Prior to performing the aerial application flight, the pilot of the helicopter surveyed the field, and observed a set of power lines along the eastern border. He completed the application uneventfully, and while completing the final pass towards the east, the helicopter struck the power lines. He reported that he did not see the lines silhouetted against the grey sky until just prior to the collision, and that he performed an evasive maneuver in an attempt to descend below the lines. The helicopter struck the ground after the collision, nosed over, and rolled onto its right side, sustaining substantial damage to the fuselage and main rotor system. This was the pilot's second revenue flight, having recently been hired as an apprentice. The pilot reported no preimpact mechanical malfunctions or failures with the airframe or engine that would have precluded normal operation.