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The Monarch Butterfly’s Nemesis: Black Swallow-wort

By Becky Gallery

Everyone recognizes the colorful Monarch butterfly, flitting about the garden. Its colorful orange and black body warns birds to stay away. What makes the monarch so unappetizing to birds? The adult monarch lays its eggs on milkweed, which the newly hatched larva eat. The monarch caterpillars feed exclusively on milkweeds, which contain toxins. As the caterpillar eats the milkweed, it takes in the toxic substance, creating a bad taste and toxicity in both the larva and the adult butterfly. When a bird or other predator tastes a monarch, it learns to associate the colorful orange with the bad taste, and avoids preying on monarchs in the future.

What happens when an adult monarch lays its eggs on a plant that is similar to milkweed, such as the invasive black swallowwort? The toxicity of black swallow-wort is different from its milkweed relative, and tests have shown that 100 percent of monarch caterpillars hatched on black swallow-wort die after eating the swallow-wort.

Black swallow-wort (Cynanchum louiseae) is found in Harpswell. A native of southwestern Europe, this plant has been identified across the northeastern part of the United States. Black swallow-wort was introduced in Massachusetts during the 1850’s as an ornamental plant, and is considered to be an aggressive, invasive plant. Because this plant is just beginning to be found in Maine, Maine’s Department of Agriculture, Conservation and Forestry has designated black swallow-wort as an Early Detection/Rapid Response invasive plant (EDRR). It is hoped that by identifying and removing these early infestations, we can eradicate the plant quickly.

The plant is a vine, with stems six feet or more in length, that twine over other plants. A herbaceous plant, its narrow, opposite leaves are dark-green, smooth-edged, and about two to five inches long. The small, purplish-brown flowers have five petals, blooming June through September, then forming slender green seed pods about one and a half to three inches in length. These pods ripen to a dark brown color, and eventually open to allow brown seeds with hairy tufts to emerge in the fall.

The swallow-wort grows in dense patches, in both full-sun and partial shade. It climbs over other plants, weakening the native ecosystem. When the female monarch  mistakes swallow-wort for milkweed, the eggs laid there will never survive. The monarch is already threatened with loss of habitat for overwintering in Mexico. The milkweed the monarch needs to nurture its larva is disappearing from our fields and now the spread of swallow-wort is threat to the monarch’s survival.

How can we help the monarch survive? The first step is to provide milkweed plants for its egg-laying females to use, and newly-hatched caterpillars to eat. Second, we can work to control existing areas of black swallow-wort, and prevent it from spreading further. This will take time and repeated efforts. The Harpswell Invasive Plant Partnership recommends a multi-faceted approach. Dig out the root crowns when the plant is small, repeatedly digging as the plants continue to sprout. Remove the flowers and un-opened seed pods, preventing more seeds from entering the habitat. Keep in mind that the seeds of years past will continue to germinate over time. This process of digging and cutting will continue for several years.

Finally, research is being done on introducing a biological control. The University of Rhode Island has conducted research on the moth Hypena opulenta. Six years of research has shown that this moth, which feeds on the swallow-wort, is a safe biocontrol agent in North America. Canada’s government has granted permission for the moth’s release into the wild, and since 2013, more that 10,000 moth larvae have been released near Ottawa and Toronto. While the moths have successfully over-wintered in Ontario, and have fed on the swallow-wort, they have not yet built up a large enough population to have a significant impact on the swallow-wort. Research continues in Rhode Island to get approval for the release of this moth in the United States.

Until the time when a natural predator can bring the swallow-wort into equilibrium, we must continue our efforts to manually reduce the spreading swallow-wort. Dig, cut and repeat. Keep the Black Swallow-wort at bay, and allow the Monarch butterfly to reign.

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October 2017