The monarch, Canada’s most beloved butterfly, has been widely known as a threatened species for many years. Historically the major threats to the monarch butterfly have been deforestation of their home turf in Mexico, removal of their favorite milkweed snack from gardens, and pesticides spoiling their food. With so many campaigns urging people to “save the monarchs!”, their populations have tentatively begun to rise again within the last few years. However a new threat is presenting itself, one much trickier to handle. The newest threat to monarch butterflies is the invasive Dog-strangling vine.
Dog strangling vine (abbreviated as DSV; see photo to the right, courtesy Toronto Wildlife) refers to two European species of swallow-wort, a perennial vine that has secured its place in Southern Ontario’s ecosystems. Its incredible growth rate coupled with aggressive sun and water use make it a formidable competitor for native plant species. When DSV enters an area, it rapidly grows dense mats of vine, quickly out-competing native vegetation as it wraps itself around trees and grasses, strangling and smothering them.
Invasive plants in and of themselves are a danger to local biodiversity, but there is one trait about DSV that makes it a special concern for our beloved monarch- it is a deceitful cousin of Ontario’s native milkweed species (pictured left; photo by Liz Cliff) upon which the monarch butterfly life cycle depends. If you aren’t familiar with the monarch life cycle, they depend heavily on milkweed plants, which they use to feed on and lay their eggs. When monarch eggs hatch, the caterpillars feed exclusively on milkweed for several weeks before forming cocoons, going through metamorphosis, and hatching as fresh adult monarch butterflies (for more info on this life cycle, check out our other blog post by Jade Anderson titled “The Endangered Monarch”!). Canada’s native milkweed plants are crucial to the reproduction of monarch butterflies, but unfortunately, DSV looks a lot like the milkweed species that monarchs prefer to munch on. Monarchs can get confused and lay their eggs on DSV rather than native milkweed plants, but DSV is not a suitable host plant for monarch caterpillars; when they hatch, they have no food since there is no milkweed to eat, and therefore cannot complete their life cycle.
What does this mean for monarchs? Well, entire viable generations of monarchs aren’t surviving the caterpillar stage because they have been laid on the wrong plant. The continued spread of invasive DSV threatens the continuance of monarchs as a species, since DSV just looks too similar to native milkweed. It grows quickly, it’s difficult to kill, and it has already been introduced all over Southern Ontario. And as this invasive species continues to spread, plant and butterfly biodiversity will feel the pressure. So what can we do about it?
Our best bet right now is to educate ourselves on what this plant looks like, where it likes to grow, and how we can manage it. One helpful tool to use is an app called iNaturalist which many nature lovers are familiar with- just snap a picture of suspected DSV, and a plant expert can confirm your suspicions. The location is then added to a database so scientists can monitor the location and spread of this invasive species, and hopefully learn more about where and how fast it is spreading.
As for managing and/or removing known populations, landowners can consult the Nature Conservancy of Canada’s website about the best and safest ways to remove DSV (https://www.natureconservancy.ca/en/what-we-do/resource-centre/invasive-species/dog-strangling-vine.html#). Be sure to follow their directions because if removed incorrectly, you could cause the plant to spread even more. If you find this plant in your garden, don’t ignore it!! Tell your friends and neighbors! Our best shot at combating this threat to monarchs is to spread the word and work together.
And of course, for those who want to take a proactive approach, planting more native milkweed species in your garden never hurts. The more the merrier! These plants are great for other pollinator species as well. Consult your regional municipality’s websites to see if they have information on how to start your own pollinator garden. The bees and butterflies will thank you, especially our monarch friends!
(Image below courtesy the Regional Municipality of York’s Twitter account)
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