Indirect defense in sorghum: Attraction of natural enemies to aphid-infested plants

The sugarcane aphid (Melanaphis sacchari Zehntner) is a new and emerging pest that can cause substantial yield loss and a reduction in grain quality in sorghum. Coupling host-plant resistance and biological control  has the potential to reduce insecticide use and cost of aphid suppression. We quantified the production of herbivore-induced plant volatiles (HIPVs) and the attraction of natural enemies to aphid-infested plants in two varieties of sorghum. We found that sugarcane aphid feeding induced the production of HIPVs including methyl salicylate, linalool, and d-limonene in a resistant variety of sorghum, but not in a susceptible variety of sorghum. Lady beetles, both Coccinella septempunctata and Harmonia axyridis, and a parasitoid wasp (Aphelinus nigritus) chose aphid-infested resistant plants over un-infested resistant plants 70% - 75% of the time (p < 0.05). Natural enemies, however, exhibited no preference between susceptible sorghum plants that were infested or free of the aphids. These results indicate that natural enemies of the sugarcane aphid are attracted to HIPVs produced by resistant varieties of sorghum and that indirect defenses should be incorporated into sugarcane aphid IPM programs.