Is the introduction of natural enemies of invading foreign species such as purple loosestrife (Lythrum salicaria) a safe and effective way to bring the invading species under control?

Viewpoint: Yes, introducing the natural enemies of invading foreign species is a safe and effective way to bring the invading species under control, as long as rigorous screening and proper release strategies are utilized.

Viewpoint: No, introducing the natural enemies of invading foreign species such as purple loosestrife is neither safe nor effective; as history shows, numerous such attempts have backfired.

Just as Sir Isaac Newton demonstrated that every action is accompanied by an equal and opposite reaction, many naturalists believe that for every pest problem there is an equal and opposite natural counterpart, which could be the basis of a biological control method. Unfortunately, we are unlikely to notice the workings of natural controls until an ecosystem has been disturbed. Biological control involves the use of natural enemies (agents) to manage invading species, i.e., nonnative organisms that have become pests (targets). Exotic animals as well as plants can become serious problems for agriculture and the natural environment when they become established in areas where they have no natural enemies. Typically, the target species are weeds, insects, snails, marine organisms, rats, snakes, rabbits, or other animals. The kinds of control programs that might be adopted depend on the nature of the area that has been threatened or damaged and the nature of the invading pest species. Very different methods might, therefore, be appropriate for gardens, greenhouses, fields, farms, wetlands, or forests. The concept and practice of biological control and integrated pest management are of great theoretical interest to researchers in ecology and agricultural science, and of practical interest to farmers, ranchers, horticulturists, land and wildlife managers, extension agents, and regulatory officials.

Biological control agents typically are plant-eating insects, parasites, diseases, or insects that attack other insects. Thus, control agents generally can be divided into three groups: microbials (fungi, bacilli, viruses, bacteria, protozoans), parasitoids (agents that parasitize their target), and predators (agents that prey on their target). In the search for safe and effective biological control agents, scientists have initiated detailed studies of the life cycles and habits of hundreds of natural enemies of various pest species. Extensive research is needed to determine the potential host specificity and environmental impact of biological control agents once they are released into a new environment.

Although insects are often thought of as major pest problems, many are quite benign and play an important role in keeping other potential pests under control. Many insect species have been suggested for use as control agents, but rigorous screening is essential and release strategies in the field must be continuously evaluated. Without detailed information about the genetics, taxonomy, and ecology of the insects used as biological control agents, their release into new areas might result in unanticipated problems. Moreover, in areas where complete eradication of an invading species is the goal, attempts to use biological controls might be disappointing. The philosophy of integrated control is based on a natural containment strategy, rather than an attempt to eradicate pest species. Because biological controls act by restoring the natural balance between species, they are unlikely to completely eradicate their target or to eliminate all of the damage caused by the pest. The goal of most true biological control programs is to bring invasive pests under control and to maintain an acceptable equilibrium between the pest and control agent. Because the control agent is a living entity that depends on the pest for its survival, the population of the agent will decrease as the pest species is brought under control. At that point, the pest population might increase until some level of balance is achieved. When insects are used as biological controls, it is usually necessary to discontinue the use of pesticides.

Advocates of biocontrol insist that it is one of the most valuable approaches for the long-term management of serious invading pests. Biological control methods have sometimes been applied without proper precautions, but with proper research and appropriate applications, biocontrols may be less damaging to the environment than pesticides, herbicides, and other toxic chemicals. The initial costs of research, screening, and testing may be very expensive, but once the appropriate agent has been established the control system should be self-sustaining. Biocontrol is not a simple matter of adding a living agent to a system threatened or already degraded by an imported pest; any given biocontrol agent must be part of a well-designed, integrated pest management strategy.

Effective agents are most likely to come from the original habitat of the target species, where it was presumably kept in check by its own natural enemies. To develop safe and effective biocontrol programs, researchers must test the natural agents that appear to control the pest in its native habitat for host specificity before bringing the agents into a new environment where they might become pests themselves. Appropriate biocontrol agents should attack the target without significantly affecting other species. The ideal agent would only attack the target, but finding agents with absolute host specificity is unlikely. The release of agents that have the appropriate host range must be closely monitored. Critics argue that it is impossible to test an agent against all nontarget species under the conditions that might be found in different sites once the agent is released.

