laboratoriesdevoted to management of aquatic vegetation sup-ported by either state or federal funding, includingfederal facilities in Florida, California, Colorado, andMississippi and a state facility in Florida. This re-alignment of scientific expertise has caused a propor-tional shift away from applied to more fundamentalresearch, simply because federal and state agencies arereceiving smaller allocations for research and devel-opment efforts. Although some academic research-ers are conducting studies on many invasive plantspecies, new management technologies that could beimplemented on a regional or national level are notbeing developed due to a lack of central coordination,low funding levels, and limited resources provided bygovernment agencies.

In many cases, little is known about the biology,life history, and appropriate control methods of an ex-otic invader, particularly with respect to its newlyacquired habitat. Well-funded and consistent researchprograms are critical to success in managing estab-lished and newly invading plant species. Successfulresearch requires highly trained individuals and spe-cialized facilities that must be supported and main-tained for extended periods. Fluctuating budgets andtheir subsequent effect on personnel training, turn-over, and continuity of effort must be avoided.

An ecologically-based approach to managing in-vasive plant species should be expanded to aggres-sively engage recently introduced species that areexpanding at exponential rates. Because invasiveplants are not restricted by political boundaries in their

growth and pattern of infestation, the federal govern-ment must provide a leadership role in rebuilding andmaintaining viable research programs that can useresources from local governments, academia, and theprivate sector. Research and development activitiesmust be coordinated within states, regions, and thenation to foster regular communication among inves-tigators, prevent unnecessary duplication of researchefforts, and target research funding. It will take anational research program based on sound science inecology, combined with the rapid development anduse of environmentally compatible herbicide technol-ogy and biological control, to stem the invasive tideof exotic plants that threaten the ecological integrityof aquatic, riparian, rangeland, wildland, and croplandecosystems in the United States.

Legislation and RegulationsInvasive plant species are regulated at all lev-

els of government. Local regulations often includecontrol of invasive plants by county weed districts ora similar entity. Some states have specific controlresponsibilities, while others have prevention pro-grams in place, such as seed, feed, and quarantinelaws. At the national level, federal land managershave the responsibility to develop weed managementprograms on their lands in states with active weedprograms and laws. The USDA Animal and PlantHealth Inspection Service has the responsibility ofpreventing the movement of undesirable plant speciesinto the United States and enforcement of the FederalNoxious Weed Act.

Effective laws, regulations, and policies are fun-damental to preventing invasive pest plants from be-ing introduced into, becoming established in, and per-sisting at sites where they cause economic loss orenvironmental damage. Laws across the jurisdic-tions of local, county, and state entities must meetthe needs of the local citizens and complementregulations at the national level. Any system mustinclude an infrastructure that ensures compliancethrough an informed public and appropriate civil pen-alties. In addition, information must be readilyavailable to all levels of government on what plantsare considered pests, how and where they can orcannot be moved, and other information to facili-tate compliance. State weed laws should be de-signed to protect their lands and citizens, withcomplementary federal laws to protect the nation’s


borders and support state management programs.The federal laws and regulations related to the

movement of invasive plants are not adequate to en-sure the protection of noninfested areas. There is nocentral database system that provides a complete list-ing of the invasiveness category of plants that moveas commodities or as contaminants. Nor is there anational source to inform the public of the federal andstate regulatory action category for individual weeds.Without a system to provide the public with informa-tion on invasiveness, there cannot be effective restric-tion in the movement of invasive plants.

With the exception of quarantines and border in-spections at points of entry for hydrilla in California,Washington, and Florida, most states have limitedstatutory authority to deal with invasive plants. Avoluntary regional program certifies weed-free haymovement between several western states. Somestate quarantine laws limit movement of commoditiesthat are not inspected or certified, but there is littleconsistency between states and enforcement is diffi-cult due to lack of staff and funding. Lack of fundingfor enforcement also limits the effectiveness of theFederal Noxious Weed Act in stopping transport, sale,and dispersal of exotic invasive weeds. Personnel tomonitor and inspect potential points of sale and dis-tribution are inadequate. This deficiency is illustratedby the recent offering for sale of giant salvinia, anaquatic species on the Federal Noxious Weed List, inmore than sixty cities in California through wide-scaleadvertising on web sites. Either due to lack of knowl-edge of the existing federal laws, or flagrant disregardfor these laws, exotic weeds are being offered for saleto the public by electronic, local retail, and magazineand mail order sources. Both state and federal regu-lations need adequate enforcement support.

