According to the pilot, he had successfully completed six aerial application passes below transmission lines that ran diagonally above a rectangular agricultural field. The pilot had also successfully completed 11 or 12 similar application flights over the same field during previous years, including 1 earlier that month. As the pilot turned the airplane toward its seventh pass, sun position and windshield spray residue resulted in his briefly losing sight of the transmission lines. The pilot subsequently regained sight of the transmission lines and continued the pass; however, the airplane struck the lines, damaging the wing spar, before returning to the originating airport. The pilot did not report any preexisting mechanical anomalies with the airplane.