The study found that when mobile handsets were placed by honeybees they had a "dramatic impact on the behavior of the bees, namely by inducing the worker piping signal." In the natural environment, "worker piping" announces the swarming process of the bee colony or is a signal of a disturbed bee colony. As you can imagine, both get the bees buzzing.
This is a point of concern given our food crops are pollinated by honeybees, and the UN already reports that the bee population is dropping all over the world. Just another reason to unplug and turn off your phone when you can.