Brindley’s Glands Volatilome of the Predator Zelus renardii Interacting with Xylella Vectors

SIMPLE SUMMARY: Zelus renardii is 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 renardii adapted to preying on vicarious indigenous species, including Philaenus spumarius (Hemiptera: Aphrophoridae), the European vector of Xylella fastidiosa. Reduviidae has several thoracic glands, pair or unpair, whose secretions may regulate adult insect behaviours, such as defence, alarm, and mating. Zelus renardii also 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 renardii reduces the production of alarm pheromones and the chance of being detected by prey interacting with P. spumarius. Alternatively, the alarm pheromone could help the predator to mark its territory, avoiding interaction with a conspecific. Evidence of the ability of Philaenus spumarius to perceive and react to the predator’s semiochemical would provide a further means to manage transmission and infection by Xylella fastidiosa. ABSTRACT: Alien 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 fastidiosa Wells, Raju et al., 1986 (XF) represents Italy’s and Europe’s most recent biological invasion. In Apulia (southern Italy), the XF-encountered Philaenus spumarius L. 1758 (Spittlebugs, Hemiptera: Auchenorrhyncha) can acquire and transmit the bacterium to Olea europaea L., 1753. The management of XF invasion involves various transmission control means, including inundative biological control using Zelus renardii (ZR) Kolenati, 1856 (Hemiptera: Reduviidae). ZR is an alien stenophagous predator of Xylella vectors, recently entered from the Nearctic and acclimated in Europe. Zelus spp. 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 with P. spumarius. The ZR volatilome includes 2-methyl-propanoic acid, 2-methyl-butanoic acid, and 3-methyl-1-butanol, which are consistent for Z. renardii alone. Olfactometric tests show that these three VOCs, individually tested, generate an avoidance (alarm) response in Z. 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 with P. spumarius. We discuss the potential effects of VOC secretions on the interaction of Z. renardii with P. spumarius.