Candy-cane stems not so sweet: ducking goldenrod stems have altered chemical defenses against non-galling herbivores

Plants exhibit a wide range of defense mechanisms to protect against herbivore damage, including physical and chemical defenses. Populations of tall goldenrod (Solidago altissima) display a genetically controlled stem dimorphism that serves as a physical defense. The majority of tall goldenrod plants grow normal, erect stems, but a small proportion have stems that bend early in the season causing the apex to point downward, giving the plants the appearance of candy canes. This ducking phenomenon has been shown to increase resistance to apex-galling herbivores. The apical buds of candy-cane stem plants are hidden by the erect stems, making them less visible and therefore less vulnerable to herbivores such as the gall fly Eurosta solidaginis.  As a result of this unique architectural defense strategy, we predicted candy-cane stem plants would have reduced selection pressure for investment in chemical defenses. Therefore, we hypothesized that these plants would be more susceptible to non-galling herbivores. In this study, we demonstrate that candy-cane goldenrod plants are more susceptible than erect plants to infestation by the specialist aphid (Uroleucon nigrotuberculatum). Not only did nymphs better colonize candy-cane plants, adult aphids also preferentially selected candy-cane stems over erect stems in choice tests. Furthermore, candy-cane plants had higher terpene concentrations in the presence of aphids than erect stem plants, which could explain the aphids’ attraction to the candy-cane stems. Our results suggest that the candy-cane stem genotype, which provides resistance to gall inducers, is more susceptible to non-galling herbivores, possibly because of altered investment in chemical defenses.