The response of eukaryotic cells to ionising radiation (IR)-induced double-strand DNA breaks is highly conserved and involves a DNA repair mechanism characterised by the early phosphorylation of histone protein H2AX (producing the active form γH2AX). Although the expression of an induced γH2AX variant has been detected in Drosophila melanogaster, the expression and radiation response of a γH2AX homologue has not been reported in economically important fruit flies. We use Bactrocera tryoni (Diptera: Tephritidae, Queensland fruit fly or 'Q-fly') to investigate this response with a view to developing molecular assays to detect/quantify exposure of fruit flies to IR and consequent DNA damage. Deep sequencing confirmed the presence of a H2AX homologue that we have termed H2AvB (i.e. variant Bactrocera) and has an identical sequence to a histone reported from the human disease vector Glossina morsitans. A linear dose-response of γH2AvB (0-400 Gy IR) was observed in whole Q-fly pupal lysates 24h post-IR and was detected at doses as low as 20 Gy. γH2AvB signal peaked at ~20min after IR exposure and at 24h post-IR the signal remained elevated but declined significantly by 5 days. Persistent and dose-dependent γH2AvB signal could be detected and quantified either by western blot or by laser scanning cytometry up to 17 days post-IR exposure in histone extracts or isolated nuclei from adult Q-flies (irradiated as pupae). We conclude that IR exposure in Q-fly leads to persistent γH2AvB signals (over a period of days) that can easily be detected by western blot or quantitative immunofluorescence techniques. These approaches have potential as the basis for assays for detection and quantification of prior IR exposure in pest fruit flies.