Simple SummaryHost specificity in an Australian species of processionary moth was studied over three years in the eastern Australia states of Queensland and New South Wales. The processionary mothOchrogaster luniferis known as a species complex, characterised by several nesting forms coexisting in the same geographical area. The canopy nesting forms associated with acacias (Acaciaspp.) and eucalypts (Eucalyptusspp. andCorymbiaspp.) were recently shown to be genetically distinct, and they were here tested for host specificity by reciprocal transplant experiments. Egg masses and subsequent offspring were moved within and among putative hosts in field settings. Early larval colonies performed significantly better on their natal hosts than on the transplanted ones, with no difference observed for mature larval colonies. The genetic separation identified in previous work is, thus, confirmed by these ecological observations on host specificity, stressing the importance of host specialization in the ongoing speciation process.AbstractThe ‘mother knows best’ hypothesis is tested in a species of processionary moth feeding on acacias and eucalypts in Australia. The processionary mothOchrogaster lunifer(Lepidoptera: Notodontidae; Thaumetopoeinae) is a social caterpillar living in large colonies on a number of tree and shrub species. Five nesting types—canopy, trunk, tree-hugger, hanging, and ground—have been described, and this study deals with canopy nesters on various species of acacias (Acaciaspp.) and eucalypts (Eucalyptusspp. andCorymbiaspp.). Reciprocal transplant experiments conducted over three years confirm the ‘mother knows best’ hypothesis, as colonies performed better on the natal host plant than on the recipient ones. Young first instar larvae were less likely to establish on a non-natal host than the mature larvae, and all acacia-sourced canopy egg masses failed to establish on eucalypts. Large larvae were able to establish on transplant hosts. This suggests a strong preference–performance link at what is likely a species level, confirming preliminary results recently published on genetic divergence. Canopy nesting forms also have a lower realised fecundity than the ground nesting form on acacias from the same geographic area, but higher than another canopy nesting form from western Australia. Further observations on ecological and genetic traits are required to draw conclusions about the separation of lineages in the canopy nesting form ofO. lunifer, by including populations from other parts of the range for both the herbivore and the host plants.