THE PILOT REPORTED THAT WHILE EN ROUTE BACK TO HIS HOME AIRFIELD FOLLOWING THE CONCLUSION OF AN AERIAL APPLICATION FLIGHT, A TOTAL LOSS OF ENGINE POWER WAS EXPERIENCED. A FORCED LANDING WAS MADE IN AN UNHARVESTED WHEAT FIELD AND THE AIRPLANE WAS SUBSTANTIALLY DAMAGED. POST-ACCIDENT INVESTIGATION REVEALED A FOREIGN MATERIAL BLOCKAGE OF THE FUEL TANK VENT LINE.