Host selection of <i>Lysathia cilliersae</i> across increasing experimental complexity and ecological realism

Abstract Assessing the host specificity of candidate biological control agents is a crucial step in classical biological control programmes. To determine an insect’s physiological host range, researchers conduct controlled laboratory assays, which provide insights into potential risks to non-target species and help anticipate unintended ecological impacts. The beetle, Lysathia cilliersae has been identified as a promising candidate for the biological control of the aquatic weed, Myriophyllum aquaticum . This study aimed to evaluate the physiological host range of L. cilliersae by testing its interactions with 18 plant species closely related to M. aquaticum , including 12 from the Haloragaceae, 1 from Penthoraceae, 1 from Crassulaceae, 2 from Onagraceae, 1 from Polygonaceae and 1 from Hydrocharitaceae. Under no-choice conditions, the beetle oviposited on nearly all tested species, except for submerged and phylogenetically distant plants. Larval development was successful on the majority of species, with the exceptions of Myriophyllum alterniflorum and Myriophyllum pinnatum . Dual- and multiple-choice assays revealed a strong preference for M. aquaticum , and this preference became more pronounced as the complexity of the tests increased, suggesting the beetle is capable of refining its host selection when presented with multiple options. Although laboratory studies tend to overestimate host ranges, semi-natural and open-field trials confirmed a similar pattern of host preference. In these settings, L. cilliersae consistently favoured M. aquaticum for oviposition, although a small number of eggs and larvae were also found on some non-target species. These findings suggest that while L. cilliersae exhibits a restricted host range under natural conditions and a strong preference for the target weed, there remains a potential risk of non-target spillover. Further research is necessary to evaluate the insect’s ability to establish and sustain populations on non-target plants under natural conditions.