Do goldenrod genotypes differ in their responses to an odor gradient from their specialist herbivore?

Plants can prime their defenses in response to airborne cues indicating increased risk of herbivory. It is unclear, however, if the intensity of these cues conveys useful information about the likelihood of attack and whether plant genotypes vary in their tolerance of risk—i.e., do different genotypes respond differentially to cue intensity? Tall goldenrod (Solidago altissima) primes its defenses in response to the male pheromone of its specialist herbivore, the goldenrod gall fly (Eurosta solidaginis). We tested whether different genotypes of goldenrod respond differentially to a gradient of priming cue concentration, with three classes of genotypes: North American cernuous genotypes, which are less likely to be attacked by the gall fly, North American erect genotypes, which are more likely to be attacked, and Japanese genotypes, which have not seen the gall fly for ~100 years in their introduced range. All classes had one genotype that was mostly resistant to galling, suggesting that resistance can persist, even with relaxed selective pressure from the gall fly. Overall, we did not detect a significant interaction between cue intensity and genotype; however, trends indicated that Japanese genotypes are less sensitive to the cue than N. American genotypes, that there is a non-linear relationship between cue strength and galling susceptibility, and that cue exposure might lead to induced susceptibility, rather than defense, in some genotypes. Increased sample sizes are necessary to draw more robust conclusions, but early data indicate a complex relationship between cue strength, plant genotype, and gall success.