The present study aimed at evaluating the host-specificity, potential efficacy and optimum densities of the two weevils (Neochetina bruchi and N. eichhorniae) as water hyacinth control agents in the Rift Valley of Ethiopia. Density-plant damage relationship was monitored for two years (2012 to 2014) in a factorial experiment by involving the two Neocthenia weevils and four densities of herbivory. Feeding multiple-choice and oviposition tests were used to assess the two weevils host specificity. The study showed highly significant (P< 0.01) impacts of the two weevils as well as, their interactions on number of ramets, number of leaves, damaged leaf area, petiole length, number of defoliated petioles and biomass of water hyacinth. Damaged leaf area (97%) and total number of defoliated petioles (2.8) were highest in plants that received three pairs of N. bruchi and two pairs of N. eichhorniae. The weevil density and water hyacinth biomass showed convex relationship that could be well-described by a negative log function (R2=0.98). The study confirmed that the weevils are sufficiently host-specific and therefore can be safely released in the Rift Valley of Ethiopia to control water hyacinth.