New Strategies for Weed Prevention
From: Exotic Pests of Eastern Forests, Conference Proceedings - April 8-10, 1997, Nashville, TN, Edited by: Kerry O. Britton, USDA Forest Service & TN Exotic Pest Plant Council
Abstract Over the past several thousand
years, Man has moved many plant species far beyond their historical native
range. Many introduced plants that have become established outside of cultivation
are benign (so far). However, some introduced species with free-living populations
pose a threat to the biodiversity of natural areas and/or diminish the production
capacity of managed or agricultural ecosystems. In the United States, 16
federal agencies have formed the Federal Interagency Committee for the Management
of Noxious and Exotic Weeds (FICMNEW). This committee has developed a National
Strategy for Invasive Plant Management. Goals of the national strategy are:
weed prevention, weed control, and restoration of degraded lands. Research,
education, and partnerships are critical to the success of the strategy.
Regulatory strategies to protect the United States and other countries from
invasive plants include: production of weed-free commodities in exporting
countries; preclearance of risk commodities at foreign ports of export;
port of entry inspections; and finally, early detection, containment, and
eradication, of incipient infestations before they spread. Currently, 10
federal noxious weeds are being eradicated from localized sites in the United
States through cooperative projects with affected states.
Throughout history, Man has intentionally and unintentionally transported
thousands of different plants and animals far beyond their natural ranges
to other parts of the world. Most of these species are beneficial to human
society or show no signs of invasiveness (so far). However, hundreds of
species now cause serious problems in agricultural and/or natural ecosystems
within the United States. In the absence of co-evolved predators and parasites
that usually keep them in check in their natural ranges, introduced species
that find suitable habitats may thrive and outcompete or displace native
species. Over the past several decades, serious problems caused by introduced
plants and animals have raised concerns over the movement of species around
the world (Elton 1958; Westbrooks 1981; Mooney and Drake 1986; Eplee and
Westbrooks 1990; Schmitz 1990; Westbrooks 1991; Westbrooks 1993; Westman
1990; Zamora et al. 1989; Schmitz 1994). While change and disruption in
ecosystems have occurred throughout history, the biological invasions that
are now resulting from human commerce are truly different with regard to
origins, rate of introduction, types of organisms, abruptness and magnitude
of change (Wagner 1993).
Recognized invasive species that pose a threat to agricultural and managed
ecosystems, or threaten the biodiversity of natural ecosystems, have been
termed biological pollutants (McKnight 1993; Westbrooks 1993). Unlike
chemical pollutants that typically degrade in the environment, biological
pollutants have the ability to grow, multiply, adapt and spread, and cause
greater problems over time.
Some examples of introduced species that have become biological pollutants
in the United States include invasive plants, such as witchweed [Striga
asiatica (L.) O.Kuntze] in the Carolinas, kudzu (Pueraria lobata
Ohwi) throughout the southeast, mile-a-minute vine (Polygonum perfoliatum
L.) in the northeast, leafy spurge (Euphorbia esula L.) in the
midwest and west, and Micoia (L.) in Hawaii. These and thousands
of other species have been transported around the globe as hitchhikers,
contaminants, or on purpose. In either case, introduced species that become
invasive typically receive little attention until they become major problems
(Eplee and Westbrooks 1990). By the time a problem is recognized, environmental
documentation is prepared, funding is obtained for control, and eradication
is often impractical. At this point, an invasive plant becomes a permanent,
expanding, and detrimental component of the invaded area.
In depauperate communities such as oceanic islands, exotic herbivorous
mammals often become ecologically dominant, lead to wholesale species extinctions
within several tropic levels, and cause severe degradation of the environment.
In mainland environments, such taxa are more likely to cause decimation
of sensitive or endangered species in addition to degradation of the environment
(Coblentz 1993). The same adverse effects are often seen when invasive plants
are introduced into a new environment.
Economic Impact of Introduced Weeds in the U.S.
