Thriving non-native plants suffocating native species in southern New England
By KEVIN PROFT/ecoRI News staff
Invasive species have run amok in Rhode Island. Japanese knotweed, which can grow 3 inches a day and reach 10 feet in height, has crowded out native plants along sunny roadsides. Phragmites, a grass that can reach 20 feet in height, have strangled marshes, riverbanks and lakefronts. Oriental bittersweet, a vine that can climb to 60 feet, can kill its host trees and anything that grows in its shadow.
Invasive species, non-native organisms that negatively impact their new habitat, out-compete native plants because their natural enemies from their native range don’t exist in their new environment. The tendency of invasives to completely stifle growth of native vegetation reduces biodiversity and destroys wildlife habitat.
The lack of diversity that results from invasive species makes the environment less resilient, by leaving it with fewer options when responding to factors such as flood, fire or climate change, according to David Gregg, executive director of the Rhode Island Natural History Survey (RINHS).
Different groups and organizations across Rhode Island are working to take back the landscape from invasives. Some efforts are localized, while others have regional impacts.
One plant at a time
The Woonasquatucket River Greenway extends about 5 miles from Lyman Avenue in Johnston to the Providence Place Mall, linking parks and neighborhoods throughout Olneyville. Bikers, joggers and walkers are treated to river views and sunny parks, and a variety of invasive plants, including knotweed, bittersweet and multiflora rose.
To control this invasion, the Woonasquatucket River Watershed Council (WRWC), a nonprofit dedicated to the restoration and preservation of the river, has hired invasive-plant specialist Eric Larson. Larson has managed invasive-plant projects at Second Beach in Middletown and Flat River in Arcadia, and is now directing his 12 years of experience at the greenway. With state Department of Transportation (DOT) funding, he began work on the three-year project this fall.
Larson said his approach to controlling invasives is a plant-by-plant undertaking. “It’s painstaking work, but it’s fun,” he said.
Larson’s primary methods are mechanical, knocking down and pulling up plants or mowing. He also uses herbicides, but said they are a last resort, especially when working near a river. In most instances, he applies minimal and concentrated doses of chemicals directly to the freshly cut stems of the invasive plants. On rare occasions, he uses foliar spraying, but tries to avoid this method as it kills nearby native plants in addition to the invasives.
“We will never get 100 percent of the invasives, but as long as we get the majority, we can reestablish native plants to stop the invasives from growing there,” Larson said.
A 1,200-foot stretch of shoreline along Lake Canonchet, across from the town beach in Narragansett, may offer a preview of what is possible along the Woonasquatucket River Greenway.
For three seasons, the volunteer group Friends of Canonchet Farm (FCF) has fought stands of knotweed, bittersweet, honeysuckle and black swallow-wort to clear the way for native species.
“This is a gateway to Narragansett and it should be beautiful,” said Kathie Kelleher, a FCF board member.
In 2012, when the program began, Kelleher said it took 10 volunteers each week to control the knotweed along the lakeshore. Three seasons later, one person a week can manage what remains in 30 minutes.
All of the work done at Lake Canonchet has been mechanical, Kelleher said. Volunteers knock down tall stands of knotweed, then pull or dig up the remaining invasives.
Native plants re-established themselves naturally once the invasives were controlled, she said. Seeds awaiting the right conditions in the understory of the knotweed, perhaps for years, sprang to life. Now, an ecosystem of rose mallow, golden rod, joe pie weed, milkweed, Turk’s cap lily and bayberry has taken hold.
FCF is planning to partner with Rhody Native to further diversify native plants along the lake, Kelleher said.
The success of the project at Lake Canonchet has spurred FCF to restore another 800 feet of lakeshore along neighboring Little Neck Pond. Work began this year.
The volunteer group plans to use herbicides along the shore of Little Neck Pond, because, according to Kelleher, there are no native plants left to damage. She said Littleneck Pond and Lake Canonchet will offer a side-by-side comparison of which method is more effective — herbicides or mechanical control.
Despite their successes, Larson and Kelleher understand the limitations of their work.
The WRWC will always need to monitor the Greenway to ensure invasive plants don’t return, according to Larson. “They’ll come back easily,” he said. Seeds from upriver will float to the banks of the Greenway and invasives from neighboring properties will attempt to re-establish themselves along the path.
