Olfactory responses of  Phortica variegata (Drosophilidae, Steganinae) the emerging vector of  Thelazia callipaeda (Rhabditida, Thelaziidae) to ecologically relevant volatiles

Background: Phortica variegata (Drosophilidae: Steganinae), native to Europe, has emerged as a major vector of ocular nematosis caused by Thelazia callipaeda (Rhabditida: Thelaziidae), following the nematode's introduction into Europe from its original habitat in Asia. Males of P. variegata transmit these nematodes through feeding on tears of mammals including wild and domestic carnivorous mammals (foxes, beech martens, wild cats and dogs), lagomorphs, and humans. Insect vectors strongly rely on volatile cues to identify suitable hosts. Due to the increasing veterinary and medical concerns, there is a growing need for attractants. Understanding the olfactory responses of P. variegatais crucial, as insect vectors rely on volatile cues to locate hosts. The identification of key attractants could facilitate the development of vector surveillance and control strategies. However, the olfactory ecology of this species remains unexplored, limiting our ability to design effective attractant-based interventions. Methods: We used gas chromatography coupled electroantennography to measure antennal responses to synthetic and natural volatile blends. A comparative analysis was performed on the antennal responses of both sexes of P. variegata and its well-studied relative, Drosophila melanogaster. Components of the synthetic blends were selected based on the odorant receptor repertoire of D. melanogaster and established mosquito attractants, with the rationale that conserved olfactory receptors among dipterans may allow P. variegata to detect similar compounds. Volatile extracts collected using active carbon adsorbent traps were also tested on the antennae and analysed using gas chromatography coupled mass spectrometry. Results: Males of P. variegatashowed higher antennal responses to phenol, 3-octanone, and sulcatone than females, indicating olfactory sexual dimorphism. Compared to D. melanogaster, the antennae of P. variegata did not respond to several common plant alcohols and terpenoids. Instead, they showed stronger responses to compounds such as anisole, ethyl propanoate, butyl propanoate, propyl acetate, 3-octanone, nonanal, and decanal, suggesting that peripheral olfaction in P. variegata may be more tuned to microbial volatiles. Conclusions: The antennal olfaction of P. variegata appears to be particularly tuned to microbial volatile emissions, suggesting that fungal and microbial substrates may play an important role in the life cycle of this species. Males show stronger relative responses to several compounds known to influence host-seeking behavior in other zoophilic dipterans, suggesting their potential as candidate attractants for future field studies.