Biological control of waterhyacinth [Eichhornia crassipes (Mart.) Solms.] by waterhyacinth weevils [Neochetina bruchi (Hustache)] and [Neochetina eichhorniae (Warner)] varies according to field environment, season, and plant stress levels. Plants at four field sites were sampled to determine if leaf scarring caused by adult weevil feeding was associated with plant biomass, biochemical and population measures, and insect densities. Leaf scar densities were consistently higher on plants from two sites at which root and dead plant part biomass were high. Scarring was correlated to root and dead biomass across all sites. Scarring was not associated with weevil densities summed across all life stages or larval gallery density. Soluble protein contents were lower in plants from the two sites with high scarring than at two other sites in Spring 2002, and scarring was negatively correlated to protein content across all sites. Protein was usually highest in immature furled leaves. Activities of soluble peroxidase enzymes were highest in old leaves. Scar densities were not associated with canopy height and shoot density. At one site, high scar densities occurred on plants with small leaf areas, which were likely growing slowly under the influence of multiple abiotic and biotic stress factors. Mechanical and natural plant removal and regrowth may have facilitated plant compensation for weevil feeding at the other site with high scarring. Temporally and spatially dynamic physical and biochemical plant traits and growth environments could limit biological control of waterhyacinth.
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