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A mottle duskywing butterfly was sighted in Ontario's Pinery Provincial Park this month following a reintroduction effort last year.Handout

A team of Ontario biologists is celebrating a key milestone in their attempt to re-establish the mottled duskywing – an endangered species of butterfly – at a site in Southwestern Ontario.

In recent days the grey-and-brown speckled butterflies have been spotted at Pinery Provincial Park, northwest of London, where a multiyear project to bring back the species is under way.

“We’ve had 10 sightings so far. It’s very exciting,” said Jessica Linton, project manager for the effort and senior biologist with the environmental consulting firm Natural Resource Solutions Inc.

Last summer, Ms. Linton and researchers at Ontario’s University of Guelph released adult mottled duskywings and deposited chrysalises and larvae in the park with the hope that some of the insects would do well enough to survive the winter.

The first indication that this happened came 11 days ago when a park visitor spotted one of the butterflies and reported it on iNaturalist, a citizen science social network that serves as a clearing house for species sightings. Ms. Linton said as soon as she heard about the report, she and her colleagues went to the park and were able to see some of the butterflies for themselves.

Prior to the reintroduction project, the last confirmed sighting of a mottled duskywing in the park occurred in 1992.

The decline of the butterfly, which is listed as endangered and at risk of imminent extinction in Ontario, goes hand in hand with the loss of its native habitat, described as oak savanna. It is a mixed landscape of hardwood forests and meadows that once covered large swaths of the U.S. Midwest, with pockets in Canada around the Great Lakes.

Pinery Provincial Park, located on the shore of Lake Huron, is one such pocket that was all but erased, starting in the 1960s when the area was reforested with pine and its natural cycle of fire and regrowth was suppressed. Later, an overabundance of white tailed deer stripped the park of the mottled duskywing’s preferred food, a shrub called New Jersey tea. Since then, park managers have worked to restore the native oak savanna habitat, using measures such as controlled burning and deer harvesting by Indigenous hunters.

Jeremy Kerr, a professor and ecologist at the University of Ottawa who is not involved in the mottled duskywing project, said shifting the park back to something like its original state was a challenging but crucial step.

“The stage had to be built before the characters could be introduced,” he said.

Ms. Linton said that even then, the butterflies would have no way of finding their way to the park on their own. The federally funded project first had to collect specimens from a location about 250 kilometres away near Rice Lake, where the mottled duskywing is still present, and rear them in captivity at the Cambridge Butterfly Conservatory, west of Toronto.

After testing the methodology on a surrogate species and working with mottled duskywings for two years, the team had raised enough of the butterflies to attempt a reintroduction last summer. Ms. Linton said that more of the butterflies would be brought to the park in July to bolster their numbers there.

If the effort continues to show promise, she said, the next step would be to try the same approach in a different location on the Lake Erie shore, where the Nature Conservancy of Canada has been returning a patch of former farmland to its native state.

“If we can create new habitat as well as trying to make do with what we have then that’s kind of a game-changer for the species,” Ms. Linton said.

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