During an aerial application flight while spraying a corn field, the helicopter’s tail section struck a powerline. The pilot did not see the powerline, which crossed over the middle of the cornfield perpendicular to the spray pass he was performing, but he knew what he had struck as soon as the helicopter came into contact with it. He had already begun to pull up out of the field when the helicopter struck the powerline and was still moving forward at approximately 50 mph. As the helicopter slowed and he reached about 200’ agl, the helicopter began a yaw to the right that developed into an uncontrollable spin in the same direction. As the helicopter began spinning faster and faster, the pilot rolled off throttle to minimize the torque induced rotations. During the touchdown, the helicopter impacted in an approximately level attitude, and then rolled over on its right side, and was substantially damaged. The pilot noted that there were no preimpact mechanical malfunctions or failures of the helicopter that would have precluded normal operation and that the accident could have been avoided if more “field recon” procedures were done to be certain that powerlines are avoided.