In addition, a more streamlined process is ur-gently needed for including new noxious weeds inexisting regulatory restrictions. Any improvement inthese processes also will require much more focusededucation programs aimed at the trades and hobbyiststhat form the largest network for importation of ex-otic species. The goal should be strong incentives tocomply based on sound environmental stewardshippractices backed up by appropriate regulatory author-ity and penalties when laws are broken. Most impor-tantly, there is a critical need for a strong federal pro-gram and for uniformity and consistency between thevarious states’ regulatory and enforcement ap-

proaches. This regulatory consistency is essential tomaximize the abilities of each state—and the UnitedStates—to keep out invading species; it is the only ra-tional way to ensure equity in the market place andthe trades related to invasive plants.

Federal land management agencies are requiredby Section 2814 of the Federal Noxious Weed Act to(1) designate an office or person adequately trainedin the management of undesirable plants to developand coordinate a management program for all weedson lands under the agency’s jurisdiction; (2) establishand adequately fund a weed management program; (3)complete and implement cooperative agreements withstate agencies regarding the management of weeds;and (4) establish integrated systems to control or con-tain weeds targeted under cooperative agreements.Federal land management agencies lack funding andstaff to implement this section of the Federal NoxiousWeed Act, but it is the section that is in most need offunding at a level that will combat invasive plants atthe point of invasion.

Many states have a weed law and state listednoxious weeds. These laws are designed to bestcomplement the needs of that state and its residents.States that do not currently have a weed law or a statenoxious weed list must be encouraged to develop aworkable plan that is consistent with surroundingstates and federal land management goals.

RECOMMENDATIONS

Develop coordinated prevention programs• Identify potentially invasive species from foreign

lands that must not be introduced into the UnitedStates and support more thorough border inspec-tions.

• Provide accountability throughout transport usinga permit system that identifies potential invasiveplants and prevents sales of invasive and noxiousspecies.

• Ensure that wholesalers and retailers are fully awareof import restrictions, proper identification ofplants, and where to report suspect invasive species.

• Require all projects promoting non-native speciesfor forage, revegetation, erosion control, and simi-lar projects to screen species and cultivars for in-vasiveness prior to use.

• Include enforcement and controls, coupled with ad-equate education, from importation to the end-user.


• Adequately document the impacts of invasive plantspecies on the economics and environment of theUnited States.

Increase education and public awareness• Expand media coverage and target social awareness

of the threat of invasive weeds as biological pollut-ants in the urban sector through a national InvasiveSpecies Environmental Stewardship program.

• Improve educational programs within the nurseryindustry and among users of areas threatened by in-vasive plants and provide funding for increased sup-port of educational programs, including a compre-hensive K-12 education curriculum.

• Improve detection and reporting systems by devel-oping an easily recognized central rapid responsecenter for reporting new invasive weed populations.

Develop coordinated mitigation and controlprograms• Use the most effective current state programs as

models to encourage regional and national pro-grams.

• Develop and implement additional integrated ap-proaches to management, including chemical con-trol methods, that are consistent across politicalboundaries and can be implemented regionally.

• Develop restoration programs that encourage theuse of beneficial species in areas of infestation.

• Develop consistent regional and area-wide manage-ment programs that encourage cooperation of allland managers and landowners and include a “strikeforce” operation to stop incipient infestations.

• Encourage a national program of coordination in thedevelopment and implementation of biological con-trol.

Increase research on the biology, ecology, andcontrol of invasive plants• Identify current centers of excellence and develop

coordinated research agendas on a regional and na-tional scale.

