Indirect Defense in Sorghum: Attraction of Natural Enemies to Aphid-Infested Plants Varies among Cultivars

Abstract Herbivore-induced plant volatiles (HIPVs) are an indirect defense used by plants to attract natural enemies and reduce herbivory by insect pests. In many crops including sorghum ( Sorghum bicolor ), little is known about genotypic variation in HIPV production or how this may affect natural enemy attraction. In this study, we identified and quantified HIPVs produced by 10 sorghum cultivars infested with a prominent aphid pest, the sorghum aphid ( Melanaphis sorghi Theobald). Volatiles were collected using dynamic headspace sampling techniques and identified and quantified using GC-MS. The total amounts of volatiles induced by the aphids did not differ among the 10 cultivars, but overall blends of volatiles differed in composition. Most notably, aphid herbivory induced higher levels of methyl salicylate (MeSA) in two cultivars. In addition, four cultivars did not change in the volatile emissions after herbivory. Dual-choice olfactometer assays were used to determine preference of the aphid parasitoid, Aphelinus nigritus , and predator, Chrysoperla rufilabris , between plants of the same cultivar that were un-infested or infested with aphids. Two aphid-infested cultivars were preferred by natural enemies, while four other cultivars were more attractive to natural enemies when they were free of aphids. The remaining four cultivars elicited no response from parasitoids. Our work suggests that genetic variation in HIPV emissions greatly affects parasitoid and predator attraction to aphid-infested sorghum and that screening crop cultivars for indirect defenses has the potential to improve the efficacy of biological control.