“This is a project that will never end because knotweed is not under control in Narragansett,” Kelleher said. A drive down Ocean Road toward Point Judith, where dense stands of knotweed crowd the roadside, confirms her statement.
Additionally, the methods being used by WRWC and FCF are labor intensive. From April to September, 61 volunteers dedicated 2,000 hours to the project at Lake Canonchet and Little Neck Pond — a stretch of only 2,000 feet.
Asked whether his methods could be used to control an invasive plant statewide, Larson said, “it would be quite an undertaking.”
Gregg, of the RINHS, oversaw just such a project between October 2011 and October 2012. The goal was to control mile-a-minute, an invasive vine, before it became permanently established in Rhode Island. When the project began, mile-a-minute had only been reported in Block Island, Cranston and East Greenwich.
The collaborative project between government agencies, private industry and nonprofits rallied community interest in invasives management from high-school students to retirees. Despite the project being well managed, the results were disappointing.
“Areas where dense mile-a-minute vine had been pulled in 2011 appeared to be completely regrown with little or no apparent reduction,” Gregg wrote in the final report. Fringe areas, where mile-a-minute was less established, showed some reduction in plant density, he reported.
“We learned we cannot control mile-a-minute that way,” Gregg said. Still, he believes local projects are valuable. “Whether they succeed or not, they are engaging people in conservation and teaching them about biodiversity.”
Richard Casagrande, a University of Rhode Island entomologist, has spent his career identifying biological control agents to combat invasive species at the region-wide level.
Casagrande searches for natural enemies to an invasive species from its home range, experiments to find those that don’t pose a risk to the new environment, and then releases them to prey on the invasive species. The biocontrol agent, often a moth, beetle or wasp, brings the invasive species into balance with the ecosystem, allowing it to coexist with native species.
URI is one of a handful of schools with a USDA-approved quarantine lab that permits this type of work. When experimenting with non-native biocontrol agents, it’s crucial that they don’t escape, because they could cause widespread negative impacts on the environment.
In partnership with other colleges and European biocontrol institutes, URI has been part of a number of successful biocontrol projects.
Purple loosestrife, an invasive wetland weed, was introduced into the United States from Europe in the 1800s. In the 1990s, two varieties of leaf-eating beetles known to attack purple loosestrife in its home range were released in North America after years of research demonstrated that the beetles wouldn’t harm the ecosystem.
URI wasn’t involved in the research that identified the beetles as biocontrol agents, but did help release and monitor them. URI released beetles at multiple sites in Rhode Island from 1995-1997 and reared and released additional beetles from 2005-2013.
In 1996, the Roger Williams Park Zoo, working with URI, released 3,000 beetles into a wetland overrun by purple loosestrife. Two years later, the purple loosestrife experienced complete defoliation, and by 2004 its population had declined dramatically, according to a report by Casagrande.
Plant height, flowering and seed production also declined. Native plants, such as sensitive fern, swamp rose mallow and slender blue flag iris, returned to the wetland naturally, while other native species such as elderberry and swamp maple were re-established by zoo staff.
The lily leaf beetle, an invasive insect that attacks both native and ornamental lilies, was introduced into Montreal in 1945 and first recorded in Boston in 1992. It has since spread throughout New England. Three parasitic wasps from the beetle’s European range were identified as suitable biocontrol agents through research conducted, in part, at URI’s quarantine lab.
From 1999 to 2013, the wasps were released throughout New England and have since diminished lily leaf beetle populations, especially at early-release sites, according to Lisa Tewksbury, research associate and lab manager at URI.
The birch leafminer, an invasive sawfly that attacks several species of birch, was first reported in Connecticut in 1933. The leafminer can damage more than 90 percent of leaves on infested trees. Since the release of a parasitic wasp, which research identified as a suitable biocontrol agent, the birch leafminer has come under complete control in southern New England and New York.
At Ryan Park in North Kingstown, between 1990 and 2008, the percentage of leaves mined fell from an average of 87 percent to an average of 2.6 percent, according to a 2009 report by Casagrande.
