Science / Medicine : Some define regional identity, others threaten our very food supply. Agricultural scientists are losing the battle against : Invading Plants
They came to America from all over the globe: from Russia, Europe, Asia, even Africa. At first, most were welcomed.
Clothed in luxurious garb of petal and leaf, many of these foreign plants were brought by settlers; others hitchhiked as seeds hidden in the food of livestock or in the soil used as ballast in old ships that was later dumped on land.
But as they traveled up, over and across the American landscape, the welcome wore out.
Today, Americans are only beginning to recognize the toll exacted by this invasion of foreign plants, and they are spending millions to get rid of them--seldom with much success.
Alien plants now are clogging waterways, disrupting natural ecosystems and strangling crops. In fact, the 10 most serious crop-threatening weeds in the country today are all foreign plants, according to the U.S. Department of Agriculture. American farmers lose more than $10 billion worth of crops each year to these “introduced weeds.” And more may be on their way.
Since 1974, the government has prohibited the importation of 104 species of foreign plants, and the Agriculture Department now wants to expand that list by 700 more.
The invaders are so successful because, in coming here, “they left their own biological control agents behind,” said Gerald Henke, a U.S. Forest Service range conservationist responsible for noxious weed control. And without predators like the insects that kept their numbers in check at home, the aliens spread unimpeded. “They keep expanding exponentially,” said Norm Reese, a USDA agricultural research scientist in Montana.
In some states, counterattacks against invading plants are well under way, with herbicides, fire, cutting, digging--all methods that, at best, only slow the plants’ proliferation.
Increasingly, U.S. scientists are looking to imported swarms of foreign beetles, flies, wasps and moths to culture here and to release upon the aliens. Sometimes as many as six different species of foreign insects are used to keep a single plant species in check--and even then, success is not assured.
As a result of the plant invasion, “it’s very likely a significant percentage of native species will be impacted,” said Bruce Sorrie, a New England botanist. “If we allow our natural areas to be degraded by alien plants, we’ll be losing the quality of our forests, losing habitat for wildlife and losing part of our natural heritage.”
It’s already happening:
* In the Southeast, kudzu, a high-climbing Japanese vine, covers trees, railroad beds, cars, buildings--”anything that sits still long enough,” according to Jim Miller of the Forestry Service at Auburn.
Growing at a rate of up to a foot a day, kudzu fetters stationary railroad cars, overtakes homes and brings down transformers and telephone wires with its weight. The vine now covers 7 million acres in Alabama, Georgia and Mississippi--and is costing those states between $50 and $175 million a year in lost timber revenues alone, according to Miller.
Kudzu’s graceful purple blossoms were intentionally introduced to the South in the 1920s as a forage crop for pigs, goats and cattle--a situation Miller likened to “taking a lion cub home for a pet.”
* In Massachusetts, the attractive purple loosestrife, a Mediterranean native with a tall of pink-purple flowers, has rendered some waterways at wildlife refuges virtually c,14p8
useless. Taking over marshes and ponds once inhabited by American cattails, bulrushes and sedge, purple loosestrife has usurped the native plants that once provided food and cover for rare native birds like the American bittern, the common moorhen, and the king and sorra rail. Where loosestrife now reigns, these birds have vanished.
* In North Carolina, multiflora rose, a bush imported from eastern Asia, has overrun more than 2 million acres of meadow. The spine-covered beauty was once encouraged as a natural fence; today, it envelopes the fields it was supposed to enclose, rendering them impassable and useless to people or animals.
Foreign plants have so insinuated themselves into the American landscape that, in some cases, they define it: Most people consider bright blue chicory, ox-eye daisies, Queen Anne’s lace, dandelions and crabgrass “typical American” plants. And Los Angeles is well-known for its palm trees.
But none of these plant species existed in pre-Colonial America. American fields, forests and marshes are filled with invaders and vagabonds. By one calculation, nearly one-fifth of the plant species in the Northeast alone were introduced, accidentally or by design, from foreign lands.
While many of these plants have been around for more than two centuries, “their numbers have skyrocketed in the past 20 years,” according to Sorrie. With an explosion in bulldozing, road building and other construction, these opportunistic aliens quickly colonize the altered landscape, taking over drained and flooded wetlands, roadsides and cleared lots. Trucks and railways hasten their spread, carrying seeds of the foreigners in loads of hay. As a result, foreign plants that gained a toehold a century or two ago have run rampant. New introductions take off almost immediately.
African witchweed, for example, was first discovered growing in North and South Carolina in 1957. Today, more than 200,000 acres are infested with the parasite. Sucking its nutrients from the roots of corn, witchweed can reduce crop yield by 50% within three years of arriving in a farmer’s field.
The USDA is spending $4.4 million a year trying to eradicate the red-blossomed, 2-foot-tall plant. “We’ve been fighting it for 30 years, and we’re just now at the point where we have the tools to eradicate it,” said Tom Flanigan, assistant staff officer with the USDA’s Animal and Plant Health Inspection Service. Twenty-seven different herbicides are needed to wipe out the African immigrant in all its habitats.
