Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2005
Publication Date: 6/1/2005
Citation: Center, T.D., Van, T.K., Dray Jr., F.A., Franks, S.J., Rebelo, M.T., Pratt, P.D., Rayamajhi, M.B. Herbivory alters competitive interactions between two invasive aguatic plants. Biological Control. 33:173-185. 2005. Interpretive Summary: Plant feeding insects have sometimes been introduced for weed biological control without knowledge of how effective they might be once released. While the field of biological control has a good record of predicting the safety of these agents, predictions of efficacy have been more art than a science. This is not surprising inasmuch as these impacts depend upon the development of large agent populations after their release which hinges upon factors (e.g., natural enemies) unknown prior to release. Furthermore, the vulnerability of the plant is likely to differ between the native range and the areas where it has become a weed. One factor that affects this vulnerability is plant competition. Also, subtle effects of the agent, which may go unnoticed during short-term experimentation, can have important, long-term consequences. Nonetheless, improvements in our ability to predict the potential effectiveness of these agents would save resources that would be wasted on ineffective agents. In this context, we investigated the use of plant competition experiments to reveal subtle, otherwise unapparent, differences between agents. We used two floating aquatic weeds, waterhyacinth and waterlettuce, and two similar species of weevils introduced to control waterhyacinth. The effect of competition magnified the impacts of both weevils and showed that one should be much more effective than the other. This prediction contradicted field observations thus suggesting that evaluations of efficacy produce a significant risk of disqualifying potentially important biological control agents. Nonetheless, evaluations under conditions of plant competition did illuminate the importance of recognizing modest effects.
Technical Abstract: We compared the effect of two congeneric specialist weevils (Neochetina eichhorniae and N. bruchi) on competition between their host Eichhornia crassipes and another floating macrophyte (Pistia stratiotes) by manipulating plant densities, presence of either or both herbivore species, and nutrient levels. Measurements of yield included biomass, clonal expansion, and flowering. Without herbivory, intraspecific competition among E. crassipes individuals was 41 times more intense than interspecific competition in terms of biomass yield. Herbivory shifted competitive outcomes, bringing intraspecific competition in line with interspecific competition. Both weevil species reduced E. crassipes biomass and flowering, but N. bruchi also lessened rosette density whereas N. eichhorniae did not. Nutrient limitation reduced plant performance but did not alter the competitive pattern. However, low nutrients moderated differences between weevil species, wherein both seemed equally damaging. We conclude that while herbivory directly and indirectly affected plant performance by altering competition between these two weeds, the competitive response depended upon the herbivore species and nutrient availability. The influence of herbivores on competitive interactions is clearly important for evaluating the invasive potential of exotic plants as well as for predicting the potential efficacy of biological control agents, but interactions between herbivory, competitors, and nutrient availability create complexities that must be considered.