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

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.

Keywords: CoDiRO; Harpactorini; OQDS; antifragility; infochemicals; leafhopper assassin bug.

Figure 1

Experimental setup for volatile collection from insects. Vials: (A) Zelus renardii alone; (B) Philaenus spumarius alone; (C) Z. renardii and P. spumarius separated by nets (created with BioRender.com, accessed on 4 April 2023). N.B.: for pictorial fold, PS is portrayed with 3× magnification.

Figure 2

Experimental setup of Xylella vector VOC production. Vials: (A) 1 P. spumarius; (B) 2 P. spumarius; (C) 5 P. spumarius; (D) 10 P. spumarius (created with BioRender.com, accessed on 4 April 2023).

Figure 3

Right partial view of ZR’s thorax and abdomen: 1 = pronotum, 2 = mesothoracic coxa, 3 = metathoracic coxa, 4 = second urite, 5 = third urite, 6 = right spiracle of the second urite, 7 = right spiracle of the first urite, 8 = first urite, 9 = hemelytron, 10 = mesothoracic flap, 11 = mesopleuron, 12 = patchy cuticle areas, 13 = meshwork evaporatorium, 14 = Brindley’s gland meshwork evaporatorium, 15 = Brindley’s gland reservoir outlet place, 16 = gutter (gouttière), 21 = abdominal finger; (a) Tessovar light macroscopy; (b) Cryo-SEM.

Figure 4

Details of right thorax and abdomen view: 10 = mesothoracic flap, 13 = meshwork evaporatorium, 14 = Brindley’s gland meshwork evaporatorium, 15 = Brindley’s gland reservoir outlet place, 16 = gutter (gouttière), 20 = secretions (?), 21 = abdominal finger; SEM.

Figure 5

Details of Brindley’s gland: 6 = right spiracle of the second urite, 14 = Brindley’s gland meshwork evaporatorium, 15 = Brindley’s gland reservoir outlet, 17 and 17b,c = trachea and tracheal branches over Brindley’s gland, 18 = Brindley’s gland reservoir, 19 = Brindley’s gland units’ cuticles; Phomi II light microscopy, SEM.

Figure 6

Brindley’s glands false-colour SEM: 6 = right spiracle of the second urite, 17 = trachea, 17a = sign of a broken tracheal branch, 17b, c = tracheal branches lying over Brindley’s gland, 18 = Brindley’s gland reservoir, 19 = Brindley’s gland units’ cuticles.

Figure 7

Venn diagram showing the volatiles shared between treatments. Zelus renardii (ZR), Philaenus spumarius (PS) and interaction between Z. renardii and P. spumarius (INT).

Figure 8

Amount of 2-methyl-butanoic acid, 2-methyl-propanoic acid, and 3-methyl-1-butanol in ZR alone (red box plots) and during ZR–PS interaction (blue box plots).

Figure 9

The final choice of Z. renardii when given a VOC (3-methyl-butanol, 2-methyl-butanoic acid, or 2-methyl-propanoic acid) or the control (empty—no VOC).

Figure 10

Average approaches of Z. renardii to VOCs: (A) 2-methyl-propanoic acid; (B) 2-methyl-butanoic acid; (C) 3-methyl-1-butanol. *** p-value < 0.001. Abbreviations: C = compound, E = empty.