This past winter was the coldest Detroit had experienced in 36 years. Across the upper Midwest, cities shivered, and more than 90 percent of the surface area of the Great Lakes froze solid.

It seemed like ideal weather to kill an unwanted insect. But it did little to stop the emerald ash borer, an invasive Asian beetle that is devastating ash trees from Minnesota to New York.

“We didn’t find a single dead larva,” said Deborah G. McCullough, a professor of entomology and forestry at Michigan State, who led a study of ash trees in Lower Michigan over the winter.

Even before the severe winter, Dr. McCullough and other scientists had come to the glum conclusion that they were going to lose the decade-long battle against the ash borer. Now they are assessing the cascade of consequences for Midwestern and Northeastern forests, both urban and wild.