Phytol, (E)-nerolidol and spathulenol from Stevia rebaudiana leaf essential oil as effective and eco-friendly botanical insecticides against Metopolophium dirhodum

Stevia rebaudiana (Asteraceae) is a medicinal plant of economic importance in the food market for the manufacture of natural sweeteners, namely steviol glycosides. The plant biomass used on an industrial level may also be the source of an essential oil (EO) of potential interest for developing novel insecticides. Here, the leaf EO chemical composition of S. rebaudiana growing in central Italy was analysed by gas chromatography-mass spectrometry (GC-MS). The EO insecticidal efficacy was evaluated against the aphid Metopolophium dirhodum (Hemiptera: Aphididae), a major pest of cereals. The EO composition was dominated by sesquiterpenes, i.e. caryophyllene oxide (20.7 %), spathulenol (14.9 %) and (E)-nerolidol (8.0 %), and diterpenes, i.e. phytol (9.2 %). The EO was effective against M. dirhodum aphids, showing an LC 50(90) of 5.1 and 10.8 mL L-1, respectively. The efficacy of the EO major constituents, namely caryophyllene oxide, spathulenol, (E)-nerolidol and phytol was also tested against M. dirhodum. Phytol was the most effective aphicide (LC50(90) = 1.4(4.2) mL L-1), followed by (E)-nerolidol (LC50(90) = 3.5(9.3) mL L-1) and spathulenol (LC50(90) = 4.3(7.5) mL L-1). The EO was minimally toxic towards non-target Eisenia fetida adults and Harmonia axyridis larvae and adults. Overall, phytol, (E)-nerolidol and spathulenol can be considered further for developing effective and eco-friendly green insecticides against aphids.