What to do with a Thousand Acres of Swallow-wort?

With 80-foot cliffs overlooking eastern Lake Ontario, 14 miles of hiking trails, a dog park, a state-of-the-art playground, a residential cottage that sleeps eight, and a globally rare ecosystem, Robert G. Wehle State Park is a gem.

This striking landscape also has a military history of helping to defend the country. Between 1895 and 1947 before it was a park, the U.S. military used this property as training grounds. The park includes remnants of the Stony Point Rifle Range, where soldiers trained for combat, as well as shoreline concrete observer posts where spotters oversaw aerial gunnery target practice.

In 1963, the U.S. Army sold this land to Louis Wehle, founder of the post-Prohibition Genesee brewery, and Thomas Nagle, a Rochester car dealer. In succeeding years, Wehle and his son, Robert, maintained the property as a cattle farm, game preserve, and rural retreat for raising of internationally-renowned hunting dogs . After Robert Wehle’s death in 2002, the state Department of Environmental Conservation acquired the land, later passing it State Parks to establish as Robert G. Wehle State Park in 2003.

Click on this slideshow below for scenery at the park:

But visitors to this park may notice something else beyond its beauty _ large areas overrun by a strange, twining vine that seems to grow everywhere that is not mowed lawn, leaving few if any other plants surviving. Before his death, Robert Wehle was trying, with limited success, to control this invasive plant, known as pale swallow-wort (Vincetoxicum rossicum).

Now, decades after it was used to help train soldiers, this park is again on the front lines of a new mission: To be part of a campaign to learn whether a small moth found in Europe and Asia can help fight this invading perennial plant, which has spread throughout the eastern U.S. and Canada.

Pale swallow-wort at the entrance to Robert G. Wehle State Park in Jefferson County. The plant has begun to turn yellow at the end of the summer.


But first, what is this aggressive interloper that drives out other plants wherever it spreads?

Also given the ominous name of “dog-strangling vine,” pale swallow-wort is a native of Ukraine that was introduced to North America in the mid- to late-1800s as an ornamental vine in herbariums and greenhouses. Once here, it began expanding into old fields, pastures, and woodland understories. Pale swallow-wort wipes out native plants in its path due to its vast root system, immense seed production and seed dispersal method (seeds look similar to milkweed seeds and can float far away in the wind), and the production of allelochemicals that inhibit growth of other nearby plants and protect it from grazing animals. Whitetail deer, which will eat most plants, avoid it.

Pale swallow-wort also poses a threat to New York’s population of native Monarch butterflies, which require milkweed to reproduce. Monarchs are known to confuse swallow-wort with milkweed and lay their eggs on it. Due to the chemical composition of swallow-wort, Monarch larvae that feed on the plant usually don’t survive.

All of these traits combine to create the ‘perfect’ invasive species and a land manager’s worst nightmare. So, what has been done and what is still being done to control this tenacious weed?

Robert Wehle noticed this plant on his property, according to anecdotal accounts. The cattle herds that he kept could have suppressed the plant’s invasion in pastures through grazing and trampling.  Wehle also utilized fire management to maintain some fields, which could have held swallow-wort at bay temporarily. Records also indicate he tried chemical herbicides to control swallow-wort infestation. This suggests that, like subsequent scientific studies conducted have shown, that Wehle found grazing and burning were not effective control techniques. 

After the land became a State Park, grazing, burning, and chemicals were no longer done.  Instead, staff began mowing areas around the entrance, maintenance shop, parking lots, rental compound, and trails frequently, which cuts back swallow-wort before it matures enough to produce seeds. But only so much mowing could be done on a 1,100-acre property.

Where mowing stops at Robert G. Wehle State Park, pale swalow-wort often begins.
Pale swallow-wort along trails in the park, above and below. The plant turns yellow in the fall.
The flowers of pale swallow-wort.