Biocontrol programs usually follow certain basic steps and testing procedures. Before a program begins, the pests that are the objects of the control efforts (the targets) must be identified and carefully studied to determine their impact on the environment and their potential vulnerabilities. Scientists need to analyze factors such as the target's current distribution, potential to spread, natural history, ecological impact, and economic implications. It is important to predict what the future impact of the pest would be in the absence of biocontrol, as well as the possibility of implementing control by other approaches. Once the appropriate agents are screened and selected, they must be released in sufficient numbers to increase the likelihood that they will attack the target, reproduce successfully, and colonize the area occupied by the target. Long-term monitoring of the released agent and the target is essential to ensure the program is safe and effective.

In the 1940s, because of the apparent success of DDT (dichlorodiphenyltrichloroethane) and other synthetic broad-spectrum insecticides, some entomologists believed that insect pests could be eradicated, but pesticide resistance developed very rapidly. Rachel Carson's book Silent Spring (1962) convinced many readers that DDT and other chemical pesticides were poisoning the water, air, fish, birds, and ultimately threatening human health. Biological control systems would presumably reduce the use of insecticides, herbicides, and other toxic chemicals. However, just as the widespread use of synthetic insecticides ignored the complexity of the natural environment, the incautious use of biological controls also could undermine the delicate ecological web that represents nature's own system of pest control.

The attempt to control purple loosestrife provides a good example of the debate concerning the use of biological agents. Purple loosestrife, which occurs naturally in Europe, has invaded wetlands throughout North America. In Europe, the growth of purple loosestrife is apparently kept under control by several insects. Advocates of biological control programs believe that the introduction of several European insect species that specifically target purple loosestrife might bring this pest species under control. The case of purple loosestrife, however, brings up one of the major obstacles to the implementation of biological control programs—the possibility that the imported control agent might itself become a pest. Despite concerns about the safety and effectiveness of biological control, various biological agents currently are being used in attempts to control exotic pests. Many scientists believe that, eventually, more specific and effective agents will be created through the use of molecular biology and bioengineering. Undoubtedly, attempts to use these genetically modified organisms in biological control programs will generate a new set of controversies.

—LOIS N. MAGNER

Viewpoint: Yes, introducing the natural enemies of invading foreign species is a safe and effective way to bring the invading species under control, as long as rigorous screening and proper release strategies are utilized.

The proper use of biological control, such as the introduction of natural enemies of invading foreign species, will restore the balance of plant and animal habitats in areas that have been overrun by an imported species. In addition, biological control (BC)—the science and technology of controlling pests with natural enemies—may be the only safe and effective solution to salvaging endangered native species and habitats from nonindigenous invaders when other control techniques are impractical or not working. For example, in one location in Illinois purple loosestrife plants are growing on a floating mat where access is nearly impossible to people, but getting at the plants is no problem for bugs.

That is not to say the use of natural enemies is the first line of defense, nor that it is guaranteed to always be without risk. On the risk/benefit issue, if a scientific approach is conscientiously used to determine exactly which natural enemies will meet a very strict list of safety criteria to protect native environments, then biological control can be the answer where nonindigenous plants have run amok. The present selected BC efforts are showing some success with purple loosestrife. BC also is being developed in the Everglades to control the devastating Old World climbing fern. Garlic mustard is another pest being studied for biological control.

However, bringing in natural enemies to return order to a runaway imported pest without enough research can be risky. The history of hasty, poorly thought-out schemes for a fast fix to a problem of invading species has given the use of one species to control another a bad reputation. Possibly the worst example of poor judgment was the introduction in 1883 of the mongoose to Hawaii to control rats, which also were imports. The mongoose sleeps in the night and hunts in the day. The rat sleeps in the day and feeds at night. They never see each other. Both are now severe pests in Hawaii and are doing damage to native species.