Weed Control Costs in the United States. Weeds cause billions
of dollars of losses annually in the United States by competition with crops
and by reducing the quality of food, feed, and fiber. During the 1950s,
annual losses due to reduced crop yield and quality and costs of weed control
were about $5.1 billion per year (USDA 1965). In 1962, $200,000,000 was
spent in the United States on herbicides alone for weed control (Montgomery
1964). In 1979, it was estimated that 10-15% of the total market value of
farm and forest products in the United States was being lost to weeds, a
loss of about $10 billion per year (Shaw 1979). During the 1980s, farmers
spent over $3 billion annually for chemical weed control and about $2.6
billion for cultural, ecological, and biological methods of control (Ross
and Lembi 1985). At that time, about 17% of crop value was being lost due
to weed interference and money spent controlling them (Chandler 1985).
In 1994, it was estimated that the economic impact of weeds on the U.S.
economy equals or exceeds $20 billion annually. In the agricultural sector,
losses and control costs associated with weeds in 46 major crops, pasture,
hay and range, and animal health, were estimated to be more than $10 billion
per year. In non-crop sectors, including golf, turf, and ornamentals, highway
right-of-ways, industrial sites, aquatic sites, forestry, and other sites,
losses and control costs totaled about $5 billion per year. Value of losses
was not available for most non-crop sites, but estimates of control costs
were determined. The importance of herbicides in modern weed management
is underscored by estimates that losses in the agricultural sector would
increase about 500% from $4.1 billion to $20 billion per year without the
use of herbicides (Bridges 1992; Bridges 1994). Since introduced
weeds account for about 65% of the total weed flora in the United States,
their total economic impact on the U.S. economy equals or exceeds $13 billion
Role of the Federal Government in Weed Management
A number of federal agencies have a variety of responsibilities for dealing
with weeds in the United States. Major areas of responsibility include:
weed regulation, research, and management. Efforts to prevent the introduction
of foreign weeds, as well as their establishment on private lands, are primarily
the responsibility of the USDA's Animal and Plant Health Inspection Service
(APHIS). APHIS cooperates with state and local agencies, as well as private
landowners/managers in eradicating newly introduced weeds on private lands.
Natural enemies of introduced weeds are imported under quarantine to control
large infestations on private lands (biocontrol). Basic research on agricultural
weeds is conducted by USDA's Agricultural Research Service (ARS). Weed research
and management on federal lands is conducted by a number of agencies, including
the U.S. Forest Service (USDA); U.S. Fish and Wildlife Service (FWLS); National
Park Service (NPS); Bureau of Land Management (BLM); Bureau of Reclamation
(BOR); U.S. Geological Survey (USGS); Bureau of Indian Affairs (BIA, U.S.
Department of Interior); Department of Defense; and the Department of Energy.
Estimated annual expenditures for weed research and control by some federal
agencies in FY97 are listed below.
*Foreign weed exclusion by USDA APHIS is a part of Agricultural Quarantine
Inspection (AQI) at U.S ports of entry. The AQI budget is about $200,000,000.00
Invasive plants grow, adapt, reproduce, and spread without respect for
agency jurisdictions or property boundaries. Therefore, an effective management
strategy to thwart alien species often includes a number of participants
and activities. Since the biology of a pest is not negotiable, the strategies
of action must consider the total biology of the species as well as political
and economic issues. There must be a recognition of need to eliminate the
alien species, a commitment of will and resources to the effort, and good,
practical science to developing control methodologies.
To be fully successful, any effort that is made in response to this serious
global problem must bring together a complex set of interests that include
private landowners, industry, and government agencies at all levels. One
of the first challenges is to create a public awareness of this issue. A
further challenge is to focus public and private resources in a partnership
approach to deal with specific weed problems while prevention and control
remain economically feasible.
In recent months, FICMNEW has developed a National Strategy for dealing
with invasive plants in a coordinated fashion. Principal goals of the national
strategy are: (1) to minimize further introductions of foreign invasive
plants in the United States; (2) to detect, report and assess incipient
infestations; (3) to prevent the movement of invasive plants from infested
to noninfested areas within the United States; (4) to eradicate or control
weeds that have already become established; and (5) to restore degraded
agricultural lands, rangelands, and other ecosystems to a healthy and productive
state. The strategy will serve as a road map to guide the nation in addressing
this growing problem.
