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Plant nutrition (bottom-up effects) impacts a plant's ability to withstand herbivory (top-down effects) and influences phytophagous insect fecundity. These factors confound efficacy predictions for biological control purposes. We investigated the importance of these forces with regard to invasibility of Eichhornia crassipes. We also examined whether one could have predicted whether the weevil species Neochetina eichhorniae or Neochetina bruchi would have been the better biocontrol agent., Three experiments compared E. crassipes performance when subjected to herbivory by these weevils across a range of fertilizer levels. A low initial plant density experiment examined growth and flowering while allowing density to increase. A high plant density experiment evaluated reduction in biomass and surface coverage. A third experiment evaluated plant growth unconstrained by space., Herbivory effects varied between treatments; interactions varied between experiments. Herbivory reduced plant density and biomass, except in low fertilizer conditions, and consistently reduced flowering. N. bruchi alone or both weevil species together reduced growth more than N. eichhorniae alone, elucidating N. bruchi as the better agent. However, N. eichhorniae better reduced biomass and plant density under initial conditions of complete plant coverage, suggesting it as the superior choice., Synthesis and applications. Bottom-up and top-down forces acting in concert affect invasibility of plant species. Plasticity to resource availability enables invaders to persist in conditions that are unfavourable to herbivores (enemy-free space). Consequently, biological control efficacy is nuanced and context specific, so predictability requires assessment of multiple parameters across a range of conditions. Overly simplistic evaluations risk rejection of effective agents capable of mediating non-native plant invasions. These results also argue that biological control practitioners should disregard 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. Aquaphyte invasions often coincide with watershed nutrification so unless weed control is carried out concurrently with measures that reduce nutrient inputs, removal of infestations will merely reduce competition thereby accelerating regrowth or facilitating replacement by other undesirable species (the 'invasive treadmill'). The suppressive effects of biological control are valuable and complimentary, even when replacement by other, presumably less problematic, species may occur. These subtler effects should be considered with protocols developed for integrated control strategies. [ABSTRACT FROM AUTHOR]

1. Nutrient availability varies widely in aquatic systems and is likely to profoundly affect the outcomes of plant–herbivore interactions in aquatic environments. This has implications in programmes aimed at the biological control of water weeds. We hypothesized that nutrient flux seasonally affects ovarian development of two specialist weevil species, Neochetina eichhorniae and Neochetina bruchi, thereby influencing population growth on the floating plant Eichhornia crassipes. 2. We also hypothesized that the weevils differed in their sensitivities to nutritional quality and interacted differently depending on host quality . 3. To test these hypotheses, we cultured E. crassipes plants under 20 fertilizer regimens then introduced reproductively dormant, over-wintering female weevils as single or mixed species populations. They were later retrieved and dissected to ascertain ovarian status. F1 progeny was extracted biweekly to compare fecundity with population growth. 4. Ovaries regained functionality and the reproductive proportion of the population increased with fertilizer rate as did the fecundity of reproductive individuals. F1 progeny increased exponentially from 7 to over 300 weevils tank−1 in the highest fertilizer treatment. 5. Mixed species treatments produced more offspring under high fertilizer rates than single species treatments (236 ± 32 mixed vs. 155 ± 25 N. bruchi or 170 ± 33 N. eichhorniae pure). Neochetina eichhorniae was more productive throughout than N. bruchi, but N. bruchi performed nearly as well in high fertilizer treatments. 6. Ratios of the two species remained consistent whether in single or mixed species populations suggesting a lack of interspecific interference. However, N. eichhorniae seemed more adaptable to a wider range of plant quality and more tolerant of lower nutritional circumstances. 7. Synthesis and applications. The coupling of reproductive status of the parental generation with the population response of the F1 generation indicates that plant quality drives population growth of both weevil species, demonstrating bottom-up regulation. This largely explains the variable performance of these weevils as biological control agents. These results emphasize the importance of considering bottom-up regulation in evaluating host range trials, predicting efficacy and in post-release evaluations. In addition, they suggest that partial herbicide treatments to accomplish phased removal of infestations could enhance the quality of the remaining plants, thereby better integrating biological control into current E. crassipes management systems. [ABSTRACT FROM AUTHOR]