Background on Purple Loosestrife

Since its introduction into New England from Europe in the early 1800s, purple loosestrife ( Lythrum salicaria ) has become a spectacular, and aggressive, perennial that is found in every state in theUnited States except Florida. Purple loosestrife favors wet meadows, floodplains, and roadside ditches, where it tends to crowd out native wetland plants. It endangers wildlife by crowding out their food supply and cover. Ducks and muskrats avoid the very dense loosestrife-covered areas. Few native insects bother it, so native birds like wrens and blackbirds cannot find enough food to stay in an environment where purple loosestrife has taken over.

Purple loosestrife was not entirely a chance introduction. Although it is believed seeds came ashore accidentally in ship ballast and tangled in the wool of imported sheep, purple loosestrife plants also were imported for use in herbal medicines. Various parts of the plant were used to treat diarrhea, dysentery, and both external and internal bleeding. Once introduced in the Northeast, purple loosestrife spread easily with the expansion of inland waterways and canals. It also was commercially distributed for its beauty, medicinal applications, and to provide a nectar source for bees.

How did purple loosestrife become such an overwhelming invader? First, it had essentially no natural insect enemies or diseases in its new environment, and unfortunately none came with it from Europe. Also, the plant is hardy in most moist environments, and can tolerate a variety of soils and nutrient regimes. It is a beautiful plant, and as a hardy perennial it was actually sold and planted in gardens for a long time. Purple loosestrife is about 6.6 ft (2 m) tall with 30 to 50 stems in its wide crown. The purple spikes of six-petaled flowers appear from July to September, forming seeds by mid-July that are shed all winter. Each of 30 stems per plant produces about 1,000 seed capsules, and each seed capsule averages 90 seeds. That equals more than two million seeds about the size of a speck of ground pepper per plant, or more than four million seeds for the 50-stem plants. The seeds are spread easily by flowing water. Purple loosestrife's sturdy root system and mature woody stems survive the winter. In addition to seed dispersal, the stems and roots of mature plants can start new plants.

Controlling Purple Loosestrife

The problem of purple loosestrife has been widely addressed. One of the leading authorities on the control of nonindigenous plants is Bernd Blossey, a research associate in the department of natural resources at Cornell University. Blossey says no conventional effective control of purple loosestrife is available except where the plant occurs in small isolated environments so it can be pulled up. It is absolutely essential to get all vegetative parts or the plants will regrow and spread. Cutting and burning do not work. Herbicides are costly, may kill desirable plants as well as the loosestrife, and also may pose some risks to animal species. They must be applied repeatedly and have not been a successful solution to date. Several areas infested with purple loosestrife in Illinois have been treated with herbicides for more than a decade with no progress made in reducing the plant population.

The obvious solution is biological control—to import the natural enemies of the species that kept it in control in its native habitat. But this is not an easy solution. The biological control of purple loosestrife has been a multiagency effort, including the Agriculture Research Service (ARS) of the U.S. Department of Agriculture (USDA), the U.S. Fish and Wildlife Service, and the U. S. Geological Survey (USGS). A scientific advisory group was formed with representatives from universities, U. S. federal and state agencies, and Canada. The problems associated with purple loosestrife are not limited to the United States; the plant has become a major pest in parts of Canada as well. The advisory group has been working since 1986. Through the efforts of this group, the first insects selected to control the spread of purple loosestrife were made available in 1992 to seven states and Canadian cooperators. Since then, trained biologists have released selected insects in about 27 states and all of the Canadian provinces.

Biological Controls for Purple Loosestrife

These insect controls were introduced after extensive research had been conducted to assure a reasonable chance for success. About 15 species were identified and tested for host specificity against 41 species of native plants and 7 species of agricultural plants. They were first tested at the International Institute for Biological Control in Europe, where both the insects and purple loosestrife were coexisting in a balanced environment. Then the selected insects were brought to the United States under strict quarantine conditions and were tested for their feeding habits and ability to survive here. The BC process is not speedy. It takes at least five years to select test insects and then they must be established where they are needed, which is not always successful. Should the insects find the new environment too desirable, native controls also must be available.

