Structural characterization of carboxyatractyloside and acaricidal activity of natural ent-kaurene diterpenoids isolated from Chamaeleon gummifer against Tetranychus urticae

Plant-borne secondary metabolites are attracting high interest for their potential use in agricultural applications, with special reference to the control of arthropod pests. In the present work, the structural elucidation of glycosylated diterpenoid carboxyatractyloside (2) isolated from the roots of Chamaeleon gummifer Cass. (Asteraceae) is reported by means of spectroscopic and spectrometric techniques. Complete identification occurred thanks to one- and two-dimensional NMR experiments, assigning the single protons and carbons, and the stereochemistry by the NOESY correlations. Carboxyatractyloside (2), together with two ent-kaurenes atractyloside (1) and atractyligenin (3), extracted from the roots of C. gummifer, have been tested for their acaricidal and oviposition inhibition activity against the two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae) Notably, compounds 1–3 were toxic to T. urticae, leading to significant mortality, oviposition inhibition, reduced hatchability of eggs, and natality inhibition. However, at the lowest dose (12.5 µg cm−2) compound 2 was the most effective, leading to mortality > 60% after 5 days exposure, inhibiting oviposition by > 70% and egg hatching by 33%; it also reduced natality by 80%. Overall, these compounds represent valuable candidates to develop novel acaricides for crop protection. Further research on how to develop stable formulations for field use, as well as on non-target effects of these compounds on pollinators and mite biocontrol agents, is ongoing.