On July 30, 2003, about 1000 eastern daylight time, a Bell 206-B3, N876M, registered to and operated by Lee County Mosquito Control District, as a Title 14 CFR Part 91 public-use flight, impacted a tree stump and rolled over in Lee County, Florida. Visual meteorological conditions prevailed and no flight plan was filed. The commercial-rated pilot and one passenger were not injured, and the helicopter incurred substantial damage. The flight was originating at the time of the accident. The pilot stated that he had landed in a marshy area to disembark a mosquito control inspector, and allow her to conduct an inspection of the area over which they were going to conduct an aerial application flight that afternoon. He stated that as he landed, he noted that the landing surface was flat with 6-inch grass, 1-inch standing water, and mud. He said he inspected the right skid from the cockpit, while the inspector departed the helicopter to inspect the mangrove, and he noted a branch or root extending 6 inches above the grass, 4 inches outboard of the right skid near the aft cross tube saddle. He said the branch was about 2 inches in diameter and rotted, and he made a mental note not to allow any right drift during takeoff. The pilot further stated that the inspector returned after having finished her inspection of the mangrove, and she reported no obstructions to the left skid. The pilot said that before the takeoff check was complete, he ascended, establishing a 2-foot stable hover in a level attitude, checked the torque, and began to increase collective for climbout. He said he noticed that the helicopter did not climb, but added that there was no roll. He said he looked over his right shoulder to check for and obstruction, and at that time he detected that a roll was developing to the right. He said he reduced collective, but it was not enough, and the roll continued in spite left cyclic being applied. The helicopter came to rest on its right side. Prior to the accident, the pilot said there were no mechanical failures or malfunctions to the helicopter or any of its systems.