Effects of salinity and nutrients on water hyacinth and its biological control agent, Neochetina bruchi

Abstract

Water hyacinth, Eichhornia crassipes (Mart.) Solms (Commelinales: Pontederiaceae), is an important aquatic weed worldwide. Previous studies demonstrate that releases of Neochetina bruchi Hustache (Coleoptera: Curculionidae) provide biological control in many locations, but not all. Notably, N. bruchi were unsuccessful at regulating water hyacinth in tidal brackish waters. Abiotic factors, including salinity and nutrients, affect water hyacinth growth, but little is known about the impact of salinity on weevil survival. We hypothesized that N. bruchi has a relatively low salinity tolerance. In a mesocosm experiment, we assessed weed growth in response to a range of salinity and nutrient concentrations. In a laboratory, we assessed adult N. bruchi mortality in response to various salinity concentrations. Results indicate that increasing nutrient concentration increases weed growth. When both nutrient and salinity levels were varied, nutrients increased leaf count, but not biomass, while salinity reduced growth and increased mortality. Increasing salinity concentrations increased adult weevil mortality; required concentrations were higher than that for weeds. Thus, these results did not provide support for the suggested hypothesis. Potential effects of salinity via other exposures to weevils need to be investigated. Elucidating abiotic factors important for weed growth and weevil survival may increase effectiveness of water hyacinth management practices.