A plan to address this issue was adopted in 2010 by State Parks, in cooperation with the state Department of Environmental Conservation. The first step was to raise public awareness of the problem. Interpretive signage was installed at most trailheads throughout the park to inform visitors. Boot brush stations were placed at the entrance/exit to the park for patrons to clean off their footwear to limit the spread of swallow-wort and its seeds off the property . The feathery seeds can easily stick to shoes, clothing and even the fur of dogs being walked.

That same year, State Parks hired an excavation company to carry out an experiment that may show promise for restoring degraded portions of Wehle’s globally rare Alvar ecosystem. Alvar is a grass- and sedge-dominated community, with scattered shrubs and sometimes trees. The community occurs on broad, flat expanses of calcareous bedrock, like limestone or dolostone, covered by a thin veneer of mineral soil.

Using a skid steer in selected areas, crews scraped away soil containing swallow-wort roots from limestone bedrock. Once most of the soil was gone, swallow-wort could not take root on bare rock. The areaa was then reseeded with native species. Other native plants showed up on their own, freed from the smothering competition from the swallow-wort. But these efforts could not be used everywhere in the park.

An area of the park reclaimed from pale swallow-wort by scraping off soil and later reseeding it with native plants.
In addition to the Lake Ontario and St. Lawrence regions, pale swallow-wort is found in other areas of the state, including the Finger Lakes and Hudson Valley. (Photo credit – New York State Invasive Species Information, http://nyis.info/)

Where does the moth come into this ongoing effort? For the last two years, Parks and its partners at Cornell University, SUNY College of Environmental Science and Forestry (ESF), Wells College, SUNY Cortland, and the University of Rhode Island have been using Robert G. Wehle State Park to study the viability of Hypena opulenta moths to suppress this invader.

This approach – the use of a natural enemy to deal with an invasive species – is known as biocontrol. In order to ensure that a new introduced species will not negatively impact other plants and animals, the effects must be extensively studied before any widespread use or release can be permitted. It cannot be overstated how extensively biocontrols are scrutinized before potential approval for release. Study can continue for years or even decades. Only after research confirms there will be little or little to no impacts to native species will federal regulators approve the biocontrol to be released.

In this case, the U.S. Department of Agriculture (USDA) in 2017 approved the release of the moths for field testing as biocontrol for pale swallow-wort. After the moths lay eggs on the swallow-wort, the larvae that later emerge eat the plant’s leaves.

For the last two seasons, Hypena moths were placed in cages in areas of swallow-wort at Robert G. Wehle State Park, as well on as nearby Grenadier Island in Lake Ontario off Cape Vincent. The cages ensured that the moths would be confined to the test areas.

Results from 2020 showed promise as one cage showed 100 percent defoliation of swallow-wort within four weeks by the caterpillars. Preliminary results from this year were not as successful. However, this is all part of the scientific process as the battle against the invasive continues with Robert G. Wehle State Park playing its part.

A Hypena opulenta moth inside the mesh cage over a patch of pale swallow-wort. The moth will lay its eggs on the plants.
After eggs hatch, the emerging caterpillars begin eating the plant leaves.
After four weeks, the caterpillars have eaten all the leaves in this cage. (All photos above credited to the St. Lawrence Eastern Lake Ontario Partnership for Regional Invasive Species Management.)

Hopefully one day we can say the tide is turning. Eradication is likely not possible, but containment could give native plants a better chance at a peaceful co-existence. If you visit the park, remember: Use the bootbrushes and check your clothes! Don’t inadvertently spread the ‘perfect invasive.’


Cover Shot – A pale swallow-wort infestation at Robert G. Wehle State Park. All photos NYS Parks unless otherwise credited.

Post by Pete Zimmer, Stewardship Specialist, Mid-State Capital District/Thousand Islands Region, NYS Parks



Resources


Learn more about the biocontrol project from the report below by the St. Lawrence Eastern Lake Ontario Partnership for Regional Invasive Species Management:

More information is also available from the New York State Invasive Species Research Institute.


The 2013 report by the U.S. Department of Agriculture also describes early efforts to contain pale swallow-wort.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.