The effect of temperate climate on potential biocontrol agents for water hyacinth, case study of Rwanda

Water hyacinth is an invasive aquatic plant that has been associated with major negative economic and ecological impacts in water systems worldwide, including Rwanda, since its establishment in the country in the 1960s. While biological control is considered the most sustainable management method, the success of biocontrol agents depends on various abiotic factors, with temperature being critical. This study assessed the suitability of potential water hyacinth biocontrol agents such as: Neochetina weevils, Megamelus scutellaris Berg (Hemiptera: Delphacidae), and Cornops aquaticum Bruner (Orthoptera: Acrididae) for regions with a temperate climate by testing their thermal boundaries. Using thermal physiology limits and CLIMEX modelling, we found that Neochetina eichhorniae Warner and N. bruchi Hustache (Coleoptera: Curculionidae) had lower thermal minimums (CTmin) of 2.4°C and 2.6°C, respectively, compared to Megamelus scutellaris (4.7°C) and Cornops aquaticum (6.2°C). CLIMEX modelling predicted the suitability of Neochetina weevils and C. aquaticum across Rwanda, while M. scutellaris appeared unsuitable for the colder northern regions of the country but appropriate for the central and eastern regions. These findings suggests that the historical failure of Neochetina weevils introduced to Rwandan water bodies in 2000 was not due to temperature extremes. Rather, other factors such as release numbers or water quality may have played a role. This study provides crucial information for future biocontrol efforts in Rwanda and similar temperate regions, highlighting the importance of pre-release thermal tolerance assessments and climate modelling to predict biocontrol agent establishment and efficacy.