The helicopter pilot reported that while maneuvering at a low altitude for an agricultural application flight he began flying along the north side of the intended spraying field. He added that he was paying attention to the trees along the field to maintain adequate clearance. He further added that the main rotor blades struck transmission line wires, and then he heard a "high pitch sound that came from the engine" and the rpm decreased. The nose pitched up and the helicopter impacted the ground and rolled onto its right side. The helicopter sustained substantial damage to the fuselage and empennage. The pilot reported that there were no preaccident mechanical failures or malfunctions with the helicopter that would have precluded normal operation. The pilot reported that he had noticed the transmission line wires earlier in flight, but later had forgotten where they were located.