The pilot stated he was returning from an aerial application practice flight, in a tail wheel-equipped airplane, to practice wheel only touch-and-go takeoffs and landings. The first four landings were uneventful. During the fifth and final landing, the airplane touched down and bounced approximately one foot into the air. The airplane stabilized and the main wheels touched down for a second time. As the tail wheel touched down approximately two-thirds down the runway, it started to shimmy “violently” and the airplane veered to the left. The pilot corrected the airplane back to the right. The intensity of the shimmy increased and despite the pilot’s control inputs, the airplane exited the right side of the runway and ground looped. The airplane’s left wing was substantially damaged. Post-accident examination of the tail wheel assembly revealed wear on the locking mechanism. Additionally, “snake-like” skid marks were found on the runway that appeared to be the result of a tail wheel shimmy.