"This is clearly very complicated. It's also clear that there are lots things killing bees," said Dennis van Engelsdorp, chief bee researcher at the Pennsylvania Department of Agriculture.
"Everything we look at presents more questions than answers," he told the Pittsburgh Tribune-Review.
Colony Collapse Disorder, reported in least 24 U.S. states and portions of Canada since 2006, is a phenomenon in which worker bees from a beehive or honeybee colony abruptly disappear.
European beekeepers report similar phenomena in Belgium, France, the Netherlands, Greece, Italy, Portugal and Spain, with possible cases reported in Taiwan.
Bees are vital to U.S. agriculture, pollinating flowering crops, including almonds, apples and blueberries.
Officials estimated honeybees add $15 billion each year to U.S. agricultural output, the newspaper said.
A single honeybee colony can contain 20,000 worker bees.
"There are many species that are threatened by everything from pesticides and herbicides to reduced plant diversity," Ohio State University ecologist Karen Goodell told the newspaper.
Other proposed causes include environmental change-related stresses, malnutrition and migratory beekeeping. More speculative possibilities include radiation and genetically modified crops with pest-control characteristics.
Notable deaths of 2014 [PHOTOS]
Mirror turtle ants thrive by going undercover