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Two invasive plants mean two more threats to the monarch butterfly

Black swallowwort (cynanchum louiseae)
Credit: Montréal Botanical Garden (Simon Joly)
Black swallowwort (cynanchum louiseae)
Two invasive plants mean two more threats to the monarch butterfly

Those threatening plants are vines with pale or dark mauve star-shaped flowers and are known as black swallowwort and European swallowwort; in Latin, Cynanchum louiseae and Cynanchum rossicum. Cynanchum is a Greek word meaning “dog choker,” and these species are also often referred to as “dog-strangling vine” – not a name that inspires confidence.

Our habitats under attack

A native of Europe, the swallowwort escaped from garden use in New England in the 19th century. Today these vines thrive in several northeastern North American habitats – hedges, woodland, alvars, etc. – and are considered invasive. Forming dense colonies, they push out native species and affect plant and animal diversity. But mostly they’re attracting attention because of their negative impact on the monarch butterfly, whose populations are in decline.

Monarchs and milkweed

The monarch depends on a type of plant, the milkweed, to complete its life cycle. Monarch caterpillars in fact feed on nothing but milkweed leaves, from which they get toxic molecules that make them indigestible and that protect them from predators, even after they become butterflies. For there to be monarchs, there has to be milkweed.

An unwanted guest

Belonging, like milkweeds, to the Apocynaceae family, swallowworts possess similar chemical compounds, which lead monarchs to lay their eggs on the leaves. No surprise there, since monarchs lay on dozens of plant species, the majority of them milkweeds. But researchers have demonstrated that monarch caterpillars do not consume the leaves of invasive swallowworts, thus condemning themselves to certain death. And the disturbing fact of the matter is that monarchs do not completely avoid these weeds, even when milkweed is present. This is what we call an “evolutionary trap”: an ecological change that triggers animals to make the wrong choice of habitat and threaten their very survival.

Wanted!

The results of the study in question, conducted in the state of Rhode Island, differ from those of studies carried out in Ontario and in New York State, where little or no monarch egg-laying had been observed on swallowworts. Even if they remain to be validated, including in Québec, these results call for caution, considering the invasive status of swallowworts. Indeed, even a low proportion of egg-laying on this plant could have adverse effects on monarch populations, especially if these plants proliferate. And that’s without counting the effects on biodiversity. So, if you notice swallowworts in your path, don’t hesitate to pull them up: monarch butterflies will be grateful!

Interesting link: Mission Monarch

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