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In the Great Basin Desert, Nicotiana obtusifolia (synonymous with Nicotiana trigonophylla) and Nicotiana attenuata co-occur, but the former is frequently less attacked by larvae of the tobacco hornworm than the latter, despite having lower nicotine and trypsin protease inhibitor defenses. Glycosides of the diterpene, 17-hydroxygeranyllinalool (HGL-DTGs) have recently been found to be important defenses of N. attenuata. Total HGL-DTG levels are 5-fold higher in N. obtusifolia than in N. attenuata, and we characterize the three major HGL-DTGs purified from N. obtusifolia leaves as: 3-0-a-L-rhamnopyranosyl( 1 -> 4)-beta-D-glucopyranosyl-17-hydroxygeranyllinalool-17-alpha-L-rhamnopyranosyl-(1 -> 4)-beta-D-glucopyranoside; nicotinoside III and its malonic acid conjugates. Using APCI- and ESI-LC-MS, we also identified mono- and diacetyl-nicotinoside III and quercetin glycosides. To evaluate the defensive value of these HGL-DTGs, we used virus-induced-gene silencing to reduce the transcript levels of geranylgeranyl diphosphate synthase and total HGL-DTG levels in both species. When fed on silenced plants, larvae gained up to about two times more mass than those that fed on empty vector control plants of both species. We conclude that HGL-DTGs function as the most important direct defenses for both N. attenuata and N. obtusifolia. (C) 2010 Elsevier Ltd. All rights reserved.