The pilot was performing an agricultural application flight in the helicopter over a cantaloupe field. During the final pass along the field border, he observed bee boxes adjacent to power poles, and he shut off the spray applicator. After passing the boxes, he turned the spray applicator back on; his next recollection was of lying upside down in the field. Examination of the helicopter at the accident site revealed that the main rotor blades struck a set of wooden power poles at the 20-foot-level. The helicopter sustained substantial damage to the main rotor blades and fuselage structure. The pilot reported that he had flown an application flight over the same field a week prior, and was aware of the location of the power poles. The pilot reported no preimpact mechanical malfunctions or failures with the airframe or engine that would have precluded normal operation.