The scuttle fly, Megaselia scalaris is an omnivorous species, capable of exploring a large variety of environments and ecological niches. It is known as an important detritivore species with maggots feeding on a variety of food of both animal and plant origin. M. scalaris was investigated as a severely-infesting pupal-adult parasitoid on the laboratory mass culture of the two tephritid species of the peach fruit fly, Bactrocera zonata (Saunders) and the Mediterranean fruit fly, Ceratitis capitata (Wiedemann) reared in laboratory of Horticultural Insects Research Department, Plant Protection Research Institute, Agricultural Research Center, Egypt. Pupae of the two species of fruit flies resulting from experiments to evaluate efficiency of certain essential oils (Clove, Lavender and Neem) as stomach poison against larval instars in treated artificial diet were separately collected till emergence. Three days post emergence, all emerged flies were investigated dead with empty body cavities and huge numbers of strange larvae and pupae were observed inside their abdominal cavity which were identified as, M. scalaris, Eurytoma martellii Domenichini, 1960 (Hymenoptera: Eurytomatidae) and Drosophila hydei Sturtevant,1921 (Diptera: Drosophilidae). The first species was major predominant, whereas the latest two species were rare. Thus, it is concluded that these species especially M. scalaris may form a threat factor for laboratory mass production of many species of insects such as tephritid fruit flies.