The objective of the current study was to determine the fumigant toxicity of Thuja essential oil against 3rd and 5th instar larvae of the greater wax moth, Galleria mellonella. Thuja essential oil was extracted from unripe fruits of Thuja orientalis plant by water distillation. The mortality of larvae at the end of larval period, pupation and adult emergence were tested at different concentrations of Thuja oil after a 24h exposure period ranging from 5 to 25µL for 3rd instar larvae and 80µL for 5^th instar. The combined effect of Thuja essential oil and larvae resulting from parents irradiated as pupae of G.mellonella with 80Gy was investigated. The results demonstrated that the mortality increased by increasing oil concentration. The percent mortality of 3rd instar larvae reached 100% when exposed to a higher concentration of Thuja oil (25µL) after 24h, while in the case of 5th instar larvae the mortality was 73.3% at (80µL). Larvae of both instars resulting from parents of G. mellonella irradiated as pupae with 80Gy and exposed to the same concentrations of Thuja oil vapor became more susceptible to the oil as a result of the combined effect of the two agents. The LC₅₀ and LC₉₀ values for each experiment were calculated. The tested plant oil prolonged the larval - pupal periods and reduced both pupation percentage of adult emergence.