Pale and black swallow-wort, invasive weeds introduced into Massachusetts from Europe in the 1800s, have spread throughout the Northeast and into Canada and the Midwest. Swallow-worts interfere with pastures, agriculture and forests, according to Casagrande. Additionally, swallow-worts are closely related to milkweeds and threaten monarch butterfly populations. Monarchs lay eggs on swallow-worts, but all larvae that hatch on the plants die.
In 2006, a URI doctoral student discovered the larvae of a moth feeding on swallow-worts in Ukraine. The six years of research that followed determined the moth was a suitable biocontrol agent against swallow-worts in North American, and resulted in the Canadian government granting permission for the moth’s release.
Since fall 2013, more than 10,000 moth larvae have been released near Ottawa and Toronto, according to Casagrande. If the moths successfully overwinter, the existing population will be bolstered by additional releases and should spread throughout the country.
Early observations show the moth is having the expected negative impact on swallow-wort in Canada, though more time and data is needed before conclusions can be drawn. “Right now, it looks good,” Casagrande said.
Proceed with caution
Casagrande believes biocontrol is the only effective way to control invasive species regionally, but the method has drawbacks.
Obtaining the appropriate permits from government agencies is challenging. Governments are rightfully cautious about allowing non-native organisms to be released into the environment, because the practice has backfired in the past.
In Australia, 3,000 cane toads from the Americas were released to control a sugarcane beetle at a Queensland plantation in 1935. The cane toad failed to control the beetle, but has multiplied into the millions, spreading throughout much of northeastern Australia. The venomous toads kill native animals that eat them. They prey on native fauna, reducing prey populations for native insectivores, according to National Geographic.
In the 1800s, mongooses were released into Hawaii to control rodent populations on sugarcane plantations. Their effectiveness against rodents has been tepid, and they now threaten the existence of native ground-nesting birds, according to National Geographic.
In 1957, the cactus moth was introduced to control a cactus on the island of Nevis in the Caribbean. The moth succeeded at controlling the pest cactus, then spread to the surrounding islands and Florida, where it was first recorded in 1989. It now threatens native prickly pear cacti and is rapidly spreading along both the Atlantic and Gulf coasts, according to the USDA.
Casagrande said scientists are aware of the biocontrol failures that give his field its controversial reputation, but he notes that many examples are from the distant past.
“When many of these agents were released, we were still practicing bloodletting as serious medicine,” he said. He noted that scientists have learned from past mistakes and are now more cautious.
“It’s not that we are reckless, it is that the weed biocontrol process in the U.S. is dysfunctional,” Casagrande said.
Asked how much time it currently takes for a weed biocontrol agent to be permitted by the U.S. government after receiving approval by experts, Casagrande replied, “infinite.” The last permit approved was originally petitioned in 2009; seven other projects are currently pending despite expert approval.
Since the regulatory process in Canada is more efficient, at least according to Casagrande, he and his colleagues apply for permits from both governments. While the United States continues to debate whether the swallow-wort moth is safe to release, it is already flying free in Canada. Casagrande said the moth could migrate to the United States before U.S. regulators act, effectively taking the decision out of their hands.
Identifying suitable biocontrol agents also is time consuming. The swallow-wort moth, for example, required six years of study and testing against 76 potential host plants before being approved.
Since 1998, Casagrande has studied two moths as biocontrol agents against phragmites, an invasive wetland reed. He is confident in the direction of the research, but said, with luck, the moths may be approved for release in five years — in all, a 20-year process.
There are limited people and funding dedicated to the work of biocontrol. Almost all biocontrol programs depend on grant funding. While invasives such as phragmites, swallow-wort, knotweed, mile-a-minute weed, Japanese stilt grass, water chestnut and tree-of-heaven all have active biocontrol programs, there are many others that scientists don’t have the time or resources to confront.
Despite these issues, biocontrol often remains the only hope for controlling invasives. In Narragansett, Friends of Canonchet Farm doesn’t attempt to fight the phragmites that grow in tall stands in the shallow water just beyond the restored shoreline. Kelleher said that, as amateurs, her volunteers are simply not able to control it.
She said that even in cases where herbicides are used in the water to kill phragmites, they still return. Some scientists have told her phragmites are something she will have to learn to live with.
Casagrande would beg to differ, but it may be some time before he is able to prove them wrong.