The load/ground crew reported that the pilot planned to make one more spray run before flying back to the home base. When the aircraft did not return to the loading area, they assumed he had returned to the base. They drove 20 miles back to the base, but found neither the aircraft nor the pilot, so they returned to the spray area. The aircraft was found where it had crashed on rolling/hilly terrain at dusk. An exam revealed the propeller had contacted the ground approximately 386 feet west of the principal impact point. There was evidence that the left wing spray boom had intermittently contacted the ground in a 10 to 15 degree circular arc to the north. The left main gear then contacted the uneven terrain. Beyond that, the aircraft crossed a gully/ditch, impacted rising terrain, and came to rest in an inverted position. No preimpact part failure or malfunction was found. This was the pilot's first spray mission for the operator. About 18 miles east at Burley, Idaho, the 1845 MDT weather was in part: 200 feet partial obscured, 3000 feet broken.