BOSTON — Several times a year, Nathan Theriault will be walking deep in the Maine woods and make a gruesome discovery: a dead moose, thin and crawling with ticks.

“They’re dying on the forest floor,” said Mr. Theriault, an outfitter and hunting guide in Eagle Lake, Me.

The moose is an iconic image in the Northeast and a crucial part of its tourism and recreational economy. But in parts of northern New England, researchers say moose are being killed by droves of winter ticks that thrive when the fall is warm and the winter comes late. By the thousands, the ticks attach themselves to moose — calves are the most vulnerable — and essentially drain their blood and strength.

Researchers say that over the last few years, ticks have killed about 70 percent of the calves they have tagged in certain regions, an indication that the tick is taking a significant toll.