Sesquiterpene synthase that catalyzes the formation of sesquiterpenes and sesquiterpenoid alcohols (PubMed:24078339). Converts farnesyl diphosphate (FPP) to germacrene D (PubMed:24078339). Converts FPP to bicyclogermacrene (PubMed:24078339). Germacrene D is the major product (PubMed:24078339). (2E,6E)-farnesyl diphosphate = (-)-germacrene D + diphosphate (2E,6E)-farnesyl diphosphate = bicyclogermacrene + diphosphate Binds 3 Mg(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family.