Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/29/1997
Publication Date: N/A
Interpretive Summary: Hydrilla is a major aquatic weed problem in the southern and eastern United States. Once established in a body of water, this exotic invasive species readily dominates and replaces native submersed plants, such as vallisneria. Two insects, a leaf-mining fly and a stem-boring weevil, were introduced for control of hydrilla. These insects caused up to 70% reduction in shoot weight of hydrilla, but no feeding damage was observed on vallisneria. As a result, the hydrilla competitive edge over vallisneria was reduced when insects were present. Hydrilla was 8.3 times more competitive than vallisneria when not under the influence of herbivory. However, in the presence of insects, competition was nearly equal between the two species. These results suggest that biological control can play an important role in integrated management of invasive weeds by reducing the competitive advantage of the target weed and thereby facilitating the re-establishment of native plants.
Technical Abstract: Experiments were conducted in outdoor tanks using addition series methods to evaluate the impact of specialized feeding by two biological control agents, Hydrellia pakistanae and Bagous hydrillae, on the competitive interactions between hydrilla [Hydrilla verticillata (L.f.) Royle] and vallisneria (Vallisneria americana L.). Competitive abilities of each plant species were determined using the reciprocal yield model of mean plant weight. Results indicated that hydrilla was the stronger competitor when not under influence of insect herbivory. In the absence of biocontrol agents, hydrilla intraspecific competition was 8.3 times stronger than interspecific competition with vallisneria. H. pakistanae interfered with canopy formation in hydrilla by removing up to 80% of plant biomass in the top 30-cm water column. Similarly, B. hydrillae caused up to 48% reduction in hydrilla plant weight in the Summer experiment. Neither insect species caused any damage to vallisneria. As a result, there was a significant shift in the competitive balance between hydrilla and vallisneria due to specialized insect herbivory, with hydrilla intra- and interspecific competition being nearly equal in the presence of the biological control agents. These results suggest that biological control can play an important role in integrated management of invasive weeds by reducing the competitive advantage of the host plant and thereby facilitating the re- establishment of native plants.