A Comparison of the Impacts of Leaf‐ And Sap‐Feeding Insects on Growth and Allocation of Goldenrod

Many plants support both leaf— and sap—feeding insects, yet comparative studies of plant responses to different types of damage are rare. I compared the impacts of a leaf—chewing beetle (Trirhabda sp.), a xylem sap—feeding spittlebug (Philaenus spumarius), and a phloem sap—feeding aphid (Uroleucon caligatum) on growth of goldenrod (Solidago altissima). My goals were (1) to measure insect impacts on plant size as a function of herbivore loads, and (2) to determine the mechanisms by which insect feeding affects plant growth, using the techniques of plant growth analysis. Plants were grown outdoors in large pots, and insects were placed on them at a range of densities. A series of four whole—plant harvests was performed, with the final harvest occurring after 3 wk of insect feeding. Insect mass gain per plant was used as the basis for the comparison, since the insects differed greatly in size. Spittlebugs were the most damaging insect, beetles were intermediate, and no effects of aphid feeding were detected. After 3 wk of feeding, both spittlebugs and beetles reduced total leaf mass, total leaf area, and root mass, and the impact of the spittlebug was 5—6 times greater than that of the beetle. Spittlebug feeding also reduced the mass of the apical buds, stem mass, and the number of lateral stems, while beetle and aphid damage had no effect on these variables. Dry mass allocation to leaves, stems, and roots was generally not affected by insect feeding, resulting in smaller but proportionally similar plants by the end of the feeding period. Plant relative growth rates (dry mass increase per unit dry mass per day) were reduced by both spittlebugs and beetles but not aphids, and the impact of the spittlebug was more severe than that of the beetle. None of the insects affected net assimilation rates (dry mass increase per unit leaf area per day), indicating that the insects did not reduce plant growth by altering plant physiology. Instead, reductions in relative growth rates were explained by changes in plant morphology; feeding by both spittlebugs and beetles strongly reduced specific leaf area (leaf area/leaf mass). This reduction of leaf area relative to leaf mass was the mechanism by which both insects decreased plant relative growth rates. This result suggests an explanation for why simulated damage frequently fails to mimic damage by actual herbivores. When leaves are artificially removed by clipping there will be an immediate change in dry mass allocation patterns, but specific leaf area will not necessarily be affected.