• Maintain specialized research and development fa-cilities, as well as trained and experienced scientistsand staff.

• Expand research and development efforts in eco-logically-based integrated weed management,which includes selective use of herbicides, culturalpractices, mechanical means, and weed-specific

biocontrol agents.• Provide national and regional guidance and coordi-

nation of relevant research and development effortsamong scientists and agencies/institutions.

Improve and harmonize current laws andregulations at all levels• Use existing effective state laws as models.• Develop a central database that provides informa-

tion on state and federal regulatory actions for allinvasive species.

• Improve the federal Noxious Weed Act, making thelisting process faster and allowing agencies to stopinterstate transport of federal noxious weeds, andstrengthen enforcement.

• Support uniformity and consistency among all lo-cal, state, and federal authorities.

• Encourage states without state weed laws to developand implement laws that are consistent with laws inother states.

CASE HISTORIES: I NVASIVE PLANT

SPECIES IN CERTAIN ECOSYSTEMS

Rangeland and Wildland EcosystemsRangeland and wildlands comprise over 50% of

the total land area of the United States. These landsprovide valuable resources to wildlife and are areasof recreational opportunities. About three-quarters ofall domestic animals depend on grazing lands. Manyranges have been used for domestic stock grazing forover one hundred years and, as a result, the plant com-position has greatly changed from the original ecosys-tems. For example, many western rangelands previ-ously dominated by perennial bunchgrasses have beenconverted to annual grasslands that are susceptible toinvasion by introduced broadleaf weeds. Over 300species have been identified as rangeland weeds in theUnited States and many of these cause significantecological and economic damage. Some of the mostdestructive are described in this paper (Figure 2).

Leafy Spurge (Euphorbia esula)Leafy spurge is an aggressive long-lived and

deep-rooted perennial. It was introduced to NorthAmerica from Eurasia in 1829. By the 1900s, leafyspurge had escaped and spread by seed and vegetativefragments throughout much of the western UnitedStates. It currently infests over three million acres in


Present (surveyed, found)1997 Status

No survey or not known to exist Absent (surveyed, not found)

Leafy spurge

Medusahead

Purple loosestrife

Russian knapweed

Spotted knapweed

Yellow starthistle

Figure 2. Areas of inf estation b y some in vasive plant species in rang eland, wildland, aquatic, and riparian ecosystems in thewestern United States (Shelle y and Petroff , 1999).


29 states. Leafy spurge is adapted to a wide varietyof sites, from riparian zones to dry hillsides, enablingthe weed to rapidly expand its range throughout theWest. Leafy spurge infestations can reduce the graz-ing capacity of rangelands by over 60%. Cattle willavoid grazing even in areas lightly infested with leafyspurge. The combined economic impact of leafyspurge due to cost of control efforts and loss in for-age productivity in Montana, Wyoming, North Da-kota, and South Dakota alone exceeds $125 millionannually. In addition to its impact on forage produc-tion, leafy spurge also degrades wildlife habitat, de-creases plant diversity, competes with native plantspecies, and reduces land values.

Yellow Starthistle (Centaurea solstitialis)Yellow starthistle is a spiny winter annual that

was initially introduced around 1850 to Oakland,California, as a seed contaminant in alfalfa importedfrom Chile. Yellow starthistle originated fromEurasia, where it is native to Balkan-Asia Minor, theMiddle East, and south central Europe. Subsequentspread was slow until the mid-1900s. By the late1950s, the weed had invaded more than one millionacres in California. Since then, yellow starthistle hasspread exponentially to infest rangelands, wildlands,pastures, roadsides, and agricultural areas. Currently,yellow starthistle is estimated to infest approximately23 million acres in the West, with 20 million acres inCalifornia alone (Gerlach, 1998). In California, thereis a potential for yellow starthistle expansion from itspresent range to nearly 40 million acres. Outside ofCalifornia, yellow starthistle has more recently be-come a significant problem and is rapidly expandingto its biological potential in several northwesternstates including Oregon, Idaho, Nevada, and Wash-ington. Small infestations were recently found in sev-eral areas in Montana and North Dakota. Because ofthe spiny nature of yellow starthistle, livestock andwildlife avoid grazing in heavily infested areas. Inaddition, yellow starthistle causes a lethal neurologi-cal disorder in horses known as “chewing disease.”Although no economic assessments have been con-ducted for yellow starthistle, millions of dollars inlosses probably occur from decreased forage yield andquality of rangeland and pastures, interference withgrazing, increased livestock and crop management andproduction costs, and reduced recreational and eco-nomic value of infested land. In wildlands, yellow

