Trade-offs between defenses against herbivores in goldenrod (Solidago altissima)

In the goldenrod Solidago altissima, most stems are erect, but “ducking” genotypes bend the tip of the apical stem downward for much of the growing season, and this morphology protects against at least two gall-forming herbivore species. Despite this advantage to defense, ducking remains a rare strategy in goldenrod, yet the costs that prevent ducking genotypes from outcompeting erect genotypes remain unclear. We tested whether ducking (an architectural defense) trades off with chemical defense against aphids (Uroleucon nigrotuberculatum). We hypothesized that signaling related to the ducking defense might interfere with investment in chemical defenses, making ducking plants more susceptible to some herbivores. To test this hypothesis, we compared aphid survival and preference on ducking and erect genotypes. We also measured terpenoid concentration in S. altissima leaf tissue to determine whether plant investment in these compounds correlated with either ducking or aphid performance. Aphids had higher survival on all three ducking genotypes than their erect counterparts and preferred ducking to erect plants in two of three genotype pairings. However, terpenoid concentrations did not track with either ducking or aphid performance and cannot therefore explain the differences between ducking and erect host-plants. Although the mechanism remains unknown, the data supported the predicted trade-off in defenses against different herbivores, which may contribute to the distribution and abundance of these two defensive strategies.