The role of nodding stems in the goldenrod-gall-fly interaction: A test of the "ducking" hypothesis.

Herbivores are among the most pervasive selective forces acting on plants, and the number of plant chemicals that presumably evolved for defense against herbivory is immense. In contrast, biologists are only beginning to appreciate the important roles that architectural traits can play in antiherbivore defense. One putative architectural-resistance trait is the nodding stem apex of some goldenrods (Solidago; Asteraceae). Individuals of S. altissima genets that undergo temporary nodding in the late spring (i.e., "candy-cane" ramets) have been shown to be more resistant than individuals of erect-stemmed genets to certain apex-attacking herbivores. We tested the hypothesis that the greater resistance of candy-cane ramets is accomplished by the ramets' "ducking" from the herbivores. In a greenhouse experiment, nodding candy-cane ramets were significantly more resistant to oviposition by the gall-inducing fly Eurosta solidaginis than were ramets of the same genets that had been experimentally straightened. The straightened candy-cane stems were just as susceptible to ovipositions as were ramets of erect-stemmed genets. Thus, ducking indeed appears to confer a resistance advantage to candy-cane genets of S. altissima.