The aircraft crash landed in trees after the engine lost power. The helicopter was being used in aerial application at the time of the accident. The operation was being done in 8 minute segments. Every 8 minutes the helicopter landed for a reload of herbicide. At about 1430-1500 hours the aircraft was refueled with 29 gallons of 100 octane fuel. At 1600 hours the engine ceased operation 3 minutes after a takeoff. The aircraft was at 50 ft AGL going about 30 mph. There was no clear area for a landing so the pilot autorotated into trees. The aircraft has an endurance of about one and one half hours. During the first interview the pilot said the aircraft ran out of fuel. No fuel was aboard the aircraft after the accident. The pilot later found an AN fitting was loose on the gas tank sump drain. He said it was not leaking when the aircraft was refueled. The pilot found an area of leaked fuel which he estimated at 5 gallons near the sump drain. The aircraft has only one fuel tank and it contains 2 unusable gallons of fuel.