Saturday, August 12, 2017

Adirondack Dog-Strangling Vine

swallow-wortThis summer, the United States Department of Agriculture (USDA) has come through with a new hope for the forces of good. Its Animal and Plant Health Inspection Service (APHIS) has opened a public comment period, ending on August 14, 2017, relating to the release of a non-native insect to control swallow-wort.

Sometimes called “dog-strangling vine,” this invasive plant from Eurasia doesn’t harm pets, but it does live up to its name as a strangler. There are two species of the perennial vine, and they are both adept at choking out wildflowers, forest seedlings, Christmas tree plantations, hay fields and other habitats. In the Eastern Lake Ontario region, it has proved capable of blanketing large tracts, hundreds of acres in some cases, to create permanent monocultures of tangled, toxic foliage.

While there are subtle differences between pale and black swallow-wort in terms of appearance, they behave the same, and there are reasons (for example, indulging my laziness and sparing you some tedium) to lump them together in an article, if not a hay field. Heck, botanists are still fist-fighting about whether swallow-worts are in the genus Cynanchum or Vincetoxicum, so let’s consider them (plants, not botanists) identical.

Swallow-wort has so many vile tricks up its proverbial sleeve, I think even Emperor Palpatine from the Star Wars saga would be jealous. It can grow in nearly all soil types, has a robust, Medusa-like root system which poisons competing vegetation, and it is endowed with super-powers where seed production is concerned. Related to the common milkweed, swallow-wort attracts monarch butterflies, which lay eggs on it. The problem there is that the caterpillars die from eating the toxic leaves. In fact, the vine is so noxious that nothing, not even insects, feed on it. Amazingly, it can bounce back from powerful herbicides like glyphosate (the active ingredient in products such as Roundup), apparently stronger than ever. No wonder biologists and agronomists have been losing sleep over it.

So it’s with much relief that the USDA posted its notice about the public-comment period on July 13th. While dog-strangling vine has withstood mowers, rototillers, flame throwers (true) and potent agrochemicals, it’s no match for this brute, which was identified by researchers from the University of Rhode Island and others in 2013. At roughly a tenth of the weight of a paper clip and a fraction of an inch long, our hero is a caterpillar, Hypena opulenta, the larva of a Ukrainian moth.

In its native range, swallow-wort is just another well-behaved plant because the tiny green Hypena opulenta caterpillar, along with many other insects, keep it in check. In fact, Carleton University biologist Naomi Cappuccino, a Canadian researcher who went to Ukraine with URI scientists to look for biological controls, said they had a hard time locating any swallow-wort; it was that scarce.

Finding biological controls for an exotic invasive plant is no easy task, and requires traveling to the plant’s home environment(s) to make field observations in all kinds of conditions. But once an insect, pathogen or vertebrate is identified, the real work begins. Many years of quarantine with carefully controlled trials are required to make sure a potential hero won’t end up going over to the Dark Side when swallow-wort gets scarce, eating all our corn and soybeans or something like that. In the past, “miracle cure” organisms have done more harm than good (witness the cane toad in Australia and the mongoose in Hawaii), so a great deal of care is taken to approve biocontrols today.

Not to take the shine off the USDA’s work, but our Canadian friends approved Hypena opulenta for release way back in 2013, and it is already putting a dent in the dog-strangler population in some places. The hope is that we will eventually have a success story like we had with purple loosestrife, which was poised to wreak great harm to freshwater wetland ecosystems before biocontrols were introduced. Even if Hypena opulenta performs admirably, it may be a while before we notice much difference. To paraphrase Princess Leia, “Help us, Hypena opulenta; you’re our only hope.” Thus far, at least.

For more information on swallow-wort and other invasive species, visit, or call your local Cornell Cooperative Extension office. To see the USDA proposal and its comment page, click here.

Photo of Swallow-Wort, courtesy Cornell University.

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Paul Hetzler

Paul Hetzler has been an ISA Certified Arborist since 1996. His work has appeared in the medical journal The Lancet, as well as Highlights for Children Magazine.

You can read more of his work at or by picking up a copy of his book Shady Characters: Plant Vampires, Caterpillar Soup, Leprechaun Trees and Other Hilarities of the Natural World

4 Responses

  1. Ray Mainer says:

    I wouldn’t know it if I saw it.

  2. JMD says:

    I consider Black Swallow-wort the most invasive plant that I battle in central Vermont. it is a terrible scourge that most property owners are oblivious to. It can decimate fields and garden landscapes, 12 foot shrubs, mowed lawns, shady forests, and full sun rock outcroppings. It thrives everywhere under all growing conditions. The seeds can spread for many miles from just one plant. Monarch Butterflies mistake it for Milkweed and it poisons them. it is a rotten miserable weed! Every time I visit the Adirondacks I look for it, and dread the day when I discover it there too.

    The most effective treatment is a 1% solution spray of Triclopyr herbicide sprayed prior to the stage of seed pod formation. At that concentration level it will not kill grass, nor many other plants (Raspberries are a notable exception), but it is highly effective against Swallow-wort. It just needs to be given a couple of weeks to work it’s magic. Do not use RoundUp, as it will not permanently kill the weed unless it is applied to the point of making the soil toxic. Even then, it is ineffective. Pulling the plants is also ineffective because of the spreading roots.

    Follow-up applications of Triclopyr will need to be made as seed that has been dormant in the soil begins to germinate. Total eradication is possible within two growing seasons as long as one is diligent.

  3. Wally Elton Wally Elton says:

    Thanks for the article, because I was not familiar with this invader. Any hope for something similar for knotweed? I admit, though, that I’m always a little uneasy about releasing any non-native creature into “the wild.”

  4. Just a note that triclopyr is a restricted pesticide in NYS and not available without a NYSDEC pesticide applicator’s license. Apparently formulations that contain a high percentage glyphosate (for example Roundup Pro at 48.7%) and a surfactant will kill swallow-wort, but I agree that triclopyr is probably a better choice, given that it is selective whereas glyphosate kills everything.
    Fortunately, swallow-wort seeds have a thin seed coat and do not seem to remain viable in the soil beyond 5 years (contrast this with dodder seeds which may be viable for 10-60 years).

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