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Hydrocotyle ranunculoides (Araliaceae) is an aquatic plant native to the Americas and invasive in Europe. This work reports laboratory and garden studies on the host range of its main natural enemy, the weevil Listronotus elongatus (Hustache) (Coleoptera: Curculionidae). The genus Hydrocotyle is in a disputed phylogenetic situation such that both Apiaceae and Araliaceae must be included in the test list, but lacking a clear criterion of phylogenetic proximity. For this reason three testing instances were applied: no-choice on excised leaves, no choice on functional plants, and a multiple choice garden experiment. A large initial test-plant list was reduced to a smaller list adjusted by a feeding intensity criterion, survival curves and oviposition levels. Feeding was observed on the excised leaves of 21 out of 36 test plants, but as tests progressed to whole plants L. elongatus showed it could only develop and thrive on its natural host, and possibly survive as adults on Hydrocotyle modesta. Results confirm that typical laboratory starvation tests can provide a dependable ranking of physiological acceptability of different plant species but severely overestimate the actual host range of a specialized herbivore. They also indicate that the phylogenetic criterion for test plant list construction must be approached cautiously if the phylogeny is uncertain or disputed. [ABSTRACT FROM AUTHOR]