Four host-specific insect species have been approved for the control of purple loosestrife by the USDA and released in the United States. These include a root-mining weevil ( Hylobius transversovittatus ), two leaf-eating beetles ( Galerucella calmariensis and G. pusilla ), and a flower-feeding weevil ( Nanophyes marmoratus ). The goal of biological control is the long-term management of an invading species, not eradication altogether. Part of the process includes continuous and long-term monitoring. BC is a relatively new science. Researchers hope to learn from their experiences with purple loosestrife, and to improve technologies for environmentally friendly, safe, and effective weed control.

There are success stories to report on the purple loosestrife biological control efforts that indicate the new science is working. An article in a USGS newsletter (2000) describes the results of a 1995 introduction of Galerucella beetles by the USGS Bureau of Reclamation in a Columbia Basin Project in central Washington State. Purple loosestrife was introduced to a new irrigation project in the 1960s as part of a university experiment, but the alien plant quickly spread over 20,000 acres (8,100 ha), ruining wildlife and waterfowl habitats. It was a nuisance for boaters, anglers, and hunters and was clogging irrigation routes. Herbicidal control was only briefly considered for the vast area. Besides environmental concerns, the cost was prohibitive.

The purple loosestrife has to be defoliated for two years by the beetles before it is killed. At the Washington site, this process requires a lot of beetles. Fortunately they are thriving in the test site and even are moving into nearby areas in Idaho that have been overrun with purple loosestrife. Biologists estimate it will take five to seven years to control purple loosestrife at a site. There is no record of insects intentionally released for the biological control of one species suddenly shifting their diet to include other plants.

Other Candidates for Biological Control

The Old World climbing fern ( Lygodium micro-phyllum ) is smothering cypress trees on the edge of the Everglades in Florida. Steve Mirsky (1999) explains the fern-covered trees "look as if they're dripping with green sequins," although this beautiful sight represents a "botanical carnage." ARS may now have identified a biological control for the climbing fern. It took a near worldwide search, but scientists at the ARS Australian Biological Control Laboratory have found a fern-fighting moth, the tiny Cataclysta camptozonale at .5 in (1.3 cm), that seems promising. According to the ARS (2000), rigorous tests were conducted on 14 other fern species, then more than 250 moths were sent to Florida for controlled tests on the Old World climbing fern and to determine whether the moth will harm native crops or plants.

Donald Strong and Robert Pemberton (2000) list recent successes using biological control for both invading plant and insect species in various parts of the world. However, they are quick to point out that "BC is not a panacea" and must be used carefully. The key to safety is finding BC organisms that feed only on the offending alien. This factor was ignored in the 1960s when the weevil Rhinocyllus conicus was introduced to control invading musk thistle, even though it was known to attack four different thistles in Europe. The weevil attacked native thistles in the United States, too. Some say the thinking at the time was that no thistle was a good thistle. Whether the weevil introduction was the result of inadequate regulations or differing values is not clear. Such a release would no longer be permitted by the USDA's Animal and Plant Health Inspection Service, according to David Ragsdale, professor of entomology at the University of Minnesota (McIlroy 2000).

Safe and Effective Biological Control

According to Cornell University researchers (Weeden et al. 2001), biological control has been successfully applied to invasions of nodding thistle in Kansas and Canada; ragwort has been controlled in California, Oregon, and Canada; and alligator weed and water lettuce have been brought into balance in Florida. The researchers have identified desirable characteristics of weed-feeding natural enemies. First on their list is the criterion that the selected insects must be specific to one plant species. The insects also must have enough of a negative impact on the alien plants to control the population dynamics of the target weed; must be both prolific and good colonizers; and must thrive and become widespread in all of the climates occupied by the pest weed.

