Carlina oxide as a new active ingredient for lure and kill against Bactrocera oleae? Insecticidal activity and electrophysiological insights

The olive fruit fly, Bactrocera oleae (Rossi) (Diptera: Tephritidae) is a key pest of olive groves worldwide. Recently, a growing research interest about eco-friendly pest management tools has been raised. Developing green pesticides from natural products, including botanicals is a fast-growing research area, contributing to reduce the use of synthetic pesticides, with beneficial effects on human health and the environment. In this scenario, essential oils can represent useful sources of compounds with multiple modes of action, making the development of resistance in targeted pests unlikely. In this scenario, herein we investigated the essential oil (EO) obtained from the roots of Carlina acaulis L. (Asteraceae) and the carlina oxide, which represents the major component of the EO as novel potential active ingredients for lure and kill formulations against the olive fruit fly. The EO was obtained by hydrodistillation of the powdered roots by using a Clevenger apparatus for 6 h. The EO chemical composition was achieved by GC-MS and NMR highlighting the presence of the following compounds: carlina oxide (94.6%), benzaldehyde (3.1%), ar-curcumene (0.4%), acetophenone (traces), benzyl methyl ketone (traces), camphor (traces) and carvone (traces). Carlina oxide was purified by silica gel column chromatography and its structure elucidated by 1H- and 13C-NMR spectroscopic analysis. The toxicity of the EO and pure carlina oxide on olive fruit fly adults was tested through ingestion tests carried out incorporating them in protein baits. Eight and nine concentrations, ranging from 156 and 39 to 10,000 ppm, respectively, were used for EO and carlina oxide, and compared with a control containing only the protein substrate. Both C. acaulis EO and carlina oxide showed insecticidal activity against the olive fruit fly. A significant dose dependent effect of both EO (F9,49=65.29, p<0.001) and oxide (F8,44=47.44, p<0.001) was observed. The LC50 was 706 ppm for C. acaulis EO, and 1052 ppm for carlina oxide. Furthermore, to shed light on the potential behavioural effects of the EO and carlina oxide to olive fruit flies, electroantennographic (EAG) tests were carried out to evaluate the capability of the male and female antennae to perceive them. In the dose range of 0.01 to 1000 µg, dose-dependent EAG responses, similar in males and females, were elicited by EO (0.03-5.67 mV) and carlina oxide (0.07-2.11 mV) indicating a strong antennal sensitivity to both stimuli. Behavioral tests investigating the impact of being exposed to EO and carlina oxide subletal doses on B. oleae adults are also ongoing. Overall, this contribution sheds light on the possible utilization of carlina oxide for the development of effective eco-friendly lure and kill formulations. However, further studies are needed on the mode of action and field effectiveness, as well as on the side-effects on non-target species.