Infestation of the gall midge Dasineura oleae provides first evidence of induced plant volatiles in olive leaves

In this study, we present the first characterization of herbivore-induced plant volatiles (HIPVs) released from infested olive leaves. The gall midge Dasineura oleae is a specific pest of Olea europaea and endemic of the Mediterranean Basin, an area in which severe outbreaks currently occurred. Little is known about the damage caused by the pest and the relationship with its host. Since gall formation and larval feeding activity may lead to the release of specific plant volatile compounds, we investigated the volatile profiles emitted from infested plants compared with healthy plants under both laboratory and field conditions. Additionally, the volatiles emitted from mechanically damaged plants were considered. A blend of 12 volatiles was emitted from olive trees infested by D. oleae. Of these, beta-copaene, beta-ocimene, cosmene, unknown 1 and unknown 3 were found to be exclusively emitted in infested plants. The emission of germacrene-D, (E,E)-alpha-farnesene, and (Z,E)-alpha-farnesene, alpha-copaene, (E)-4,8-dimethylnona-1,3,7-triene, (E)-beta-guaiene and heptadecane significantly increased in infested trees. Linalool, beta-copaen-4-alpha-ol, beta-bourbonene, beta-cubebene, beta-elemene, beta-copaene and delta-amorphene were found only in the field trial and showed differences depending on the level of infestation and the plant stage. (Z)-3-Hexenol, (E)-4-oxohen-2-enal, and 2-(2-butoxyethoxy)-ethanol, were exclusively emitted from the leaves after mechanical damage. In a field trial in Italy, we also demonstrated spring synchronization between adults of D. oleae and O. europaea trees. Analyses of morphoanatomical malformations of gall leaves showed that tissue alterations occur at the spongy parenchyma causing an increase of the leaf blade thickness. We speculate that tissue alterations may lead to HIPV release, in turn potentially attracting D. oleae natural enemies.

Automated summary

This study characterized the herbivore-induced plant volatiles released from infested olive leaves for the first time. The gall midge D. oleae, a specific pest of O. europaea, was found to cause specific plant volatile compounds to be emitted from infested plants. The study also showed increased emission of certain volatile compounds in infested trees, with potential implications for attracting natural enemies of D. oleae.