A Biological Control of Nonindigenous Plant Species program was established at Cornell to advance the science of biological weed controls. The main focus of the program is to document the ecosystem effects of invasive species and to develop and implement biological control programs. In addition, the program includes the long-term monitoring of ecosystem changes after the release of controlling agents. Research also is being conducted to determine those factors that increase the competitive ability of invasive plants.

Where safeguards are applied as outlined in Cornell's scientific approach to biological control and USDA protocols are respected, the introduction of natural enemies of invading foreign species such as purple loosestrife is a safe and effective way to bring the invading species under control.

—M. C. NAGEL

Viewpoint: No, introducing the natural enemies of invading foreign species such as purple loosestrife is neither safe nor effective; as history shows, numerous such attempts have backfired.

Freedom to Destroy

One reason a foreign species becomes invasive in its new environment is that the new environment contains none of the natural enemies that keep the species in check in its native environment. It would therefore seem feasible to introduce a natural enemy of the invading foreign species into the new environment to bring that species into check. However, biocontrol efforts can go wrong. Some have gone terribly wrong, wreaking havoc on native bird, animal, and plant populations, causing total extinction of numerous species, and bringing others to the verge of extinction.

Biological control, the act of intentionally introducing a natural enemy of a pest in order to control that pest, was first used in 1889 in a California citrus grove (now Los Angeles) where the cottony cushion scale was devastating the citrus industry. By releasing only 129 vedalia beetles imported from Australia, the scale was brought under control as the beetles proliferated and the industry was saved. There are other biocontrol success stories, but there are also horror stories.

Dangers of Introducing a Foreign Species

Introducing a foreign species into a new environment is a dangerous proposition, even if it is for biocontrol purposes. While some foreign species will live in harmony with native creatures, others are opportunistic and will take control. Of the more than 4,000 plants introduced (intentionally or accidentally) into the United States, only 10% are generally considered invasive. However, this 10% has done untold damage, both economically and environmentally. Invasive species have often overrun native vegetation, leading to a loss in diversity of many species that rely on native vegetation.

Worldwide, introduced species have caused about 46% of all species loss. "At first the invasion doesn't look like much," explain the authors, "but, if left unchecked, can alter a region's natural, cultural, and aesthetic values irrevocably." By the time a foreign species is known to be invasive, it is often too late to halt its destructive progress.

Preintroduction Testing—How Reliable?

Before a foreign species is introduced, it is essential that extensive testing and analysis be conducted to predict the impact the species may have on its new environment. S. J. Manchester and J. M. Bullock (2000) believe the early identification of "problem" foreign species might make their control easier, but that attempts at predicting which species will become invasive have been highly unsuccessful.

One way to help reduce the threat from foreign species is by host-range testing—attempting to determine how many different hosts may be susceptible to attack by the proposed pest predator. Any species other than the target host (the pest) is called a nontarget host. Every effort must be made to ensure that nontarget species will not be adversely affected by the pest predator. According to Keith R. Hopper (1999), a research entomologist with the U.S. Department of Agriculture and a scientist in the Beneficial Insect Introduction Research Unit, University of Delaware, it is virtually impossible to test all nontarget species in the area of intended introduction.

Also, the host range may evolve over time. A nontarget host that the pest predator did not attack in the laboratory—or even in the environment while the target host was prolific—may become a target as the availability of the target host decreases and the pest predator proliferates. Regardless of the narrowness of the host range, few species are monospecific (feed on one specific host). Seldom, explains Hopper, can a list of species suitable for feeding be "neatly" separated from a list of species that appears unsuitable. Even if nontarget species are at a lesser risk, the risk assessment for individuals in that group must be translated into a risk factor for the entire population. Predicting the overall impact on the target host is difficult, let alone the impact on nontarget species.

