Interspecific hybridization is a common phenomenon in plants, and it often causes substantial changes in phenotypic and physiological traits, resulting in various resistance abilities to herbivores and pathogens among hybrids. Compared to their parents, hybrids exhibit lower resistance, higher resistance, or intermediate resistance. Qualitative or quantitative changes in primary and secondary compounds in hybrids are important factors affecting their herbivore resistance. Hybrid breeding technology has been widely applied in the Eucalyptus industry, and achieved remarkable success worldwide in recent years. As Eucalyptus hybrids vary in their ability to resist herbivores, they provide an ideal system to investigate the mechanisms underlying herbivore resistance in hybrids. To elucidate the herbivore resistance variance mechanisms, two Eucalyptus hybrids, DH201-2 (E. grandis × E. tereticornis) and G9 (E. grandis × E. urophylla), and an important Eucalyptus pest, Leptocybe invasa, were chosen for study. The resistance of DH201-2, G9, and their pure parental species (E. grandis and E. urophylla, E. grandis and E. tereticornis, respectively) against L. invasa were compared. In parallel, comprehensive measurements were taken to assess differences in leaf traits (leaf thickness, water content, and specific leaf area), primary compounds (C, N, soluble sugar, and soluble protein), and secondary compounds (total phenolics and condensed tannins) among the two hybrids and their parental species. The results showed that gall number on DH201-2 was significantly more than on its two paternal species, while gall number on G9 was significantly less than on its two parental species. Leaf thickness of DH201-2 and G9 were similar to E. grandis, but their leaves were thinner than those of the other parental species. Leaf water content in DH201-2 was significantly higher than that in E. tereticornis but similar to that in E. grandis, whereas G9 contained significantly lower water content than its two parental species. Similarly, both the specific leaf area of DH201-2 and G9 were significantly larger than their parental species. Considering the primary compounds, DH201-2 contained significantly more soluble sugar and soluble protein than its parental species, and more N than E. tereticornis. Although the content of soluble protein was significantly higher in G9, soluble sugar and N content were similar and lower than those in its two parental species, respectively. Both total phenolics and condensed tannins in DH201-2 were significantly lower than those in its parental species; however, they were significantly higher in G9 than in its two parental species. Therefore, we conclude that compared with the respective parental species, DH201-2 is more susceptible and G9 is more resistant to L. invasa. Different expressions of nutritional substances that are closely related to the development of L. invasa (i.e., water content, soluble sugar, and N content), and secondary defensive substances such as total phenolics and condensed tannins in Eucalyptus hybrids, together generate variable resistance abilities to L. invasa. In the context of the wide development of Eucalyptus hybrid breeding and the gradual increase in Eucalyptus pests over the years, more research focusing on the resistance variance mechanisms underlying Eucalyptus hybrids is required. These studies could provide valuable scientific guidance not only for screening herbivore-resistant hybrids through crossbreeding but also for the sustainable development of future Eucalyptus plantations.