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Dog Strangling Vine

Image of Dog Strangling Vine plantDog Stranging Vine (Cynanchum rossicum, formally of the family Vincetoxicum) is a European member of the milkweed family also known as Pale (or European) Swallowort.  Dog Strangling Vine (DSV) has become invasive in many parts of eastern North America and is particularly abundant in the GTA and parts of Ottawa.  It is commonly found along roadsides and railways, as the seeds are picked up by passing vehicles and railcars and distributed throughout the province. Both DSV and its close relative Black Swallowort (Cynanchum nigrum, also known as Cynanchum louisaea and sometimes referred to as dog strangling vine) form dense colonies in a variety of habitats, where they outcompete other vegetation and literally strangle plants with their twining growth habit.

Leaves of DSV are opposite and short stalked, oblong to oval with pointed tips, 5-10 cm long and dark green (Dickinson et al, 2004), although it has been our observation that the leaves are lighter and more pointed when the plant is growing in full sun than when it grows in shade.  The pale swallowort flowers are longer than they are broad and pink to light purple in colour, whereas the black swallowort flowers are more triangular in shape (with microscopic hairs) and dark purple with a yellow center.  Pale swallowort spreads to new areas primarily by seed, whereas black swallowort forms deep rhizomes which aid its spread (data from the USDA Natural Resources Conservation Services website, see below).

monarch butterfly image

In addition to its disruptive growth pattern, DSV carries substances that repel or kill native herbivorous insects.  These plants are particularly problematic for the Monarch Butterfly populations as the females lay their eggs on the DSV (due to their close relation to milkweeds) but the larvae cannot mature on these plants due to their inherent toxicity.  There are several other species of insects that also oviposit on milkweed that may also be affected by this invasive species.  These plants are not considered weeds in their native European habitats as there are a variety of insects that are herbivorous on the leaves and/or seeds, suggesting that biocontrol may be a suitable means of control or that native species may be found that can tolerate and inhibit DSV's invasive growth.




Control methods:

Pulling
Pulling or digging out small DSV plants by their roots is an effective measure in the early years of establishment. You must be diligent because if only the above ground portion of the plants are removed these plants will readily re-sprout from the remaining root system. This measure should be applied when the soil is moist and the roots are easily loosened to avoid soil disturbance. Soil disturbance may also activate the seed bank.

Limiting dispersal of seeds
As the DSV seeds disperse by wind, gardeners and home owners must be vigilant and survey land especially in the fruiting season (August, September and October). This plant turns bright yellow and pods are easily visible during late summer. Remove the pods, place in bags and discard in a green bin.  Do not put these into compost.  The fall is the best time for above ground removal of the plant colony to stop seed dispersal.

Mulching
Where DSV growth is dense, mulching with multiple layers of newspapers, leaf or twigs and cloths are fairly effective when used around the base of trees and shrubs.  Although some plants grow through the mulch, growth is considerably less than in un-mulched areas nearby (see page regarding Field Trials). Mulching with bio-degradable plastic is also very effective if the plastic is covered with wood chips. Contact us for information on obtaining biodegradable plastic.

Chemical control
Repeated application of the herbicides glyphosate (Roundup) and triclopyr (Brush-B-Gon or Garlon 3A) are both effective as foliar sprays during the seasons of larger infestation.  The first application should be done at the onset of flowering.  There should be a follow-up treatment in two to three weeks.  For dense populations additional treatments may be required for the next couple of years. Always be aware of the potential risks in the use of pesticides because of their toxicity and impacts on non-target species.
 
Biological control
 A number of species of insects feed on white swallowort (C. hirundinaria) in Europe including the weevil Otiorhynchus pinastri, the diptera Euphranta connexa,  the gall midges Contarinia vincetoxici and C. asclepiadis, and several species of leaf beetles including Chrysochus asclepiadeus.  Herbivory of these insects on black or pale swallowort has generally not been confirmed.  There are currently studies underway through CABI investigating a selection of insects as potential biocontrol agents. Only a few native species have been reported to feed on dog strangling vine, including milkweed longhorn beetle (Tetraopes tetraopthalmus), small milkweed bug (Lygæus kalmii), large milkweed bug (cf. Oncopeltus fasciatus) and Oleander aphids (Aphis nerii), although there has been no indication that the insects thrive on this plant or that they cause any significant damage (TRCA, 2007).  Shotgun damage on DSV leaves has been found to be caused by two separate species to date - by the colorado potato beetle (Leptinotarsa decemlineata) (TRCA, 2007) and also by the tarnished plant bug (Lygus lineolaris) (USDA NRCS website), but this damage also has not been shown to have a significant impact on plant growth.  Our own studies also identified an additional insect creating this type of damage (see herbivory).

Plants with vigourous early season growth caste dense shade and can help in controlling DSV. Dense tree cover can also act as a barrier for seed production and dispersal. Tree planting could be an effective method to block the major corridors.

Planting of trees and shrubs that inhibit the growth of DSV by releasing chemicals (allelopathy) is another option for biological control. Black walnut (Juglans nigra) is well-known for its allelopathic effects (secretion of Juglone).  Our own observations and those of other researchers (TRCA, 2007) have described instances where black walnut did appear to inhibit DSV within its drip-line.


Please email us directly if you have seen any instances of insect damage on DSV in your area, and include any information you may be able to provide regarding the type of damage, extent of the damage, and possible insect candidates.  If you have confirmed herbivory by a particular insect, please let us know!



Other sources of information:

Toronto and Region Conservation - http://www.rougepark.com/unique/reports/RP_DSV_Report_Dec07.pdf

Invasive Plants of the Eastern U.S. - http://dnr.state.il.us/stewardship/cd/biocontrol/16swallowworts.html

USDA Natural Resources Conservation Services website - ftp://ftp-fc.sc.dgov.usda.gov/CT/invasives/swallow-wort.pdf

Controlling swallow-worts the sustainable way - http://www.cabi.org/default.aspx?site=170&page=1017&pid=4277




References:
Dickinson, T. Metsger, D. Bull, J. and Dickinson, R. 2004. The ROM Field Guide to Wildflowers in Ontario. Royal Ontario Museum and Canadian Publishers (co-published). Toronto, Ont. Canada.
TRCA 2007 (see above)