Host range estimation using historical records and field-based host specificity tests was performed with Tachardiaephagus somervilli, an encyrtid parasitoid selected for the biological control of the invasive lac scale Tachardina aurantiaca on Christmas Island, Indian Ocean. T. aurantiaca is tended by the invasive ant Anoplolepis gracilipes and its honeydew supply is implicated in formation of ant supercolonies that threaten native biodiversity and disrupt ecosystem processes in island forest. Historical data indicated that T. somervilli and other Tachardiaephagus spp. have narrow host ranges restricted to lac scales (Kerriidae). Furthermore, none of the 40 encyrtid species known to parasitize lac scales has a host range extending beyond the Coccoidea, and no encyrtid species is known that bridges the phylogenetic distance between their kerriid hosts and the families of any endemic Hemiptera found on Christmas Island. No-choice host specificity tests conducted in the field in the native range of Tachardina aurantiaca showed that none of the eight test species (Coccoidea) were parasitized by T. somervilli. These results predict that release of T. somervilli on Christmas Island will have negligible risks of non-target impacts. More broadly, host-specificity testing in the native region should play a greater role in assessment of potential biological control agents.