Ecological
Management of Invasive Weeds
There
are an estimated 18,000 native plant species in the United States. In
addition, 25,000 exotic plant species have been introduced; approximately
5,000 of these have become established in natural and managed ecosystems.
Many of these introduced species are now serious pests in U.S. pastures
and rangelands. Former Secretary of the Department of Interior, Bruce
Babbitt, reported that the nation spends $5 billion each year in attempts
to control invading weeds.
The problem
of invasive weed species
Dealing
with the threat of exotic weeds
The problem with using herbicides for weed control
Alternative
weed management stategies
The problem of invasive weed species
Alien weeds, like
knapweed and purple loosestrife, are serious agricultural and environmental
problems because they often displace valuable forage plants that livestock
and wildlife depend on. Some weeds are also toxic to livestock and wildlife.
The displacement of native plant species and the toxicity of some alien
weeds can seriously threaten endangered species of plants and animals.
The Nature Conservancy reports that 42% of the endangered and threatened
species in the U.S. are in trouble due to predation and competition from
exotic species.
Dealing with the threat of exotic weeds
How should we deal with the problem of exotic weeds? The answer to this question in complex. First, the U.S. government needs to develop and implement innovative policies focused on reducing and preventing invasions of new weed species. A few million dollars invested in prevention would be well spent if it can curtail a potential loss of billions of dollars to agricultural and environmental weed problems in the future.
Indeed, preventing new invaders from taking root in ecosystems is especially important, because once invading weeds are well established in natural and managed ecosystems, they are essentially impossible to exterminate completely. Even with herbicides, biological control agents, and selective grazing by livestock, there is no fully effective, environmentally-safe method of weed control. An examination of these various methods of weed control will serve to illustrate the problems associated with their use.
The problem with using herbicides for weed control
Herbicides are frequently sprayed over pastures and rangeland in an attempt to reduce invading weeds. Sometimes herbicides can be relatively effective in reducing the number of targeted weeds. However, there are major disadvantages and damages associated with the application of herbicides. First, herbicides and other pesticides are associated with several public health risks from acute poisonings to chronic effects like cancer and asthma. For instance, there are about 110,000 non-fatal human pesticide poisonings each year in the United States. In addition, use of certain pesticides has been linked with rising rates of specific human diseases, such as breast cancer; extensive exposure to some herbicides and other pesticides has also been shown to have adverse respiratory and reproductive effects ranging from asthma to sterility.
When spraying pesticides over rangeland with aircraft, there are often serious drift problems. Even the new ultra low volume technology (ULV) under ideal weather condition results in only approximately 25% of the herbicide reaching the target area. In fact, it is estimated that less than 0.1% of the pesticide applied ever reaches the target pests; therefore, more than 99% of the applied pesticide is released and left to impact the surrounding environment. And in addition to the public health problems mentioned above, there are also numerous environmental problems associated with herbicide use.
First, herbicides not only destroy the target weed, but often reduce a number of non-target plant species as well. In addition, an increase in toxic poisons in some plants -- including potassium nitrate and cyanide that occur naturally in some plants -- has been documented following herbicide use. These poisonous chemicals have been demonstrated to be toxic to livestock as well as wildlife, so increasingly the levels of these poisons is surely detrimental to the animals that live among these plants. Furthermore, some herbicide-tolerant plants may be physiologically affected by the herbicide. For example, several herbicides that did not kill certain non-target plants did significantly increase (up to 3-fold) the attack of pest insects on the non-target plants. Similarly, plant pathogens often increase in abundance (up to 5-fold) on non-target plants exposed to herbicides. Finally, yet another disadvantage of herbicides is that their necessary annual applications get to be very expensive.
Alternative weed management stategies
There are some safer -- though less widely used -- options for weed control. In some cases, the use of biological control agents, like insects, can selectively remove one weed species from a pasture, range, and/or natural ecosystem with minimal ecological effects. The effectiveness of this technology has been demonstrated for more than a century. For instance, an introduced biological-control insect proved highly effective against the serious klamath weed infestation in California. Biological pest control using natural enemies in the United States provides an estimated $12 billion/year in benefits. Not only is the use of biological controls economical, but once established, these insect species provide permanent, effective control of the weed or other pest.
Finally, in some situations, selective grazing by various livestock species can be an effective method of weed control. The introduction of a particular livestock animal, like geese, goats, cattle, and sheep, can reduce pest weeds and make the pasture and/or range land more productive.
The safe, effective management and control of established exotic-weeds requires input from and the joint effort of scientists from several distinct disciplines, including biological control specialists, chemical control specialists, wildlife ecologists, animal science specialists, economists, and the public. When properly employed, this integrated pest management (IPM) has major benefits. The basic premise of IPM centers on employing first biological and other non-chemical pest controls, with the use of chemical pesticides only as a last resort. IPM has the potential to limit the adverse public health and environmental effects of chemical pesticide use. Since pesticide effects on public health and the environment cost the United States a conservatively estimated $9 billion per year, this should be a much welcome change.
In conclusion, dealing with invading weed species is not a simple problem and requires the joint effort of scientists and the public, plus a significant investment in research, to search for environmentally and economically sound weed controls.
David Pimentel
College of Agriculture and Life Sciences
Cornell University
Ithaca, NY 14853-0901