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Dog-strangling vine in Ontario: Caterpillars to the rescue!

Dog-strangling vine in Ontario: Caterpillars to the rescue!
By Naomi Cappuccino, Carleton University

Dog-strangling vine (DSV), also known as pale swallow-wort, was introduced from Europe in the late 1800s. DSV has become a highly invasive weed in the Ottawa Valley, Toronto area and elsewhere in eastern North America. DSV is native to the grasslands of European Russia and Ukraine. Although it is a rare species in its homeland, in North America it forms extensive, dense mats of intertwined stems that choke out other vegetation. It thrives in a variety of habitats including hedgerows, fields and woodlots. Once it has invaded an area, it is extremely difficult to eradicate.

DSV is an herbaceous vine, which means that its above-ground parts die back to the roots in the fall. It cannot climb into the canopy the way that Virginia creeper or native grape vines do. Still, it can achieve heights of 2.5 metres over the course of a growing season, which is more than enough to smother young trees in restoration sites. DSV poses a threat to Christmas tree plantations and no-till agriculture as well.

DSV has a number of characteristics that make it especially invasive. It produces plentiful wind-borne seeds resembling those of its relatives in the milkweed family. The seeds have extremely high emergence rates and survival, aided by the multiple embryos that emerge from many of them. The growing plant invests considerable energy in its roots, which produce chemicals that alter the soil and slow the root development of competing plants. Potent toxins are also present in the leaves, protecting them from plant-feeding insects. The lack of insect damage gives DSV an advantage over native species, which normally support hundreds of species of insect herbivores. An insect-free plant also has consequences for the food chain. No insects translates into no food for birds and other insect eaters.

Control of DSV has proven to be difficult. Mowing the stems is a good way to prevent further spread by keeping the plants from producing seeds; however, the plants will use the energy stored in their extensive fibrous root systems to re-sprout. Digging to remove plants is really only feasible on a small scale. It is not necessary to remove all of the fibrous roots, but the plants will re-sprout if the buds at the root crown are left behind. It takes several applications of herbicides such as Roundup® to control DSV, and, of course, non-selective herbicides kill the surrounding vegetation as well.

We are optimistic that biological control will provide a solution and eventually help turn this highly invasive plant into a benign member of Ontario’s plant communities. Biological control of weeds involves importing natural enemies—usually insects—from the original geographic range of the plant. Surveys of insects in Europe revealed several species that are found only on DSV, including beetles, caterpillars and seed-feeding flies.

In early July 2014, we released Hypena opulenta caterpillars in Ottawa and Toronto to combat DSV. Like its host plant, Hypena opulenta is native to Ukraine. Before it could be released as a control agent, it underwent stringent testing to make sure it would not feed on other plants, especially DSV’s relatives in the milkweed family, some of which are endangered. As soon as a panel of expert reviewers was convinced that the risk of Hypena attacking other plants was vanishingly small, the release was approved.

Hypena produced a second generation in the summer of 2014 and spread at least 200 metres from the release site. We are currently waiting for evidence that the species successfully overwintered in Ottawa. Hopefully, the caterpillars will be well adapted to our climate, and their populations will grow. DSV will never be entirely eradicated, but we hope that someday it will revert to what it is in Europe—a rare plant!

This article was based on a presentation made to the Lower Ottawa Valley Chapter at their AGM.

More information on DSV and biological control news can be found here: http://dogstranglingvine.weebly.com/

Biologist, Dr. Naomi Cappuccino, is Associate Professor at Carleton University. The work in her lab, in collaboration with colleagues at Agriculture and Agri-food Canada’s Central Experimental Farm, involves the biological control of insect pests of agricultural crops and garden plants using introduced parasitic insects, and the control of invasive weeds using plant-feeding insects.

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