The pilot of the helicopter was flying over an agricultural field at dawn performing frost control. He flew three passes over the field at a low altitude in order to distribute warm air onto the crops. During the passes, he observed power lines in the vicinity. On the fourth pass he descended to about 50 feet agl and lost sight of the power lines. Concerned that he may collide with them, he began to turn right. Halfway through the turn the main rotor blades stuck the power lines. The helicopter descended to the ground, sustaining substantial damage to the fuselage and rotor system. The pilot said there were no mechanical malfunctions or failures.