Retrospective studies are often the only true way to evaluate the effectiveness—or destructiveness—of the introduced natural enemy. "All natural systems are dynamic," explains Hopper, noting that dynamic systems are a moving target. "Introductions in particular may take a long time to reach some sort of equilibrium or at least relatively steady state," he says. The rule of thumb for an introduced agent to become established in its new environment is approximately 10 years, a timeframe that may be much too short to evaluate its impact on the target, let alone on the nontargets. Yet for researchers, 10 years is a long time. "Such long time horizons and large spatial scales often put evaluations of target, as well as non-target, impacts beyond the resources available to most researchers," says Hopper.

By the time the introduced enemy is well established over a wide area, control sites (sites without the introduced enemy) may be difficult or impossible to find, as the enemy will have infiltrated all of the areas containing its target host. Because control sites are important in assessing the introduction's results, a lack of those sites will make accurate assessment nearly impossible. "A major problem with extensive surveys for non-target impacts is that negative evidence is hard to quantify and publish," says Hopper. "Showing in a convincing way that small but significant impacts have not occurred is much more difficult and time consuming than showing that large impacts have occurred."

Donald R. Strong and Robert W. Pemberton (2000) caution that, while biological control is a "powerful technique," it is not a cure-all. They note that there are very few safeguards in place anywhere in the world protecting native habitats from the intentional introduction of foreign species. They also explain that in the United States biocontrol is governed by a "hodgepodge" of ancient laws that were meant for entirely different purposes; native invertebrates and insects are relatively unprotected; and the past importation of herbivores (plant-eating animals) for the control of weeds has proven "problematic."

Grave Biocontrol Disasters

In 1872 the Indian mongoose ( Herpestes auropunctatus ) was introduced into Hawaii and Jamaica, Puerto Rico, and other parts of the West Indies to control rats in sugarcane. While the mongoose controlled the Asiatic rat, it did not control the European rat. It also found native ground-nesting birds and beneficial native amphibians and reptiles to its liking. R. W. Henderson (1992) found this exotic mammal caused the extinction of at least seven species of reptiles and amphibians in Puerto Rico and other islands in the West Indies alone, and is a major carrier of rabies. Some researchers estimate this biocontrol effort gone wrong costs Puerto Rico and Hawaii $50 million annually by destroying native species, causing huge losses in the poultry industry, and creating serious public health risks.

The Eurasian weevil ( Rhinocyllus conicus ) was introduced into the United States in the 1960s to control a Eurasian thistle that had previously been introduced into the United States and had become a serious pest plant in farming areas. Even before the weevil was released, there were predictions that it might also threaten native thistles. The weevil was released anyway, and does indeed attack native thistles.

Bird species introduced into the United States for biocontrol purposes include the house or English sparrow ( Passer domesticus ) and the common myna ( Acridotheres tristis ). The English sparrow, introduced in 1853 to control the canker worm, proliferated to the point where it diminished fruit crops by eating fruit-tree buds; displaced native birds such as wrens, cliff swallows, purple martins, and bluebirds from their nesting sites; and spread more than 30 different diseases among humans and livestock. The myna, introduced in 1865 to help control cut-worms and army worms in sugarcane, became a major disperser of Lantana camara seeds, an introduced weed species harmful to the environment.

Three small predators from the Mustelid family—the ferret, stoat, and weasel—were introduced into New Zealand in the late 1870s to control rabbits, which had been introduced into the country decades earlier and had quickly become a serious agricultural pest. Mustelids, the stoats in particular, have a huge negative impact on native New Zealand species, especially the indigenous kiwi, killing an estimated 15,000 brown kiwi chicks annually—a whopping 95% of all kiwi chicks. Ferrets find ground-nesting birds easy prey and pose a serious threat to the endangered black stilt, of which only 100 birds remain. The weasel, a small animal and relatively low in number, tackles prey much larger than itself and has negatively affected lizards, invertebrates, and nesting birds.

