Swallow-worts Saboutage the Monarch Butterfly

Toni Leland

Our world is full of imposters and jokers, and the plant community is no different.

Sneaking into pastures, urban areas, forests, and our own gardens, the highly invasive swallow-worts belong to the milkweed family (Apocynaceae) and, therefore, are attractive to Monarch butterflies.

Why is this a problem?

Black and pale swallow-wort are highly toxic to the Monarch caterpillar, but the female butterfly often can't differentiate between the swallow-wort and milkweed, especially if they are growing in the same area. She lays her eggs on the swallow-wort and as soon as the caterpillars hatch and begin to feed, they die. Swallow-worts are so toxic that livestock—including goats!—will not eat the plants. 

The black swallow-wort vine originated from the Mediterranean regions of France, Italy, Portugal, and Spain, while the pale swallow-wort comes from Ukraine and southeastern Russia. This is another instance of a plant being purposely introduced into North America in the late nineteenth century, with unfortunate consequences.

Identifying Black Swallow-wort

According to details from the University of Rhode Island Biocontrol Lab, Swallow-worts are perennial vining plants with dark green leaves. Black Swallow-wort (Vincetoxicum nigrum) produces dark violet flowers, and Pale Swallow-wort (Vincetoxicum rossicum) produces flowers that are pale pink. The plants have a strong root system and, in late summer, produce slim, light green seedpods.
 

Black Swallow-wort growing up the side of a house in town. ©Toni Leland

How do we control it?

For the homeowner who discovers swallow-wort on their property, there are only a couple of remedies. Early detection and removal are the best management practices for small scale invasions. Dig or pull every plant and place in a plastic bag for disposal. Do not compost! For large infestations, chemical control is most effective, according to the Plant Conservation Alliance Alien Plant Working Group. The link contains complete instructions on effective application of herbicides.

Biological Control?

The University of Rhode Island Biocontrol Lab made the first releases of the moth, Hypena opulenta, in 2017 in the states of Massachusetts and Rhode Island. The moth was discovered on research studies in Europe and Ukraine, and determined to be the most likely candidate for biocontrol of swallow-worts. As with all experimental biological control studies, time will tell.

Because the swallow-worts disperse seeds in the same way as milkweed, these invasive vines can suddenly appear in even the most well-tended gardens. If you have a pollinator garden, be sure to include plenty of milkweed to give our lovely Monarchs enough hosts to continue the generations. And patrol that garden for swallow-wort.
 

Monarch on Common Milkweed (Asclepias syriaca) ©Toni Leland

Monarch caterpillar on Common Milkweed ©Toni Leland