Catalyzes the conversion of (2E,6E)-farnesyl diphosphate into (-)-delta-cadinene. Cyclization mechanism involves an intermediate nerolidyl diphosphate leading to a helminthogermacradienyl cation. (2E,6E)-farnesyl diphosphate = (-)-delta-cadinene + diphosphate kcat is 0.00114 sec(-1). Belongs to the terpene synthase family.