On April 20, 2013, at 1130 eastern daylight time, a Cessna model 172G airplane, N3682L, was substantially damaged during a forced landing following a partial loss of engine power during cruise flight near Reading, Michigan. The airline transport pilot was not injured. The airplane was registered to and operated by a private individual, under the provisions of 14 Code of Federal Regulations Part 91, without a flight plan. Day visual meteorological conditions prevailed for the personal flight that departed Lenawee County Airport (KADG), Adrian, Michigan, at 1100, and was destined for Joliet Regional Airport (KJOT), Joliet, Illinois.The pilot reported that about 30 minutes into the cross-country flight, at 2,500 feet mean sea level, the engine speed inexplicably decreased from the selected cruise power setting to 1,900 rpm over a few seconds. The pilot was unable to restore full engine power and performed an off-airport landing in a nearby agricultural field. The airplane came to an abrupt stop when it encountered an area of deep mud during the landing. The main wing spar was substantially damaged when the right wingtip impacted terrain during the abrupt stop. Maintenance inspectors with the Federal Aviation Administration examined the airplane before it was recovered from the accident site. Visual inspection of the engine compartment revealed that the throttle control had separated from the carburetor's throttle arm. The corresponding retention hardware was not recovered during the investigation. The engine recording tachometer indicated 3,078.96 hours at the accident site. According to maintenance logbook information, the airplane had undergone an annual/100-hour inspection two days before the accident flight at 3,078.40 hours tachometer time. The aviation mechanic that performed the inspection, who also held an inspection authorization certificate, reported that after completing a field overhaul of the engine he could not locate the cotter-pin used to safety the airframe throttle control to the carburetor throttle arm. The mechanic elected to use safety wire to secure the throttle control assembly instead of the specified cotter-pin.