Sept. 4 2018 10:57 AM

Invasive “black sheep” of milkweed family lures the butterfly, but poisons its young.

Monarch butterfly caterpillar
Steven Katovich, USDA Forest Service,

As if the endangered Monarch butterfly didn’t have enough problems, officials in Kingsley, Michigan are concerned about an invasive plant species that, bent on regicide, poisons its caterpillars, according to a story published on the website of the Traverse City Record Eagle. It’s called black swallow-wort, a vine with heart-shaped leaves that can be found growing along roadsides, pastures and gardens.

It is, ironically, a member of the milkweed family, the only plant that sustains the Monarch species. In another touch of irony, the vine was first identified in the Kingsley Branch Library's butterfly garden.

When in bloom, it sports tiny, dark purple flowers with five petals covered in fine hairs. Even if it didn’t pose a threat to the butterflies, it would be of concern, as it will outcompete native plants and wildflowers.

Emily Cook, outreach specialist for the nonprofit Northwest Michigan Invasive Species Network, said the curious thing about the plant is how it attracts Monarchs. Even when common milkweed is close by, the butterflies seem to prefer the invasive plant. Researchers are trying to figure that out.

Adult butterflies can feed on the black swallow-wort’s nectar without harm. It’s when they mistakenly lay their eggs on the plant that the trouble begins. Hatching caterpillars start eating whatever plant they find themselves on, and when they munch down on this one, death follows quickly. "It's toxic to caterpillars when eaten," Cook says.

The organization is trying to determine whether the troublesome plant is also in other spots in its four-county service area, which includes Benzie, Grand Traverse, Leelanau and Manistee counties.

Unfortunately, this weed is quite difficult to get rid of. It propagates in two different ways, from seeds and as well as underground rhizomes. Because of its complicated root system, you can’t just yank it out of the ground; pulling it promotes its spread. The best method to kill off the plants and minimize the risk of unintentional spreading, according to Cook, is to use an herbicide.

The network's officials want residents across the region to look around their yards for the poisonous plant with the purple flowers. While it’s hoped that more of the plant won't be found outside of the Kingsley area, they want to be notified if and when it is, so the network can assist property owners with removal.

Black swallow-wort can also harm other insects and mammals. Joanne Foreman, invasive species communication coordinator for the Michigan Department of Natural Resources, says haemolytic glycosides in the plant’s roots are antibacterial and antifungal in a way that prevents other plants around them from growing and thriving. And the sap in those roots is toxic when eaten by mammals, including livestock.