/ We hypothesized that repeated herbicidal (maintenance) control of water hyacinth infestations in Florida suppressed biological control agent populations, especially the weevils Neochetina eichhorniae and N. bruchi. We therefore sampled water hyacinth and weevil populations at 54 sites distributed statewide. Half were under maintenance control, half were not treated with herbicides. General site conditions were assessed, demographic data were collected on weevil and plant populations, the reproductive condition of the weevils was determined, and plant nutrient and proximate composition of water hyacinth leaves were analyzed. Water hyacinth infestations under maintenance control were minimal when compared to unmanaged sites. Likewise, on a population basis, all weevil cohorts were much lower due to the paucity of plants. Plants at unmanaged sites, where weevil intensities were much higher, suffered high levels of stress and showed low growth potential. Lower percentages of the female weevils were reproductive at unmanaged sites when compared to managed sites, so densities of reproductives and immatures were similar at both site types. Reproductive status of the weevils improved with increased plant quality. Plant quality, in turn, declined as stresses arising from weevil feeding increased. Plant quality was positively correlated with plant growth potential and flower production. Thus, maintenance control improved plant nutritive quality thereby inducing reproductive vigor of the weevils, but ensuring plant regrowth and the need for future control. This suggests that biological and herbicidal controls should be integrated, using herbicides to maintain water hyacinth infestations below management thresholds but in a manner that conserves biological control agent populations. This approach would lead to improved plant nutritional quality that would, in turn, stimulate reproduction in biological control agent populations. KEY WORDS: Eichhornia crassipes; Neochetina eichhorniae; Neochetina bruchi; Phytophagy; Integrated control; Aquatic weeds

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Summary1. 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. F 1 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. F 1 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 AE 32 mixed vs. 155 AE 25 N. bruchi or 170 AE 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 F 1 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 postrelease 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.