Cross over to Bar Island at low tide in June and July, and soon you’ll discover fields of majestic purple, blue and pink lupine wildflowers of Acadia, beckoning to be admired and photographed.

Western lupine blooms in early June on Bar Island. The flower is popular with tourists who sometimes stop to pose for photos next to the purple, blue or pink flowers.

Squeezed along the edge of the fields stands a less showy plant – common milkweed – which could be missed if not for a sign calling this place a “Milkweed Habitat.”

These fields may be the site of a coming showdown between lupine and milkweed that could affect the fate of monarch butterflies in Acadia National Park.

As photogenic as the spiky tall flowers of the western lupine are, they are invasive non-native plants, threatening to crowd out the homelier milkweed critical to the lifecycle of the monarch, which recently became candidate for listing under the US Endangered Species Act. The faceoff between lupine and milkweed and the monarch could eventually come to a head on the island, just off the coast of Bar Harbor.

An effort by the National Park Service to protect the soon-to-be-endangered Monarch butterfly includes spreading seeds for milkweed on Bar Island and staking signs to inform the public.

“If the western lupine is encroaching on critical habitats in the park…it would be a very high priority to remove it,” said Amanda S. Pollock, Acadia public affairs officer, in an email. In years past, the park’s Invasive Plant Monitoring Team “removed lupines encroaching on a significant area of milkweed to protect habitat for the soon to be listed as endangered monarch butterfly.”

Pollock said the team has managed lupine encroaching on milkweed on Bar Island, Fernald Point, and a small area near Great Meadow Drive. She said the team has removed lupine on Bar Island “but only a section of the field, near the milkweed and the path.”

The park has not planted any mature milkweed, but resource managers have spread seed in areas where milkweed would likely grow well, including Bar Island, Pollock added.

Which plant’s tendency to spread will win out: Invasive western lupine, on the left, or the later-to-bloom milkweed on the right? Monarch butterflies exclusively lay their eggs on milkweed, as the developing caterpillars need the milkweed sap for chemical defense against predators. This scene played out on Bar Island in Acadia National Park in early July.

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