In Australia, the most disastrous example of biocontrol gone wrong is the introduction of the cane toad ( Bufo marinus ) by the Australian Bureau of Sugar Experimental Stations. Sugarcane was introduced into Australia in the early 1800s and is now a major industry. However, along with the sugarcane came pests to that crop: the grey-backed cane beetle and the Frenchie beetle. In 1935, 102 Venezuelan cane toads, imported from Hawaii where they were reported to have successfully controlled the sugarcane beetle, were set loose in a small area in North Queensland. Hillary Young (2000) says this "misguided attempt" not only failed to control the cane beetles (ultimately controlled with pesticides), but the toads "successfully devoured other native insects and micro-fauna to the point of extinction. Adding insult to ecosystem injury, the poisonous toad instantly kills any predator that attempts to eat it, particularly the quoll, Australia's marsupial cat, and giant native lizards. Its population continues to proliferate, outcompeting native amphibians and spreading disease."

In a two-year program that ended in 1998, scientists at the Commonwealth Scientific Industrial Research Organization (CSIRO), Australia's biggest scientific research organization, failed to find a way to control the cane toad. One pair can lay between 20,000 and 60,000 eggs per breeding season, and the tad-poles develop faster than most native frog tad-poles (Australia had no toads until the cane toad's introduction), thus outcompeting native tadpoles for food. Cane toads are poisonous at all stages in their life cycle; eat virtually anything from dog food, mice, and indigenous plant and animal species to their own young; and grow to almost 10 in (25 cm) long and more than 4 lb (2 kg). They have spread thousands of miles, moving at a rate of almost 19 mi (30 km) a year.

According to the Environment News Service article "Australia Declares Biological War on the Cane Toad" (2001), these toads found Australia to be a virtual paradise compared to their native Venezuela, and are 10 times more dense than in their native habitat. By the year 2000 the species had migrated to an area near Sydney, New South Wales, where the last of the endangered Green and Golden Bell frogs exist. In early 2001 the toads were observed for the first time in Kakadu National Park in the Northern Territory, a 7,700 sq mi (20,000 sq km) environment providing habitats for a huge variety of rare and indigenous species. This delicately balanced environment, one of the few sites the United Nations Educational, Scientific, and Cultural Organization's World Heritage lists as having outstanding cultural and natural universal values, is now facing irrevocable damage from an introduced foreign pest predator. The CSIRO is continuing its research into ways to halt the toad's progress.

Testing for Enemies of the Purple Loosestrife

In the search for a way to control the invasive foreign plant purple loosestrife ( Lythrum salicaria ) in the United States and Canada, researchers identified approximately 15 species of insects and 41 species of native North American plants for testing host-range specificity. They determined that, although some insects could feed on some native plant species, no natives were "preferred" when in the same area as the purple loosestrife, and in no instances did insects complete their life cycle on any native plant. In 1992 researchers began rearing and releasing two European beetles ( Galerucella pusilla and G. calmariensis ) in several U.S. states and all Canadian provinces. Also, a European root-mining weevil ( Hylobius transversovittatus ) and a flower-feeding weevil ( Nanophyes marmoratus ) have been approved for release.

Although the biocontrol plan for purple loosestrife is considered a model program, range testing all native species and predicting what the introduced species will do in the environment over the long term is virtually impossible. David Ragsdale, professor of entomology at the University of Minnesota, says in an on-line article (McIlroy 2000) that insects (usually short-lived) are "wired differently than a long-lived animal like a mammal or bird." Insects gain an adaptive advantage by narrowly selecting a host rather than using their short life spans moving from host to host as they feed. The article also states, "There is no case where an introduced insect has either exterminated the target weed, or unexpectedly switched hosts to become a serious pest of other plants," citing the work of P. Harris (1988).

No Such Thing as Risk-Free

Bernd Blossey, director of Biological Control of the Non-indigenous Plant Species Program, Cornell University, cannot unconditionally exclude the possibility of an unexpected host switch in insects, although he believes this event is extremely unlikely (McIlroy 2000).

