Separate and interactive effects of competition and herbivory on the growth, expansion, and tuber formation of Hydrilla verticillata

This study examined the interaction and main-effect impacts of herbivory by the leaf-mining fly Hydrellia pakistanae and plant competition from Vallisneria americana on the growth, expansion and tuber formation of Hydrilla verticillata in a 2 × 2 factorial design experiment. The study was conducted in 14,000-L tanks, over two growing seasons. Each tank represented a single experimental unit and contained 32 1-L pots. At the beginning of the experiment half of these were planted with H. verticillata while the other half were either left empty or planted with V. americana (the competitor). H. pakistanae fly larvae (the herbivore) were added to tanks as appropriate. No significant interactions were identified between herbivory and competition on any parameter of H. verticillata growth analyzed (i.e., total tank biomass accumulation, total number of rooting stems, total tuber number, total tuber mass, and tuber size), indicating that the factors were operating independently and neither antagonism nor synergism was occurring. Both competition and herbivory impacted the growth of H. verticillata. H. verticillata plants grown in the presence of V. americana developed less total biomass, had fewer total basal stems, had fewer tubers and less tuber mass per tank, and produced significantly smaller tubers relative to control plants. Herbivory also suppressed H. verticillata biomass accumulation and tended to suppress the number and total mass of tubers produced in each tank. Both factors showed 30–40% reduction of total H. verticillata biomass, although the mechanism of impact was different. Competition suppressed expansion of H. verticillata into adjoining pots but had little impact on its growth in pots where it was originally planted. Herbivory resulted in a general suppression of growth of H. verticillata in all pots. Although herbivory significantly impacted H. verticillata biomass, it did not result in competitive release for V. americana under the current experimental conditions. We conclude that management activities that promote competition or herbivory will impact the growth and expansion of H. verticillata. Furthermore, since these factors operated independently, the combined use of both factors should be beneficial for suppression of H. verticillata dominance.