Bottom-up effects on top-down regulation of a floating aquatic plant by two weevil species: the context-specific nature of biological control

Authors: CENTER, TED - Retired ARS Employee; Dray, F Allen; Mattison, Elizabeth; Tipping, Philip; Rayamajhi, Min

Submitted to: Journal of Applied Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/9/2013
Publication Date: 6/1/2014
Citation: Center, T.D., Dray Jr, F.A., Mattison, E.D., Tipping, P.W., Rayamajhi, M.B. Bottom-up effects on top-down regulation of a floating aquatic plant by two weevil species: the context-specific nature of biological control. Journal of Applied Ecology.51(3):814-824. 2014.

Interpretive Summary: We examined the effects of fertilizer level (i.e., plant nutrition) and initial plant density on several plant population parameters using two weevil species and their host plant, waterhyacinth (Eichhornia crassipes). Both weevil species were effective at retarding flowering at all levels of plant nutrition. Final plant density and biomass were also affected by herbivory, but plant nutrition affected which weevil species was most effective. This dynamic was further modified by the initial plant density so that treatments with the weevil Neochetina bruchi were more effective under higher fertilizer levels when starting plant densities were small, whereas the weevil Neochetina eichhorniae was more effective at lower fertilizer levels with small starting densities and at higher fertilizer levels with larger starting densities. Neither weevil species was effective at the lowest fertilizer level.

Technical Abstract: 1. Plant nutrition (bottom-up effects) impacts a plant’s ability to sustain herbivory (top-down effects) and affects phytophagous insect fecundity. These factors potentially confound efficacy predictions for biological control projects. We investigated the relative importance of these two forces with regard to invasibility of Eichhornia crassipes. We also examined, a posteriori, whether it would have been possible to predict which of two introduced weevil species, Neochetina eichhorniae or N. bruchi, would have been the superior choice for biological control of E. crassipes. 2. We conducted three experiments comparing E. crassipes performance when subjected to herbivory by these weevils, independently and together, across a range of fertilizer levels. A low initial plant density experiment examined plant growth and flowering while allowing for density to increase. A high plant density experiment evaluated reduction of biomass and surface coverage. The third experiment evaluated plant growth with minimal spatial constraints, maintained by reducing coverage to 20% whenever it exceeded 50%. 3. Herbivory effects varied between weevil species and among fertilizer treatments; interactions were apparent but their nature varied among experiments. Herbivory reduced plant density and biomass except in low nutrient conditions. Herbivory retarded flower production across all fertilizer levels. Otherwise, N. bruchi alone, especially in high fertilizer regimens, or the species together reduced growth more effectively than N. eichhorniae alone, suggesting that N. bruchi was the better agent. However, N. eichhorniae was better at reducing biomass and plant density in high fertilizer levels under initial conditions of full plant coverage, suggesting that N. eichhorniae was the superior choice. 4. Synthesis and applications. Bottom-up and top-down forces acting in concert affect invasive characteristics of plant species. Plasticity in growth responses to resource availability can enable invaders to persist in conditions unfavorable to herbivores (enemy-free space). Consequently, biological control efficacy is nuanced and context specific, so predictability requires assessing multiple parameters across a range of environmental and ecological conditions. Overly simplistic evaluations risk rejection of effective agents capable of mediating adverse impacts of non-native plant invasions. These results also argue against the concept that a single best agent can be identified to control an invasive plant that exploits a broad range of habitats and environmental conditions.