Pest control programs that chase one foreign species with another create the potential for the pest predator to become a pest itself, and maybe an even worse pest. In a funny little children's song an old lady accidentally swallows a fly. She then swallows a spider to catch the fly, then a bird to catch the spider, then a cat to catch the bird, then a dog to catch the cat, then a cow to catch the dog, then a horse to catch the cow. The end of the story? She dies, of course. Introducing a foreign species involves risks to the native environment; the ultimate risk is death to the unique native species in that environment.

—MARIE L. THOMPSON

Further Reading

Agricultural Research Service. "IPM/Biological Control." Quarterly Report of SelectedResearch Projects. April-June 2000. <http://www.ars.usda.gov/is/qtr/q200/ipm200.htm>.

"Australia Declares Biological War on the Cane Toad." Environment News Service. 6 March 2001. The Lycos Network. <http://ens.lycos.com/ens/mar2001/2001L-03-16-10-html>.

Biological Control of Nonindigenous Plant Species. 21 September 2001. Department of Natural Resources, Cornell University. <http://www.invasiveplants.net/>.

Carson, Rachel. Silent Spring. Boston:Houghton Mifflin, 1993.

Cox, George W. Alien Species in North America and Hawaii: Impacts on Natural Ecosystems. Washington, D.C.: Island Press, 1999.

Devine, Robert. Alien Invasion: America's Battle with Nonnative Animals and Plants. Washington, D.C.: National Geographic Society, 1998.

Harris, P. "The Selection of Effective Agents for the Biological Control of Weeds." Canadian Entomology 105 (1988): 1495-503.

Henderson, R. W. "Consequences of Predator Introductions and Habitat Destruction on Amphibians and Reptiles in the Post-Columbus West Indies." Caribbean Journal of Science 28 (1992): 1-10.

Hopper, Keith R. "Summary of Internet Workshop on Research Needs Concerning Non-target Impacts of Biological Control Introductions." Online posting. 5 October 1999. University of Delaware. <http://www.udel.edu/entomology/khopper/Summary.txt>.

Malecki, R. A., et al. "Biological Control of Purple Loosestrife." BioScience 43, no. 10 (1993): 680-86.

Manchester, S. J., and J. M. Bullock. "The Impact of Nonnative Species on UK Biodiversity and the Effectiveness of Control." Journal of Applied Ecology 37, no. 5 (2000): 845-46.

McIlroy, Barbara, comp. "Q & A: Biological Controls for Purple Loosestrife." Invasive Plants in the Upper Valley. January 2000. Upper Valley of New Hampshire and Vermont. <http://www.valley.net/~invasiveplants/faq.html>.

Mirsky, Steve. "Floral Fiend." Scientific American November 1999. <http://www.sciam.com/1999/1199issue/1199scicit2.html>.

Pimentel, David, et al. "Environmental and Economic Costs of Nonindigenous Species in the United States." BioScience 50, no. 1 (January 2000): 53-65. <http://www.findarticles.com/cf_0/m1042/1_50/58563640/p1/article.jhtml>.

Strong, Donald R., and Robert W. Pemberton."Biological Control of Invading Species—Risk and Reform." Science 288 (16 June 2000): 1969-70. <http://www.casa.com/hotttopics/insect/key/gen21.html>.

United States Geological Survey. "Bugging Purple Loosestrife." People, Land, and Water July/August 2000. <http://www.usgs.gov/invasive_species/plw/loosestrife.html>.

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KEY TERMS

ALIEN SPECIES:

Any biological material capable of propagating that is not native to that ecosystem; also described as nonindigenous species.

BIOCONTROL:

Intentionally introducing a natural enemy of a pest to control that pest.

ENTOMOLOGY:

A division of zoology that deals with insects.

HOST-SPECIFIC:

An organism that is attracted to only one particular species.

INDIGENOUS SPECIES:

A plant or animal species that is native to a particular ecosystem.

MONOSPECIFIC:

Specific to just one thing, in this case, one species.

NONTARGET HOST:

Unintentional target that a pest predator may attack.

PEST PREDATOR:

An organism that preys on a pest organism.

TARGET HOST:

Intentional target for attack by the pest predator.