Regulatory Strategies for Exclusion of Foreign Weeds
One aspect of the mission of APHIS is to prevent the entry of certain
foreign pests into the United States. Foreign pests regulated by APHIS include,
but are not limited to invasive plants, insects, plant diseases, animal
diseases, and mollusks that are of foreign origin. Plant Protection and
Quarantine (PPQ) is an operational section of APHIS that has the responsibility
for implementing the exclusion of such pests from the United States. Regulatory
strategies for protecting the United States by preventing the entry of harmful
non-indigenous species include:
- prevention (requiring or encouraging the production
of pest-free com modities in foreign countries to minimize the world movement
of recognized pests)
- preclearance (inspection/certification of certain
commodities at the port of export, prior to being shipped to the United
- exclusion (port of entry inspections and treatments,
designed to detect or remove prohibited pests in imported commodities,
and to mitigate pest risk of contaminated shipments)
- detection (conducting surveys and communicating with
scientists and state agencies for early detection of incipient infestations
of prohibited foreign species)
- containment (establishment of regulatory rules and
progams to prevent the spread of prohibited species from infested areas)
- eradication (total elimination of incipient infestations
of prohibited species by appropriate means)
- biological control (utilizing biological agents to
control certain pests if they cannot be eradicated)
Plant Taxa Listed as Federal Noxious Weeds
In 1976, 26 taxa of foreign weeds were designated as Federal Noxious
Weeds (FNWs). The FNW list now includes 94 taxa with 89 species, all species
of the parasitic genera Aeginetia, Alectra, and Striga;
plus all species of Cuscuta and Orobanche that are not native
to the United States. Melaleuca (Melaleuca quinquenervia [Cav. T.
Blake]), a tree in the myrtle family from Australia that is causing major
problems in the Florida Everglades, was added to the FNW list in 1992. Tropical
soda apple (Solanum viarum Dunal), a serious new weed of pastures
in Florida, was added to the list in 1995.
Detection of Noxious Weeds at Ports of Entry
Between 1976 and 1988, resource materials available to APHIS personnel
in enforcing the Federal Noxious Weed Act included a list of target species,
a short list of high risk commodities, and sampling procedures for inspecting
commodities for noxious weeds. At that time, greasy (raw) wool, soil-contaminated
equipment, aquatic plant shipments, and seed shipments, had been recognized
as high risk vectors for introducing foreign weeds (Westbrooks 1989; Westbrooks
and Eplee 1991).
In the mid- to late-1980s, a Noxious Weed Inspection System (NWIS) was
developed to enhance the ability of PPQ Officers to detect weed contaminants
in high risk commodities at ports of entry. The purpose of the system is
to provide officers with information on potential associations of target
weeds and commodities that originate in habitats where such weeds could
be expected to grow. NWIS is based on the principle that certain weeds are
likely to be associated with certain commodities from certain countries.
NWIS is comprised of a Federal Noxious Weed Inspection Guide, a Federal
Noxious Weed Identification Guide with monographs, line drawings, and range
maps on all listed species, and a Noxious Weed Seed Collection. Each PPQ
work station at U.S. ports of entry has one set of NWIS materials (Westbrooks
1989; Eplee and Westbrooks 1991; Westbrooks and Eplee 1991; Westbrooks 1993).
New Weeds Within the United States
Strategies for Early Detection, Reporting, and Rapid Response.
If noxious weeds do enter the United States, despite regulatory efforts
to exclude them, the next goal is to detect, contain, and eradicate incipient
infestations before they become entrenched and start to spread. A critical
element in this process is early detection. At present, new plant species
that are collected in the United States are typically stored at one of the
600+ public or private herbaria that exist around the country. Generally
speaking, weed scientists and other plant specialists learn about such new
state and national records through word of mouth or through notes published
in botanical journals. Experience has shown that if an infestation is detected
early, it can be generally contained and eradicated at a relatively low
cost compared to what it will cost for control once it becomes established.
One way to enhance early detection and reporting of new infestations
of weeds would be to create a Weed Detection Network in each state. Such
a network could be established by creating communication links between plant
collectors, herbarium curators, and appropriate state and federal agencies.
Botanists, farmers, county agents, and land managers, are just some of the
people who need to be encouraged to report new plants that they oberve.
To facilitate action on such reports, a state weed team in each state
could be established. Such a team would be comprised of state and federal
officials from agencies and institutions that are involved with weed management
and research in a particular state. The goal of a state weed team would
be to develop a coordinated plan of action and to leverage available resources
and expertise for dealing with important weeds of common concern. Having
one interagency spray crew to cover multi-jurisdictions would be far more
efficient and cost effective than having separate county, state, and federal
crews in a particular area.
