Stem dimorphism and herbivory in goldenrod: Recurved (“candy-cane”) stems as a resistance mechanism against galling insects

Solidago altissima (Asteraceae), or tall goldenrod, displays a peculiar stem dimorphism: while the majority of the stems are straight (erect), a substantial minority (~10-15%) are recurved, with the apical bud facing downward such that the stem resembles a candy cane. We hypothesized that this “candy-cane morphology” might help plants resist herbivory by reducing oviposition by gall-inducing flies that tend to prefer taller stems. In a field survey of 121 S. altissima genets, stems of the 11 candy-cane genets were 55% as likely (P = 0.008) to be ovipunctured by the goldenrod ball gallmaker, Eurosta solidaginis (Tephritidae), than were ramets of erect-stem genets. Similarly, in a common-garden study of 26 genets (4 candy-cane and 22 erect-stem genets, replicated 15 times) candy-cane stems were 78% as likely to be ovipunctured (P = 0.01). Even so, in neither study did the lower ovipuncture rate in candy-cane genets translate into lower ball-gall formation. However, a mean of 29% of the ramets of erect-stem genets formed rosette galls of a second herbivore, Rhopalomyia solidaginis (Cecidomyiidae), compared to only 11% of the ramets of candy-cane genets. While candy-cane stems are significantly shorter than erect stems during the spring and early summer when they are susceptible to gall-fly oviposition, they straighten up and are of the same height as erect stems by flowering. Therefore, the candy-cane morphology affords goldenrods increased resistance early in the season while minimizing potential costs of decreased competitive stature for light or pollinators later in the season.