Honeybees are one of the most important insects in the world, and a new study suggests that they are at a perilous risk of dying off in large numbers. Scientists are stunned at the struggle facing U.S. honeybee populations, with a reported 40 percent of colonies lost in the last year.

According to the USDA, it is normal for bee colonies to perish over the course of a year. In a survey of American beekeepers, they said that they expected to lose up to 18.7 percent of colonies. Two thirds of beekeepers reported losses higher than expected and 42.1 percent of colonies were lost across the board between the spring of 2014 and the spring of 2015.

The economic value of bees has many beekeepers, farmers, and agricultural investors worried about the $15 billion worth of crops that honeybees pollinate each year. Bees traditionally die off in greater numbers during the winter, when conditions are harsh and food is scarce. The scientists expected this pattern, but were shocked when they examined colony loss during the summer months.

Summertime is typically prime time for bees, with an endless buffet of flowers and balmy temperatures. For the first year ever recorded, however, colony loss in the summer exceeded colony loss in the winter. Scientists know there must be another explanation for the bee deaths.

Colony collapse disorder, or CDC, has a history of killing off bees in large numbers, but researchers don’t believe it’s responsible in this instance. They don’t know what is responsible for sure, but they’re looking at pesticides, hive nutrition, and mite infestations to begin cracking the case. Until then, beekeepers will struggle to keep their colonies alive as they pollinate farms across America.