COMPETITIVE INTERACTIONS BETWEEN HYDRILLA (HYDRILLA-VERTICILLATA) ANDVALLISNERIA (VALLISNERIA-AMERICANA) AS INFLUENCED BY INSECT HERBIVORY

Two experiments (winter and summer) were conducted in outdoor tanks us ing addition-series methods to evaluate the impact of specialized feed ing by two biological control agents, Hydrellia pakistanae Deonier and Bagous hydrillae O'Brien, on competitive interactions between hydrill a [Hydrilla verticillata (L.f.) Royle] and vallisneria (Vallisneria am ericana Michx). Competitive abilities of each plant species were deter mined using the reciprocal-yield model of mean plant weight. In the ab sence of the biocontrol agents, intraspecific competition from hydrill a on itself was 8.3 times stronger than interspecific competition from vallisneria. Hydrellia pakistanae interfered with hydrilla canopy for mation by removing as much as 80% of the plant biomass in the top 30 c m of the water column. Damage by H. pakistanae also caused a 43% reduc tion in hydrilla tuber production during the winter experiment. Simila rly, B. hydrillae caused up to a 48% reduction in hydrilla plant weigh t in the summer experiment. Neither insect species damaged vallisneria . As a result, there were significant shifts in the competitive balanc e between hydrilla and vallisneria due to selective insect feedings. I n the presence of H. pakistanae, hydrilla intraspecific competition wa s nearly equal to interspecific competition from vallisneria, indicati ng that hydrilla had lost its competitive edge over vallisneria. Bagou s hydrillae also produced similar, but smaller, shifts in the relative competitive abilities of hydrilla and vallisneria. These results indi cate that biological control agents can disrupt the competitive balanc e between plant species in favor of native species, thus adding anothe r element to the weed biological control strategies.