Biocontrol of Water Lettuce (Pistia stratiotes L.) Using the Weevil Neohydronomus affinis Hustache and Fungal Pathogens

Pistia stratiotes L., is an invasive aquatic plant that poses environmental and economic challenges. This study investigates the effectiveness of biological control using the weevil Neohydronomus affinis Hustache and two fungal species, Alternaria murispora, and Alternaria destruens, to manage P. stratiotes. The experiments were conducted under greenhouse conditions. The effect of single and combined releases of N. affinis, A. murispora, and A. destruens on P. stratiotes visual quality and dry biomass was assessed over five weeks. Neohydronomus affinis population density was monitored throughout the treatment period. Among the single-agent treatments, A. murispora and A. destruens at 10¹⁰ conidia mL⁻¹ significantly reduced visual quality and dry biomass by over 50%. The most effective combination treatments included A. murispora + A. destruens at 1010 conidia mL⁻¹, and combinations of N. affinis with both fungal strains at 1010 conidia mL⁻¹. These reduced visual quality and dry biomass by more than 50%. The population density of N. affinis increased across all treatments. The most significant growth occurred when combined with A. murispora and A. destruens at 10¹⁰ conidia mL⁻¹, with densities rising from 9.1 to 36.3 over four weeks, indicating a synergistic effect. This combination helps control P. stratiotes while boosting N. affinis populations.