Perhaps the greatest irony of the alien invasion is that many of the noxious plants arrived as pampered, invited guests, intentionally imported and carefully nurtured by colonists and their followers. Even folk hero Johnny Appleseed was guilty of this mistake. Besides leaving behind orchards, he also introduced a nasty weed, dog fennel, which plagues Midwestern farmers to this day.
Foreign weeds are much more difficult to control than native ones because the aliens have no natural controls here, explained Ray Taylorson, USDA plant physiologist. And, worse, “by the time a new alien weed is discovered here,” he added, “it’s usually already firmly established and often extremely difficult to wipe out.”
By the time kudzu was recognized as a problem, for instance, many patches had developed root systems weighing up to 300 pounds, stretching 20 feet underground.
Managers of the Parker River National Wildlife Refuge on Plum Island in Massachusetts are still trying to figure out what to do about the purple loosestrife choking their three fresh water impoundments. And saving those pools from total takeover “isn’t just a matter of dollars--it’s how to go about it,” said Bob Secatore, assistant manager of the 4,650-acre bird sanctuary.
In Idaho, the Russian common crupina has rendered 30,000 acres of rangeland useless because cattle will not eat the plant, and it out-competes native forage crops. Since 1982, the USDA has spent $100,000 each year trying to eradicate it. While pesticides can kill 99% of the crupina in any given range, the remaining 1% can re-infest the entire area within five years.
But there is a glimmer of hope. Some creative approaches appear to be paying off.
Hydrilla is an aquatic weed that was imported from Central Africa and Southeast Asia as an aquarium plant and then spread through 200,000 miles of California and Florida canal and river systems until many waterways were completely clogged. Two years ago, after three years of USDA-funded research, a hydrilla-eating fish, the grass carp, was introduced into the Californian Imperial Valley Irrigation District. “It’s working well,” according to Flanigan. The carp are sterile hybrids, he added, so the fish cannot become pests themselves.
USDA researchers are also looking to foreign insects for help in controlling foreign weeds. But scientists have to be extremely careful because there is always the risk that the insects themselves can become pests.
For instance, in an effort to check the growth of the musk thistle, a thorny weed from Europe with bright pink flowers that grows up to 8 feet tall, researchers looked at 89 species of foreign insects known to feed on the plant. Of those, only 26 were detrimental to the weed. But 21 of those species could also damage the U.S. artichoke crop, a plant distantly related to the musk thistle.
Another risk is that such insects can compete with or threaten wildlife.
In 1969, the USDA selected a European weevil, Trichosirocalus horridus, as its insect warrior against the thistle. Since, 3 million of the weevils have been released in 22 states. Because the weevil’s larvae chew through the thistle’s seed head, they slow the weed’s spread.
Unlike most foreign weeds, the musk thistle reproduces almost exclusively by seeds. But sometimes one type of insect is not enough to control the spread of a plant, which may reproduce via seeds, roots and shoots.
Thus, six insect species--including a leaf-eating moth, a root-burrowing beetle, and a seed-eating fly--are being used in concert to attack the cypress spurge, an alien now found throughout the Western United States. Five different bugs are being evaluated for use in controlling the spread of leafy spurge.
Some insect controls have proved useful. One European beetle, Chrysolina quadrigemina, has cut the spread of the Klamath weed (a.k.a. goatweed) by 90%. The Eurasian plant’s oil glands sicken cattle, and was such a serious threat to Western rangelands that, during World War II, banks would not loan on California properties that were infested with it.
Other introductions, however, have yet to prove their worth. A leaf-mining moth from Russia, released to combat the Russian thistle in Montana several years ago, may not have even “established” in the state, much less made an impact on the thorny weed. And the introduction of a hawk moth--one of the initial efforts against Cypress spurge--failed because it was accidentally introduced with its own virus.
The nonprofit Nature Conservancy has established Natural Heritage programs in 36 states that catalogue native and introduced species. In the last two years, the organization has begun to establish computerized management abstracts on methods to effectively destroy aliens competing with natives.
But if Americans cannot wipe out the problem aliens from foreign lands, there is at least this one small consolation: we gave Europe poison ivy.
SOME GOOD AND BAD IMPORTED PLANTS
1. CALIFORNIA PALM
Washingtonia filifera
Grow 20-60 feet high.
2. PURPLE LOOSESTRIFE
Lythrum salicaria
Grow 2-4 feet tall. River banks, ditches and marshes.
3. CYPRESS SPURGE
Euphorbia cyparissias
Calcareous soil. Found in Eastern states.
4. OX-EYE DAISY
Chrysanthemum leucanthemum
18 inches tall. Basic soils and pastures.
5. QUEEN ANNE’S LACE
Daucus carota
1-3 feet tall. Open places. Pacific states.
6. DOG FENNEL
Anthemis cotula
1/2-3 feet tall. Pacific states.
7, 8. DANDELION
Tapiscia taraxacum
Many species. Mostly nothern hemisphere.
9. KUDZU VINE
Pueraria lobata
Grow 40-60 ft. per season. Eastern and Southern states.