starthistle reduces wildlife habitat and forage, dis-places native plants, and decreases native plant andanimal diversity.

Spotted Knapweed (Centaurea maculosa)Spotted knapweed is an aggressive perennial na-

tive to Central Europe, Russia, and western Siberia.Spotted knapweed was initially introduced into theUnited States in the late 1800s as a seed contaminantof alfalfa. Today it is a widespread weed probleminfesting over six million acres. Although spottedknapweed can be found throughout the country, it isparticularly problematic in the western states fromWashington to North Dakota and south to NewMexico, Arizona, and California. In Montana alone,spotted knapweed currently infests 2.1 million acresof rangeland and wildlands and has the potential toinvade up to 33 million acres. The impact of spottedknapweed on the livestock industry exceeds $42 mil-lion a year (Hirsch and Leitch, 1996). In addition, ithas been shown to reduce plant diversity, increase soilerosion and stream sedimentation, and dramaticallyreduce wildlife habitat. For example, in a Montanastudy, spotted knapweed infested wildlands reducedelk use by 98% compared to an adjacent uninfestedarea (Sheley and Petroff, 1999).

Russian Knapweed (Acroptilon repens)Russian knapweed is an aggressive long-lived

perennial that can form near monospecific stands ininfested areas. It is native to Eurasia and was firstintroduced into North America in 1898 as a contami-nant of alfalfa seed. Today Russian knapweed cov-ers about 1.5 million acres and is a problem in 21western states, particularly Wyoming, Idaho, and Utahand in some eastern states, such as Virginia. It con-tinues to expand its range at about 8% a year on allfronts, including into midwestern, northcentral andsouthwestern states. Russian knapweed is unpalatableto livestock and it appears to produce chemicals thatinhibit the growth of competing vegetation (Stevens,1986), probably accounting for its complete domi-nance in many infested habitats. Lands have beenabandoned in some areas because infestations are soexpansive and pervasive. However, a more typicalimpact is reduction in livestock carrying capacity ofthe range by 50% or more. Although there are noestimates on the economic effect of Russian knap-weed, it has a similar or even more significant per acre


impact than spotted knapweed. Because it is a peren-nial and has the ability to form monocultures, theimpact of Russian knapweed on ecological systemscan be even more detrimental than that of yellowstarthistle or other knapweeds.

Medusahead (Taeniatherum caput-medusae)Among the numerous introduced annual grasses

that have invaded rangelands and wildlands,medusahead poses the greatest threat. It was intro-duced from Eurasia into southern Oregon in the late1800s and has since spread to cover much of the grass-lands of northern California, Oregon, Idaho, and Ne-vada. Medusahead presently occupies over one mil-lion acres of rangeland and is expected to spreadfurther north, south, and east. Medusahead competesaggressively with other plants and provides little for-age for livestock and wildlife. Furthermore,medusahead seeds contain awns that can cause physi-cal injury to the eyes and mouths of animals. Highsilica content of the foliage contributes to low foragequality and a slow rate of vegetative decomposition.Older infestations are characterized by a thick layerof thatch that excludes most native herbaceous spe-cies and native perennial grasses and retards their re-establishment. The thatch layer is highly combustibleand increases the frequency of fire within these sen-sitive habitats. While there are no reliable estimatesfor the economic impact of medusahead on grasslands,infestations reduce grazing capacity for livestock andwildlife by up to 80%.

