The parasitoid Dolichogenidea gelechiidivoris eavesdrops on semiochemicals from its host Tuta absoluta and tomato
Semiochemicals such as herbivore-induced plant volatiles (HIPVs) and host chemicals serve as communication signals for parasitoids searching for oviposition sites. The braconid koinobiont endoparasitoid Dolichogenidea gelechiidivoris (Hymenoptera: Braconidae) efficiently parasitises larvae of Tuta absoluta (Lepidoptera: Gelechiidae), a major pest of tomato (Solanum lycopersicum). However, the attractive compounds used by the parasitoid to locate T. absoluta on host plants are not known. We therefore performed behavioural assays and chemical analyses to investigate the chemical basis of interactions between the parasitoid, the tomato plant and T. absoluta. Y-tube olfactometer bioassays revealed that D. gelechiidivoris was attracted to T. absoluta larvae-infested tomato plant volatiles and preferred volatiles of plants with a high infestation level than those with a low infestation level. The parasitoid was also attracted to volatiles of larval frass and to the sex pheromone of T. absoluta. Coupled gas chromatography–mass spectrometric analyses were performed on plant and frass volatiles. We found both qualitative and quantitative differences in volatile emission between healthy and T. absoluta larvae-infested tomato plants, where volatile emission rate increased with increasing infestation level. The most characteristic volatile compounds which distinguished T. absoluta larvae-infested plants from healthy plants were α-pinene, sabinene, β-myrcene, 2-carene, α-phellandrene, 3-carene, α-terpinene, β-phellandrene, (Z)-β-ocimene, (E)-β-ocimene, allo-ocimene, (E)-β-caryophyllene and methyl salicylate. With the exception of caryophyllene oxide, all larval frass volatile compounds were also found in tomato plant headspace volatiles. Olfactometer bioassays using synthetic compounds revealed that D. gelechiidivoris was attracted to α-pinene, β-myrcene, α-phellandrene, α-terpinene, β-ocimene, methyl salicylate and (E)-β-caryophyllene, and the 7-component blend of these attractants elicited the greatest attraction in the parasitoid. These findings open new avenues for exploiting these attractants as kairomone-based lures to recruit and retain the parasitoid in tomato fields for the biological control of T. absoluta.