The leaf-beetle Galerucella birmanica is a potential biocontrol herbivore for water chestnut, Trapa natans in North America. During our assessments of effectiveness and safety of G. birmanica in quarantine, we found a narrow fundamental host range with complete development only possible on all T. natans morphospecies present in North America and Brasenia schreberi. Phylogenetic relatedness did not fully explain feeding and oviposition preferences of G. birmanica in no-choice or multiple-choice trials. Instead, feeding and oviposition on non-target plants appears better explained by host-associations of congeneric Galerucella species, underscoring the importance of including additional factors in selecting host-specificity test plant species. To further gauge potential risk to B. schreberi, we used a comparative performance metric proposed by Paynter et al. (2015) and found G. birmanica scored low, further indicating low risk to B. schreberi. Our findings suggest that widespread and safe biocontrol of T. natans in North America appears promising if G. birmanica is granted release approval by regulatory agencies.