Tropical Soda Apple (Solanum viarum)Tropical soda apple is a relatively recent arrival

to the United States. It is a perennial shrub native toArgentina and Brazil that was first detected in Floridain 1987. Since then, tropical soda apple has spreadrapidly throughout Florida to infest at least 500,000acres of pasture and has also invaded about a dozenother southern states. Tropical soda apple has thepotential to invade any area with a tropical or subtropi-cal climate. The foliage and stems have numerouslong thick spines and are not palatable to animals.However, grazing animals can consume fruits andtransport seeds in their digestive tract to otheruninfested areas. Tropical soda apple in Floridarangeland forms a dense conopy that shades out othercrops and vegetation and reduces cattle production tozero (Mullahey et al., 1996). The impact on the live-

stock industry exceeds $11 million a year. In addi-tion, hundreds of thousands of dollars are spent eachyear to control this pest plant in open range, pastures,and forest edges and on Florida roadsides.

Kudzu (Pueraria lobata)Kudzu is one of the most recognized invasive

plant species throughout the southern and easternUnited States. It was first introduced as an ornamen-tal vine from Japan in 1876 and was later used as ahay crop. In the 1930s, kudzu was widely distributedby the Soil Conservation Service for erosion control.It has since invaded more than seven million acresfrom the mid-Atlantic states to Florida and west toTexas. Because of its viny morphology, kudzu canclimb over existing vegetation and develop into largecurtain-like stands that prevent light penetration toother species. Kudzu can displace virtually all otherplant species dramatically altering wildlife habitat. Itcan grow over road signs and buildings, obscuringvision and reducing motorist safety. The economicimpact of kudzu to the public is difficult to estimate,but is likely to be in the hundreds of million dollars.

Aquatic and Riparian Ecosystems Virtually every aspect of human activity relies

on adequate and high-quality water resources. Inva-sive aquatic plants cause significant ecological andeconomic impacts on critical and dwindling aquatic,wetland, and riparian systems (U.S. Congress, 1993).Overabundant growth and biomass create dense sur-face mats that reduce light penetration to understoryspecies and curtail water circulation, thereby reduc-ing dissolved oxygen concentrations and water qual-ity. These weeds can limit the growth of desirablenative vegetation, alter fish and wildlife communities,and reduce overall biodiversity (Haller and Sutton,1975; Newroth, 1985; Nichols and Shaw, 1986; Smartet al., 1994). Exotic infestations impair aesthetic val-ues of water bodies; reduce property values; interferewith potable, industrial, and irrigation use of water;hinder recreational swimming, fishing, and boating;and cause indirect impacts on human health(Bergstrom et al., 1993; Gangstad, 1986; Gangstadand Cardarelli, 1990; Newroth, 1985; U.S. Congress,1993).

Aquatic and riparian ecosystems are highly sus-ceptible to invasion by exotic plants because they of-fer suitable habitats for a wide range of species and


because they are exposed to human activities that fa-cilitate introductions of these species. A brief discus-sion of some ecologically and economically importantexotic aquatic pest plants in the United States follows(Figure 2 [Purple loosestrife] and Figure 3).

Hydrilla (Hydrilla verticillata)Hydrilla is an Asian, African, and Australian na-

tive first found in Florida in the 1950s. This rooted,submersed plant has spread to all seaboard states fromMaryland to Washington. Florida, Texas, and Ala-bama have the largest infestations, with a combinedtotal of several hundred thousand acres. In the late1970s, hydrilla colonized 500 miles of key irrigationchannels in southeastern California and threatened theviability of this billion dollar agriculture productionarea. Hydrilla has aggressively displaced native plantsthrough its ability to use low light levels and by theproduction of long-lived tubers, which can remainviable in sediments for several years. Hydrilla hasbeen controlled to some extent in most southeasternstates since the 1960s. Specific eradication programshave been in place on hydrilla in California for the past20 years and in Washington for the past 5 years. Costsfor control of hydrilla in the United States now ap-proach $25 million annually.

