Butterfly watchers in New Brunswick are fearing the worst, after a freak winter storm struck a Monarch sanctuary west of Mexico City last week.

"This is a heartbreaking development," said Quispamsis conservationist Jim Wilson.

Every year, Wilson eagerly awaits the butterflies' mid-summer arrival, which comes mid-July, near the end of an epic migration.

That journey had just begun, when temperatures plunged well below freezing last Thursday in El Rosario.

Snow-crusted butterflies appeared to be still clinging to trees in photos posted on Facebook by Homero Gomez Gonzalez, the chair of the sanctuary there.

He said it was better to see them still perched on tree trunks, rather than lying dead on the ground.

Orley "Chip" Taylor, founder of Monarch Watch at the University of Kansas, said that could be a good sign, although sometimes fatally damaged butterflies don't fall until later.

"I've been down there when the butterflies are dead on the tree trunks and they look alive but if you pick them up, you realize they've frozen in place," he said.

Monarchs spend most of the winter months clumped together in masses in order to keep warm and conserve moisture. (Submitted by Jim Wilson)

Taylor said it will be difficult to get an accurate assessment of the death toll because no one will know how many butterflies had already left the region.

Still, he's convinced it won't be as devastating as weather events in 2002 and 2004.

"When there were literally millions of dead butterflies on the forest floor," he said.

"Butterflies that you could wade through, five or six inches deep. In those cases, we had 70 to 80 per cent loss. Clearly, we're not dealing with a 70 to 80 per cent loss."

Very few individual Monarchs make the round trip between Canada and Mexico.

Rather, females lay their eggs along the way and successive generations continue the journey.

As they move north, the population spreads outward.

That means they are most susceptible to singular crises, when the population is at its most concentrated in the biosphere in Mexico.

"These massive aggregations make them vulnerable," says zoologist, John Klymko, of the Atlantic Canada Conservation Data Centre.

A sky full of wintering Monarchs that became active on a mild day in early March during a visit to Mexico in 2006. (Submitted by Jim Wilson) Still, Klymko said he's seen numbers in New Brunswick that are out of sync with counts in Mexico.

For example, he said Mexico recorded a terrible return in the winter of 2012, but the following spring and summer, the Maritimes saw large numbers of the butterflies.

"I wouldn't want to read too much into a single event like this," he says.

Wilson has visited El Rosario and he said it is a sight to behold when nearly a billion butterflies gather there.

"You have to remember they've been down there visiting that area for thousands of years," he said.

"So they would have adapted to adverse conditions."

Wilson said scientists should be going into the area within the next few days and reporting back more details about what happened.