Invasive plants have become a major threat throughout the world, in particular in islands where the biodiversity is important and even unique. Since the 17th century, many weeds or invasive plants have been introduced in Réunion Island. Among them Rubus alceifolius, a giant bramble, is the most invasive and threatening to the endemic vegetation. After years of unsuccessful and even detrimental controls with herbicide applications and/or mechanical removal, a new biological control program has been launched, based on the release of a biocontrol agent, Cibdela janthina, a sawfly. Modelling Rubus-Cibdela interactions is not only helpfull to formalize knowledges but also to guide control strategies and decision-making, in particular before field releases. The aim of this talk, is to present a minimal mathematical model and some results based on ongoing experiments in Réunion Island.