Influence of Butomus umbellatus L. lineage and age on leaf chemistry and performance of a generalist caterpillar.
Invasive plants may display age or genotype-specific defenses against herbivory. Thus, the importance of herbivory for preventing or mitigating plant invasions may depend on genetics and age-structure of invader populations. We used an excised-leaf experiment to test whether constitutive herbivore defenses in Butomus umbellatus L. (flowering rush) plants decline with age between colonizing (1st year) and established (2nd year) plants and whether the decline was parallel between introduced diploid and triploid genotypes. Leaves from colonizing and established plants were fed to the generalist herbivore fall armyworm (FAW; Spodoptera frugiperda) and then short-term FAW development was measured. Additionally, leaf dry matter content (DMC), % carbon (%C), % nitrogen (%N), carbon:nitrogen (C:N) ratio, and total leaf phenolics were determined for leaves from a subset of test plants. FAW fed leaves from diploid plants grew 27% less than those fed leaves from triploid plants but were only 1% smaller when fed leaves from second-year versus first-year plants of either cytotype. DMC of diploid leaves was 11% greater overall (11% DMC) but the difference in cytotypes depended on age (1% difference in colonizing plants vs. 21% difference in established plants). In general, diploid plants were characterized by 29% higher total phenolics, 17% less leaf nitrogen, and 21% higher C:N ratio, regardless of age. Measured traits were, on average, 27% more variable for diploid than triploid plants, possibly a reflection of increased genetic diversity of diploid flowering rush populations used in the study. These results demonstrate that diploid B. umbellatus plants are, overall, better defended against generalist herbivory than triploid plants. Differences in herbivore performance on B. umbellatus lineages, and to a lesser extent plants of varying ages, may therefore contribute to spatial or temporal heterogeneity in the dominance of genotypes in areas where they grow together.