Eurasian Watermilfoil (Myriophyllum spicatum)Eurasian watermilfoil is a native of Europe and

Asia and has been in the United States since at leastthe 1940s. This submersed plant now occupies lakesand reservoirs in 45 states and three Canadian prov-inces. In the past 15 years, watermilfoil has spreadto Minnesota where it now infests 130 lakes; to Ver-mont, where it infests over 40 water bodies; and it wasfirst found in South Dakota during the summer of1999. The plant expanded from just five counties inWisconsin in the 1960s to 54 counties in the 1990s.Eurasian watermilfoil thrives in flowing and standingwaters and tolerates a wide range of temperatures. Inthe western United States, it has been found from asfar south as the Imperial Valley in California to as farnorth as Washington. High-elevation sites are alsovulnerable; Lake Tahoe has serious infestations ofEurasian watermilfoil. Like many aquatic submersedplants, Eurasian watermilfoil propagates primarilythrough vegetative growth and disperses easily byshoot fragments. The main cause for the continuedspread of Eurasian watermilfoil is related to boating

activities and transport on boats between aquatic sites.Several million dollars are spent each year to controlEurasian watermilfoil in the northern tier states.

Hydrilla

Water hyacinth

Giant salvinia

drainages introduced drainages eradicated

Figure 3. Areas of inf estation b y some in vasive plantspecies in aquatic ecosystems in theUnited States (U . S. Geological Sur vey, 1999).


Egeria (Egeria densa)Egeria is a native of South America and is one

of the most widely sold aquarium plants. Presently,only male plants are found in the United States, so alldispersal and spread is through cuttings and frag-ments, which can readily start new populations. Thedistribution of egeria is not well documented; how-ever, recent surveys show that it infests more than3,000 acres in the Sacramento-San Joaquin Delta ofCalifornia. It is widespread in Washington and Or-egon as well. Impacts include interference with irri-gation and boating-related activities and disruption ofnative habitats, including displacement of nativeplants. California has recently enacted state legisla-tion with authority to spend up to $1 million annuallyfor management of egeria. However, the latest costestimates for this program now project a requirementof over $2 million per year.

Water Hyacinth (Eichhornia crassipes)Water hyacinth, a floating tropical to semitropi-

cal plant native to South America, is considered oneof the world’s worst weeds, currently creating seriouseconomic and public health problems in lakes in cen-tral Africa, Mexico, and Latin America. It was intro-duced into the New Orleans area in the late 1800s andhas spread to most coastal areas, from North Carolina,south and west to Texas and California. Waterhyacinth’s rapid growth and ability to smother vast,open water areas enable it to impair almost all water-related commercial and recreational activities. Densecover formed by water hyacinth blocks light and re-duces the ability of microscopic plants and rootedsubmersed plants to produce oxygen in the water.Vast areas invaded by this plant become uninhabitableby fish and many invertebrates. In addition, it reducesthe amount of open forage areas for waterfowl. Wa-ter hyacinth continuously sloughs old roots and leaves,which greatly accelerate build up of organic matter onthe bottoms of infested waters. Annual costs of man-agement programs range from $500,000 in Californiato nearly $3 million in Florida. Before these programswere initiated, water hyacinth covered in excess of500,000 acres in Louisiana and 125,000 acres inFlorida. Now, control programs limit coverage to afew thousand acres.

Salvinia or Giant Water Fern (Salvinia molesta)Salvinia, which ranks as one of the most detri-

