More than 40% of U.S. honeybee colonies died in a 12-month period ending in April, extending a troubling trend that has scientists scrambling for a solution and professional beekeepers struggling to stay in business.

The Agriculture Department said in its annual honeybee survey released Wednesday that beekeepers are starting to lose large numbers of bees during both the summer and winter—presenting scientists with a new wrinkle since die-offs had generally occurred during the cold winter months.