Thursday, October 24, 2013

Strangle-Vine: Invasive Swallow-Wort

swallow-wort1The invasive plant sometimes called dog-strangling vine doesn’t harm pets, but it lives up to its name as a strangler, choking out native wildflowers as well as Christmas tree plantations and fields of prime alfalfa. In Northern New York, in Jefferson County, a nearly 1,000-acre tract on an island lies blanketed under this perennial Eurasian vine.

Dog-strangling vine grows in almost any soil type, has a prodigious root system, and is particularly good at making and dispersing seeds. It is so toxic that no North American bird, mammal or insect will eat it, and it bounces back from the most powerful herbicides. No wonder biologists and agronomists have been losing sleep over it.

So it’s with much relief that researchers at the University of Rhode Island announced they’ve recently located and vetted a worthy opponent for the invasive vine. While dog-strangling vine, more commonly known as swallow-wort, has withstood mowers, rototillers, flame throwers (true) and potent agrochemicals, it’s no match for this brute. At a tenth the weight of a paper clip and a fraction of an inch long, our hero is Hypena opulenta, the larva of a Ukrainian moth.

In its native range, swallow-wort is just another well-behaved native plant because the tiny green Hypena opulenta caterpillar, along with other insects, keeps 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” and doing more harm than good, which has happened in the past.

This caterpillar won’t be USDA approved for release here until some time in 2014 at the earliest, but in Canada, 500 Hypena opulenta larvae were released near Ottawa in September 2013 in a cold-hardiness trial. The hope is that we’ll have a success story like the recent one with purple loosestrife, in which biological controls stopped it from harming freshwater wetland ecosystems. Even if Hypena opulent performs admirably, it may be a decade or more before we see the decline of the dog-strangler. To paraphrase Princess Leia, “Help us, Hypena opulenta; you’re our only hope.” Thus far, at least.

More information on black swallow-wort, pale swallow-wort, and other invasive species can be found online.

Swallow-wort photo courtesy of Purdue University Extension.

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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

3 Responses

  1. SwilliAm says:

    I’m not sure how much of a foothold dog-strangling vine has gained in the Adirondacks, but last summer I came across a very large expanse of it on Valcour Island, in Lake Champlain.

    It’s especially pernicious because it’s very shade tolerant and so, like garlic mustard, it invades the densely shaded forest floor. Let’s hope this biological control works!

  2. Bill Ott says:

    Mr Hetzler,
    I just got back last night from 2 weeks in the 5-Pond area, particularly along the Robinson River. I took with me homemade, laminated, identification sheets of the 8 most common invasive plants likely to be found in the region. However, due to the lateness of the season and my unfamiliarity with similar native species, I found my sheets to be rather useless. Where would one find a guide for an amateur. I go places not regularly visited that could be devastated by invasives.
    Bill Ott, Lakewood, Ohio

    • Paul Hetzler says:

      Mr. Ott,

      Sorry for the long delay, but I haven’t been checking comments.

      I think you’re not giving yourself enough credit. As you said, it was late in the season, when and many plants have senesced to the point that their distinguishing features are hard to see. Another thing is that, given the places you visit, it’s very unlikely that you’d encounter some of the most pernicious invasive species.

      However, as you said, one introduction could be devastating if it was left unchecked in a remote area, and I’m glad you’re keeping an eye out for invasives. In my experience, the full-color laminated ID guides are usually very good–I suspect you’ll have no trouble with ID if you do encounter something. But don’t be frustrated if you don’t; be glad.

      If you want to email me at ph59 [at] cornell [dot] edu, I’d be happy to try and direct you to a more comprehensive guide.

      Best regards,

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