Effects of Herbivorous Insects and Soil Fertility on Reproduction of Goldenrod

Abstract

Plant responses to herbivore damage vary enormously, but the causes of this variation are not well understood. We examined how both insect feeding style and soil fertility influence the severity of herbivore impacts, by measuring the effects of feeding by a leaf—chewing beetle (Trirhabda sp.), a xylem sap—feeding spittlebug (Philaenus spumarius), and a phloem sap—feeding aphid (Uroleucon caligatum) on growth and reproduction of goldenrod (Solidago altissima). Our goals were to determine (1) whether these three insects differentially affect plant fitness, (2) whether soil fertility affects a plant's ability to compensate for damage, and (3) whether soil fertility and insect feeding style interact in determining herbivory impacts. Plants were grown outdoors in large pots at two levels of soil fertility, and insects were placed on them at a range of densities. Insects fed on the plants for °3 wk in spring, and then the plants were grown insect free for the rest of the season. We used insect mass gain per plant as the basis for comparing the effects of three insects, since the insects differed greatly in size. Spittlebug feeding reduced plant growth rates 3 times more than beetle feeding; aphid feeding had no detectable effect. The plants grew faster at higher soil fertility, but there was no interaction between insect impact and fertilizer level. Stem heights at the end of the season were increased by fertilizer and not affected by herbivory. Spittlebug feeding, and to a lesser extent aphid feeding, decreased the number of lateral stems, and again there was no interaction with soil fertility. Soil fertility had strong effects on plant sexual and asexual reproduction. Plants at the high level of soil fertility showed a three—fold increase in total seed production, a 5% increase in mean achene mass, almost a two—fold increase in rhizome mass, and increased percent nitrogen in seeds and rhizomes. Total seed production was reduced by herbivory. The pattern was the same as for plant growth rates; the spittlebug caused the strongest decrease, beetle feeding was second, and aphid feeding caused slight reductions. There was a strong interaction between insect impact and soil fertility. Total seed production was reduced only at the high level of soil fertility for all three insects. Spittlebug and beetle damage also delayed flowering, but only at the high level soil fertility. Rhizome mass, mean achene mass, and percent nitrogen in seeds and rhizomes were not affected by herbivory. The common assumption that plants are better able to compensate for herbivore damage at higher levels of soil fertility must be reconsidered. This study also suggests that sexual reproduction may be more sensitive to damage than asexual reproduction in perennial plants.