mental invasive aquatic plants worldwide, was notbelieved to be in the United States until it was discov-ered in South Carolina in 1995. This small popula-tion was eradicated, but the species was later foundin Texas in 1997. Currently there are known popula-tions in Alabama, Texas, and Florida, and, in Augustof 1999, salvinia was found in the Lower ColoradoRiver system near Blythe, California along the Ari-zona border. This small fern has the capacity for pro-lific growth and may double its population within 3to 4 days during summer months. The buoyancy ofthe plant also enables it to form floating layers ofvegetation that may range from 0.5 to more than 2 feetthick. Entire lakes, ponds, and reservoirs have beencompletely covered with salvinia within one or twoyears following infestation, blocking light needed forphotosynthesis by beneficial, native macrophytes andmicroscopic algae that form the base of the food web.Due to the dense cover that prevents light penetrationand limits oxygen, areas beneath salvinia become vir-tually devoid of aquatic life. Waterfowl can no longeruse these infested sites because their access to openwater and food is lost. The salvinia infestation threat-ens not only the Colorado River system but also thevast acreage of irrigated crop production in Arizona,California, and Mexico. There is great potential foradditional spread throughout the United States fromestablished infestations and from nurseries and mailorder suppliers that sell salvinia.

Purple Loosestrife (Lythrum salicaria)Since its introduction into the northeastern

United States in the early 1800s, this European peren-nial has encroached on inland wetland and ripariansystems, degrading native cattail, sedge, and wild ricecommunities that are critical to continental waterfowlproduction. Tens of thousands of acres have beeninfested with major populations occurring throughoutthe states bordering the Great Lakes, along the east-ern flyway, and in western provinces in the prairiepothole region. Purple loosestrife is spreadingthrough irrigation systems, wetlands, and river sys-tems in the West and is found extensively in the Co-lumbia River basin. Important native wetland plantcommunities have been suppressed or replaced bypurple loosestrife, which is also a prolific seed pro-ducer. Purple loosestrife has showy purple spike flow-ers with a long bloom period, thus the spread of thisweed has been enhanced by the ornamental plant and


apiary industry with its use in horticultural plantings.Many large wetland areas have been so completelyinfested that they are beyond the scope of currentlyavailable, economically viable control techniques.

Saltcedar or Tamarisk (Tamarix spp.)Saltcedar or tamarisk species are native to north-

ern Africa, the Arabian Peninsula, Iran, and India.They were introduced to the United States in Arizona,where they escaped cultivation and are rapidly infest-ing riparian areas. Infestations currently exist as farnorth as in Colorado, Wyoming, Washington, andMontana. The plant uses large quantities of waterfrom the riparian zone and can displace native veg-etation by competing for water. It also changes soilquality. The leaves of tamarisk exude salt, whichaccumulates on the soil surface following rainfall.This increases surface soil salinity and displaces salt-sensitive native vegetation, including cottonwoodsand willows. Saltcedars also increase fire danger inriparian zones and quickly reestablish after a fire.Biodiversity has been shown to decline significantlyfollowing infestation (Hughes, 1993).

Cropland EcosystemsHundreds of species of invasive plants are found

in cropland ecosystems throughout the United States.The cost of invasive weed control in agriculture isestimated to be greater than $20 billion per year(Bridges, 1994). Weeds cause greater expenditure infuel, equipment, labor, and control than all other pests(insects, rodents, and plant diseases) combined. Theenvironmental and economic impacts of weeds willworsen if established species are permitted to continueto spread and new species are allowed to enter, estab-lish, and spread throughout agricultural ecosystems.Many of these weedy species also have the ability tocross over from croplands to natural ecosystems andcause significant damage in these areas. Some of themost serious cropland weeds are described.

Velvetleaf (Abutilon theophrasti)Velvetleaf thrives in highly fertile soils and is

found in cultivated fields, gardens, fence rows, andwaste areas. It is considered the most serious crop-land weed in the United States (Stoller, 1993). It wasintroduced from Asia and is now widely distributedthroughout the country. Velvetleaf is common in theeastern United States and the Midwest and is also

found on the Pacific Coast and sparingly in the inlandWest, generally in warmer regions. Germination iscontinuous through the growing season and seeds re-main viable for longer than 50 years in the soil, mak-ing control of this invasive species extremely difficult.

