Across Michigan, Monarch butterflies are facing a deadly threat from an invasive vine: Black swallow-wort. The Kudzu-like vine mimics the milkweed plant and its leaves are toxic to Monarch larvae. Conservation experts are concerned by the rate of spread and the severity of infestation.

The Kudzu of the North

Black swallow-wort with open seed pod.

“I like to call it the Kudzu of the North,” says Jessie Schulte, a land management expert at Kent Conservation District. Waxy pointed leaves grow opposite from narrow, elongated seed pods. The vine grows thick like Kudzu, wrapping around whatever it might find. Black swallow-wort is native to Eurasia.

A thick infestation can produce over 2,000 seeds per square meter of plant growth. And not only do the seeds disperse quite effectively by air, but the plant spreads underground via stems called rhizomes. The flowers are self-pollenating. Black swallow-wort overwhelms and crowds out native species quickly and effectively.

To better understand why black swallow-wort is dangerous, we need to touch on the underlying biology. Monarchs have various sensors on their body known as chemoreceptors. A chemoreceptor translates chemical stimuli from outside the body into action potential in the nervous system. In other words, chemoreceptors allow the butterfly to taste and smell. Humans have various chemoreceptors as well. Think tastebuds.

Black swallow-wort flowering.

What we think of as taste and smell are really just electrical signals sent to our brain telling us that a certain chemoreceptor has detected a particular chemical.

Monarch chemoreceptors recognize a chemical released by milkweed, drawing the butterfly to the plant where it will deposit eggs. Monarch larvae feed on milkweed exclusively.

Black swallow-wort deceives the Monarch’s chemoreceptors by mimicking the favored milkweed. If given a choice between the two, a Monarch will land and deposit their eggs on the swallow-wort, Master Naturalist Ginny Wanty explains.

Why is this a problem? Black swallow-wort is toxic to Monarch larvae. It is also toxic for cats, dogs, and livestock.

What if you find black swallow-wort?

Report it through the Midwest Invasive Species Information Network (MISIN). It is best to leave treatment work to trained professionals. Well-meaning attempts to handle invasive species without proper training can cause them to spread if done incorrectly. Early detection and prevention is key. And you can help make a difference.

If you’ve made it this far and you’re unsure of what we’re discussing. Sorry. Check out this short post answering the question: what is an invasive species? And check out our guide to invasive species in Michigan.