By KEVIN PROFT/ecoRI News staff
PROVIDENCE — A few years ago, Anna Browder and Mary Dennis, two East Side residents, noticed something nefarious showing up in the Blackstone Parks Conservancy. It lurked along the edges of the 45-acre, wooded park that sits on the shore of the Seekonk River, and was especially problematic along Paterson Street. Dennis and Browder then spotted it in neighboring properties, as far west as Butler Avenue. They decided something had to be done to control it.
The intruder? Black swallow-wort, a vine famous for its ongoing role in the monarch butterfly’s population decline.
Browder and Dennis, both certified through the University of Rhode Island in invasive plant management, began to remove the dense patch of swallow-wort from the edge of the park between the Paterson Street playground and Angell Street. They also passed out a flier to neighbors with identification pictures, management methods, such as removing seed pods, and their phone numbers.
During the next few years different efforts were made to eradicate, or at least control, the vine in the park. The Providence Parks Department applied an herbicide, but Browder said it had no effect on the plant, possibly because the herbicide was improperly applied or applied out of season. Browder cut back the swallow-wort and dug up large portions of its root system to see if it could be killed off; while she was unable to follow through on monitoring the site due to a health issue, she said it appeared that where the root was dug up, the plant didn’t resprout.
After learning about Browder's effort to control the invasive weed, a neighbor to the park volunteered to regularly mow in the area along the edge of the park where the plant grew. This proved to be an effective management method, but the volunteer has since moved away.
Despite all of these efforts, the plant still grows along the edge of the park on Paterson Street, spreading across the ground or climbing up bushes. During a recent visit, seed pods were hanging from the vine, ready to fall to the ground to further establish the plant and damage the ecosystem.
The conservancy is leading a park-keeping event Aug. 18 from 6-7:30 p.m. to again combat the black swallow-wort near the Paterson Street Playground.
Black swallow-wort grows in dense patches in sun and partial shade. It climbs up, outcompetes and replaces other plants, weakening local ecosystems. Monarch butterflies, which lay their eggs only on milkweed plants, are tricked into laying their eggs on swallow-wort, a close relative of milkweed. None of the larvae hatched on this plant survive.
Already threatened by the logging of overwintering grounds in Mexico, and a lack of suitable habitat due to herbicide use on industrial farms in the United States, swallow-wort offers one more obstacle for monarchs to overcome on their annual migration from Mexico to Canada and back again.
Biological control of swallow-wort could prove to be a more effective long-term management method than labor-intensive and localized control efforts, such as have been practiced at the Blackstone Parks Conservancy.
Biological control combats invasive species at the region-wide level. Natural enemies of a species from the plant’s home range are identified and researched to ensure the assassin doesn’t pose a threat to the new environment. The biocontrol agent, often a moth, beetle or wasp, is then released to prey on the invasive species and bring it into balance with the ecosystem.
In 2006, a URI doctoral student discovered the larvae of the moth Hypena opulenta feeding on swallow-worts in the Ukraine. Six years of research followed and determined the moth was a safe biocontrol agent against swallow-worts in North America, leading to the Canadian government granting permission for the moth’s release into the wild.
Since fall 2013, more than 10,000 moth larvae have been released near Ottawa and Toronto. The moths have successfully overwintered in their new environment and the larvae feed on and damage swallow-wort as desired. So far, though, they haven’t yet built up a large enough population to have a significant impact, according to Lisa Tewksbury, manager of URI’s biocontrol laboratory.
“Getting good data on how well they are doing is difficult because in many cases, the moths fly away and lay their eggs somewhere (outside the boundaries of the release area),” she said.
It could take 20 years to get balance between an invasive species and biocontrol agent in an ecosystem, Tewksbury said.
But localized effects around release sites can be seen in as few as three years. At the Roger Williams Zoo, for example, a beetle species was released, in 1996, to control purple loosestrife, an invasive species that clogs wetland habitat. By 1999 the loosestrife in the release area was almost completely defoliated. Since then, the loosestrife has gone through cycles where it recovers, then becomes defoliated and weak again, said Tewksbury, and the loosestrife growing in open water seems to have been eliminated permanently.
The swallow-wort moth has yet to be released in the United States, because of a cautious and many-stepped federal permitting process. Tewksbury is optimistic that the moth will soon be cleared for release, and is already rearing moths in the URI quarantine laboratory for release in May or June of next year.
When the moth is cleared for release in the United States, the first release will happen on Naushon Island in Massachusetts, between New Bedford and Martha’s Vineyard. The island is home to black and pale swallow-wort, both invasive to the region. Requests for releases have also been received from groups in New York, Rhode Island, Massachusetts, Maine and Michigan, according to Tewksbury.