Yellow Foxtail (Setaria glauca [S. pumila]), GreenFoxtail (S. viridis), and Giant Foxtail (S. faberi)

The foxtails are considered the second most se-rious groups of weeds in U.S. agriculture and causelosses in spring-seeded alfalfa, row crops, and smallgrain crops. They are also found along roadsides andin other disturbed sites. Yellow and green foxtail arenative to Eurasia but are common throughout most ofNorth America, with green foxtail more common thanyellow in the western United States. Giant foxtail isa native of China and occurs over a large area of theeastern United States. These plants are responsiblefor reductions in crop yield, increased cleaning costs,and expensive control measures.

Yellow Nutsedge (Cyperus esculentus) and PurpleNutsedge (C. rotundus)

Nutsedges are found in cultivated soils, pastures,gardens, lawns, and waste places. Yellow nutsedgeis one of the most troublesome weeds in the world. Itis a native of both North America and Eurasia and iswidespread in eastern North America and along thePacific Coast. Purple nutsedge is a native of Eurasiaand is found in the southeastern United States and westto California, occasionally moving northward. Thescaly rhizomes of these plants often terminate in hardtubers or nutlets, which are persistent and difficult tocontrol. Nutsedges spread by seed and by tubers.

Johnsongrass (Sorghum halapense)Johnsongrass was introduced from the Mediter-

ranean region as a potential hay or forage crop in the1800s and has become one of the most widespread andtenacious weeds in the United States. It was oncebelieved to be a warm season grass but has adaptedto cooler climates. Johnsongrass forms hydrocyanicacid when stressed by frost or moisture and becomestoxic to livestock. Johnsongrass is a highly invasivespecies that has spread rapidly and occurs in nearlyevery southern state. Hundreds of millions of dollarshave been spent for control, while millions more havebeen lost from diminished crop production and diffi-culties associated with crop harvest operations.


Witchweed (Striga asiatica)Witchweed is a parasitic plant that attaches to

and devastates agronomically important crops such ascorn, sugarcane, sorghum, and more than sixty othergrass species. The plant originated in Africa and theMiddle East and it was discovered in the United Statesin 1956. Witchweed is now found most prevalentlyinfesting cropland in North and South Carolina. Be-cause of its recognized threat to agricultural produc-tivity, an eradication effort is currently in place in theSouth. More than $225 million has been spent onwitchweed containment, research, and eradication toprotect the $30 billion crop industry of these south-ern states.

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National Research Council. ISSN 1070-0021

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Pest Control. U.S. Congress,Office of Technology Assess-ment, Washington, D.C.

U.S. Department of Agriculture, Ag-ricultural Research Service.1991. Biological Control

Quarantine: Needs and Pro-

cedures. U.S. Department ofAgriculture, Agricultural Re-search Service, Washington,D.C.

U.S. Department of Agriculture, Ag-ricultural Research Service.1996. Proceedings of the In-vitational Workshop on

USDA Activities in Biological

Control. U.S. Department ofAgriculture, Agricultural Re-search Service, Washington,D.C.

U.S. Geological Survey. 1999. Nonin-digenous Aquatic Species.Available: http://nas.er.usgs.gov/

Westbrooks, R. G. 1998. Invasive

Plants, Changing the Land-

scape of America: FactBook. U.S. Department ofAgriculture, Federal Inter-agency Committee for theManagement of Noxious andExotic Weeds, Washington,D.C. 109 pp.

Whitson, T. D., L. C. Burrill, S. A.Dewey, D. W. Cudney, B. E.Nelson, R. D. Lee, and R.Parker. 1991. Weeds of the

West. Western Society ofWeed Science, Pioneer ofJackson Hole, Jackson,Wyoming.

Wilcove, D. S., D. Rothstein, J.Dubow, A. Phillips, and E.Losos. 1998. Quantifyingthreats to imperiled speciesin the United States. Bio-

science 48:607–615.

Partial support for this issue paperwas provided by the Aquatic Ecosys-tem Restoration Foundation.

Additional copies of this issuepaper are available for $3.00.Kayleen A. Niyo, Ph.D. ManagingScientific Editor. World WideWeb: http://www.cast-science.org.