Stable isotopes for reliable identification of wild and mass-reared Queensland fruit flies in sterile insect technique programs

Queensland fruit fly is one of the most economically important horticultural pests in Australia. Sterile insect technique (SIT) is now being reconsidered and upscaled to combat this pest so reliable discrimination of released sterile Q-flies from wild flies in monitoring traps is important for effective SIT operations. Stable isotopes provide a permanent chemical marker to discriminate sterile and wild flies when dye marking is unclear. In this study, we compared the isotopic ratios of carbon and nitrogen between Q-flies reared on different larval diets and wild flies collected from diverse locations in Australia and New Caledonia. Finally, we conducted a release-recapture study to corroborate differences in stable isotope C and N ratios in laboratory-reared and wild Q-flies. The delta N-15 values obtained from wild and laboratory Q-flies showed high variability that is likely related to the food source of the larval and/or adult stage and do not offer an effective means to discriminate between sterile and wild Q-flies. The delta C-13 values of examined wild Q-flies ranged from – 27.46 to – 24.37 parts per thousand VPDB, whereas those from laboratory-reared, released and recaptured Q-flies ranged from – 25.73 to – 19.26 parts per thousand VPDB. Differences in delta C-13 values resulted in 100% correct classification of wild flies and 96.88% correct classification of released flies. Measurements of intrinsic delta C-13 values offer a precise tool to discriminate between sterile and wild Q-flies in SIT programs, regardless of the composition of the larval or adult pre-release diets.

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