Simple SummaryZelus renardiiis one of the alien insects recently acclimatised to the Mediterranean basin. The trivial name “leafhopper assassin bug” preludes the reduviid prey preference. In the Mediterranean,Zelus renardiiadapted to preying on vicarious indigenous species, includingPhilaenus spumarius(Hemiptera: Aphrophoridae), the European vector ofXylella fastidiosa. Reduviidae has several thoracic glands, pair or unpair, whose secretions may regulate adult insect behaviours, such as defence, alarm, and mating.Zelus renardiialso possesses a pair of Brindley’s glands, each consisting of about one hundred elements secerning in a reservoir with an outlet that opens at the thoracoabdominal limit. Stressful events elicit the production and secretion of a semiochemical bouquet, acting as alarm pheromones. This bouquet comprises 2-methyl-propanoic acid, 2-methyl-butanoic acid and 3-methyl-1-butanol as significant components, effectively repel conspecifics and suggest the role of Brindley’s glands as alarm pheromone foci.Zelus renardiireduces the production of alarm pheromones and the chance of being detected by prey interacting withP. spumarius. Alternatively, the alarm pheromone could help the predator to mark its territory, avoiding interaction with a conspecific. Evidence of the ability ofPhilaenus spumariusto perceive and react to the predator’s semiochemical would provide a further means to manage transmission and infection byXylella fastidiosa.AbstractAlien species must adapt to new biogeographical regions to acclimatise and survive. We consider a species to have become invasive if it establishes negative interactions after acclimatisation.Xylella fastidiosaWells, Raju et al., 1986 (XF) represents Italy’s and Europe’s most recent biological invasion. In Apulia (southern Italy), the XF-encounteredPhilaenus spumariusL. 1758 (Spittlebugs, Hemiptera: Auchenorrhyncha) can acquire and transmit the bacterium toOlea europaeaL., 1753. The management of XF invasion involves various transmission control means, including inundative biological control usingZelus renardii(ZR) Kolenati, 1856 (Hemiptera: Reduviidae). ZR is an alien stenophagous predator ofXylellavectors, recently entered from the Nearctic and acclimated in Europe.Zelusspp. can secrete semiochemicals during interactions with conspecifics and prey, including volatile organic compounds (VOCs) that elicit conspecific defence behavioural responses. Our study describes ZR Brindley’s glands, present in males and females of ZR, which can produce semiochemicals, eliciting conspecific behavioural responses. We scrutinised ZR secretion alone or interacting withP. spumarius. The ZR volatilome includes 2-methyl-propanoic acid, 2-methyl-butanoic acid, and 3-methyl-1-butanol, which are consistent forZ. renardiialone. Olfactometric tests show that these three VOCs, individually tested, generate an avoidance (alarm) response inZ. renardii.3-Methyl-1-butanol elicited the highest significant repellence, followed by 2-methyl-butanoic and 2-methyl-propanoic acids. The concentrations of the VOCs of ZR decrease during the interaction withP. spumarius. We discuss the potential effects of VOC secretions on the interaction ofZ. renardiiwithP. spumarius.