Electroantennographic responses of the parasitoid Psyttalia concolor (Hymenoptera: Braconidae) to olive fruit volatiles

Abstract: The olive fruit fly, Bactrocera oleae, is a key pest of olives worldwide. The braconid parasitoid Psyttalia concolor has been used in Italy and other Mediterranean areas to manage olive fly populations, with patchy results. Therefore, it is crucial to elucidate the olfactory cues routing host location behaviour of this biocontrol agent. Herein, the electroantennographic (EAG) responses of adult P. concolor to 27 volatile compounds, including hydrocarbons, alcohols, aldehydes, ketones, and monoterpenes, previously identified from olive fruits were recorded. All tested compounds triggered EAG responses in both sexes, indicating that the olfactory receptor system of P. concolor is adapted to perceive a rather wide variety of olive fruit volatiles. As a general trend, EAG response profiles of P. concolor males and females were similar. However, significant sexual variations in the EAG amplitude to some compounds were found, indicating possible differences in the number of antennal sensilla and/or receptor neurons tuned to individual compounds, probably because of different ecological roles. The largest EAG amplitudes were evoked, in decreasing order, by salicylaldehyde, (E)-2-octenal, 1-heptanal, octanal, 3-hexanone and (E)-3-hexen-1-ol in males, and by salicylaldeyde, octanal, (E)-2-octenal, 1-heptanal, (E)-2-eptenal, nonanal, and 1-octanol in females. The weaker antennal stimulants were n-pentacosane, n-heptadecane and n-hexacosane in males, and β-pinene, α-pinene and n-tricosane in females. Short chain aliphatic aldehydes are generally emitted by plant tissues after herbivore damage (host-induced plant volatiles). Therefore, the high olfactory sensitivity of P. concolor males and females to volatile aldehydes reported in this study, strongly suggests a possible role of these compounds during host habitat location by the adult parasitoids.