The sterile insect technique (SIT) as an eco-friendly and reliable strategy has 
been used to control populations of insect pests of agricultural, veterinary and 
human health importance. Successful applications of SIT rely on the high-level 
ecological fitness of sterile males. A suitable and stable gut microbiome can 
contribute to the ecological fitness of insect by influencing their 
physiology, biochemistry and development processes. Here, we show that a shift 
in the gut bacterial composition and structure by sterilizing irradiation, 
characterized by a decrease in the major gut microbiota community 
Enterobacteriaceae, an expansion of the minor members (e.g., Bacillaceae) and a 
higher richness and diversity, is tightly linked to radiation-induced ecological 
fitness (male mating competitiveness, flight capacity, survival rate and life 
span) decline in Bactrocera dorsalis (Hendel) sterile males. Function prediction 
of gut microbiota indicated that changes in microbiome taxonomy tend to drive 
microbiome functional shifts. A higher nutrient consumption of the flourishing 
minor gut microbiota may cause a decline in nutrients and energy metabolic 
activity of host and then result in the reduced ecological fitness of irradiated 
flies. Furthermore, we found that a gut bacterial strain Klebsiella oxytoca 
(BD177) can restore ecological fitness by improving food intake and increasing 
haemolymph sugar and amino acid levels of irradiated B. dorsalis flies. Our 
findings suggest that gut symbiont-based probiotics can be used as agents for 
reversing radiation-induced ecological fitness decrease.