AbstractThe release of classical biological control agents has reduced the economic, environmental and social problems caused by water hyacinth, Eichhornia crassipes; however, additional control measures are needed in some locations. Water hyacinth plants were treated with different densities of eggs of the weevil Neochetina eichhorniae Warner, one of the main control agents, under different nutrient regimes in a controlled experiment. Plants were destructively sampled and the development of N. eichhorniae was assessed. The survival of first and second instars declined as larval density increased. Plant nutrient status did not directly affect the mortality rate of larvae, but at higher nutrient concentrations larvae developed faster and were larger at a given developmental stage. It is argued that the density dependence operating in N. eichhorniae occurs through an interaction between young larvae and leaf longevity. Consequently, events which disrupt water hyacinth leaf dynamics, e.g. frost or foliar herbicides, will have a disproportionately large effect on the control agents and may reduce the level of control of the host.