Abstract Scale insects of the genus Apiomorpha Rübsaamen (Hemiptera: Coccoidea: Eriococcidae) induce sexually dimorphic galls on Eucalyptus, but the Apiomorpha pharetrata species-group is unusual in that nymphal males aggregate on the surface of the maternal gall where they induce a compound structure. Although originally described as distinct species on the basis of differences in gall morphology, A. pharetrata (Schrader) and A. thorntoni (Froggatt) were subsequently synonymised, primarily because the adult females of the two are morphologically indistinguishable. The two gall types of A. pharetrata sensu lato are allopatric and found on different host eucalypt species. To test the hypothesis that gall morphology may be determined by the host species on which the female feeds, we reared crawlers of field-collected females from both gall types on eucalypts in a glasshouse. Both gall types were induced on the same eucalypt species, with the gall type matching that of the maternal gall from which the crawlers had emerged. This indicates that it is the insect, not the host, that determines the gall morphology in these taxa. In addition, insects from the two gall types were different chromosomally and could be distinguished by one fixed allozyme difference. Thus, Brachyscelis (= Apiomorpha) thorntoni (Froggatt) revised status is removed from synonymy with Brachyscelis (= Apiomorpha) pharetrata (Schrader), and recognised as a distinct biological species. However, the extent of chromosomal variation among the populations of both A. pharetrata and A. thorntoni suggests that there may be further cryptic species present.