Diomus guilavoguii Duverger, 1994, an adventive species, is recorded from Guangzhou (Guangdong Province), China for the first time. Larvae of D. guilavoguii were collected in association with an invasive mealybug, Paracoccus marginatus Williams & Granara de Willink, 1992, infesting papayas, cassava and several ornamental plants. However, little has been known about the biology of D. guilavoguii, especially the morphology of their larvae since their original descriptions. Diomus guilavoguii Duverger, 1994, native to Conakry, Guinea (Africa), is recorded as established in Guangdong Province for the first time. However, it is unclear when and how D. guilavoguii spread from Africa to Guangzhou, Guangdong Province. Both the adult and larva feed on the invasive mealybug Paracoccus marginatus Williams & Granara de Willink (Hemiptera, Pseudococcidae) that infests papaya and ornamental plants. In this paper, the external morphology and male genitalia of adults are re-described. The detailed descriptions of larva and pupa are also provided for the first time. The status of D. guilavoguii and D. hennessyi Fürsch, 1987 are discussed.