Eucalyptus plantations have suffered severe damage from scarab grubs in recent years. To investigate the actual scarab species that damage Eucalyptus trees, continuous closed-net monitoring and monthly soil-digging surveys were conducted in Eucalyptus plantations in Lancang County, China, from 2024 to 2025. The primarily affected roots were covered with nylon mesh bags until the insects reached adulthood. A few adults were successfully collected from the damaged roots. The scarab species that infests Eucalyptus trees has been identified as Megistophylla grandicornis (Fairmaire, 1891). It exhibited a single generation annually in local Eucalyptus plantations. Adults emerge from late April to June, and larvae cause damage from July to November. Eucalyptus trees with severely damaged roots exhibit reduced growth vigor and are highly prone to windthrow and death, leading to substantial losses in forestry production. These preliminary results provide foundational data for recognizing Megistophylla grandicornis as a new root pest of Eucalyptus and establishing targeted larval-monitoring protocols in Eucalyptus plantations.