“We expect the colonies to die during the winter, because that’s a stressful season,” said Dennis vanEngelsdorp, an assistant entomology professor at the University of Maryland who directs the survey for the bee partnership. “What’s totally shocking to me is that the losses in summer, which should be paradise for bees, exceeded the winter losses.”

Image Bees at the Vermont Farm Show. Honeybee deaths last summer exceeded winter deaths for the first time. Credit... Andy Duback/Associated Press

Bees are not in danger of extinction, but their health is of major concern to agriculture, where honeybees’ pollination services are estimated to be worth $10 billion to $15 billion a year.

Nobody knows with certainty why honeybee deaths are rising. Beekeepers once expected to lose perhaps 10 percent of their bees in an average year. But deaths began to spike in the middle of the past decade, when a phenomenon in which bees deserted their hives and died en masse, later named colony collapse disorder, began sweeping hives worldwide.

Those mass die-offs have abated somewhat in recent years, experts say, but colonies remain in poor health, and overall death rates remain much higher than in the past.

Dr. vanEngelsdorp said increasingly poor nutrition could be a factor in the rising summer death rate. Rising crop prices have led farmers to plow and plant millions of acres of land that was once home to wildflowers; since 2007, an Agriculture Department program that pays farmers to put sensitive and erosion-prone lands in a conservation reserve has lost an area roughly equal to half of Indiana, and budget cuts promise to shrink the program further. Dr. vanEngelsdrop and other scientists cite two other factors at work in the rising death rate: a deadly parasite, the varroa mite, and pesticides.