Once a state weed team is informed about a new infestation, it will need
input from technical specialists on how to proceed. One way to do this would
be to establish a National Rapid Response Weed Team. The purpose of such
a team would be to provide technical support to federal, state, and local
agencies, in evaluating new infestations of introduced weeds. The national
team which would consist of recognized weed regulatory and control specialists
from participating federal agencies, would cooperate with weed scientists,
botantists, and state plant regulatory officials in affected states. Such
an interdepartmental team would provide a shared pool of expertise that
is not normally available to individual agencies. When this or a similar
system is adopted nationwide, we will be in a much better position to detect
new weeds and to respond to them appropriately. Early detection, reporting,
and rapid response, are three major goals of the APHIS Noxious Weed Policy
Implementation Plan, the U.S. Department of Agriculture Strategic Plan for
Weeds, and the National Strategy for Invasive Plant Management.
Federal/State Noxious Weed Eradication Projects
A Few Success Stories. Currently, about 45 species of the 94
taxa that are listed as FNWs are known or reported to occur in the United
States to a limited degree. Over the past 40 years, APHIS and its predecessors
have been involved in cooperative federal/state efforts to eradicate a number
of these species from the United States. These include:
- Witchweed (Striga asiatica [L.] O. Kuntze). 177,000 ha infested
in NC and SC; now reduced to 11,000 ha in 17 counties in NC, and in three
counties in SC
- Branched broomrape (Orobanche ramosa L.). 283 ha infested in
Karnes County, TX
- Goatsrue (Galega officinalis L.). 16,000 ha infested in Cache
- Mediterranean saltwort (Salsola vermiculata L.). 550 infested
in San Luis Obispo County, CA
- Hydrilla (Hydrilla verticillata (L. f.) Royle). 310 km of canals
infested in the Imperial Irrigation District, Imperial Valley, CA; now
- Japanese dodder (Cuscuta japonica Choisy). 1 ha infested in
the SC Botantical Garden, Clemson, SC
- Small broomrape (Orobanche minor Smith). Spot infestations in
Washington County, VA; Pickens, Abbeville, and Aiken Counties, SC; and
in Baker County, GA
- Catclaw mimosa (Mimosa pigra L. Var. Pigra). 405 ha infested
in Martin and Palm Beach Counties, FL
- Asian common wild rice (Oryza rufipogon Griffith). A rhizomatous
red rice; 0.5 ha infested in the Everglades National Park, FL
- Wild sugarcane (Saccharum spontaneum L.). A rhizomatous wild
sugarcane; 13 spot infestations along the southeastern shore of Lake Okeechobee
in Martin County, FL, totalling less than 1 ha
Most of the early weed eradication projects (e.g., witchweed, goatsrue,
and hydrilla) involved large acreages. However, in recent years, there has
been a general trend toward projects that are smaller in scope and duration
(1-2 ha; 3-5 yr). This measure of success is mostly due to increased networking
between weed scientists and botantists in recent years. Weeds detected early
can be eliminated for less money in less time.
Strategies to Prevent the Spread of Established Invasive Plants
The first line of defense against introduced invasive plants
is early detection of new infestations. As already noted,
the work of amateur and professional field botantists is critical in early
detection and reporting of new plant species as they are observed.
The second line of defense against invasive plants is to contain
and eradicate incipient infestations as soon as they are detected.
The third line of defense against invasive plants is to prevent
movement into noninfested areas.
The fourth line of defense against invasive plants is to develop
effective and environmentally sound methods and procedures for control of
Harmful non-indigenous plants are biological pollutants that threaten
agricultural production and the biodiversity of natural ecosystems in the
United States. Federal agencies in the United States, through FICMNEW, are
developing a coordinated national strategy for dealing with invasive plants.
One role of USDA APHIS in biological protection of ecosystems is to prevent
the introduction of foreign invasive plants into the United States. APHIS
also cooperates with affected states to combat incipient infestations of
Federal Noxious Weeds before they become widespread. The most effective
way to deal with invasive plants is to prevent their introduction from other
countries, to detect incipient infestations at an early stage, and to implement
an effective eradication program before they begin to spread to other farms
and states. Money spent on weed prevention is a wise investment that will
help to minimize future losses and control costs that are typically associated
with widesprad weeds.
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