Bio-efficacy of Salvia officinalis essential oil, nanoemulsion and monoterpene components as eco-friendly green insecticides for controlling the granary weevil

Abstract

In the pursuit of searching for novel and environmentally benign insecticides to minimize the environmental risks of synthetic pesticides, essential oil (EO) of Salvia officinalis L., the so-called common sage, was hydrodistilled and analyzed using gas chromatography–flame ionization detection (GC–FID) and Gas chromatography-Mass spectrometry (GC–MS). The monoterpenes, α-thujone (26.8%), camphor (19.9%), and 1,8-cineole (13.4%) were identified as the key fractions. Oil-in-water nanoemulsion (NE) was developed adopting a green low energy method, then characterized for thermodynamic stability. The EO, the NE and monoterpenes showed insecticidal bioactivity against the granary weevil, Sitophilus granarius L. The NE caused the strongest contact bioactivity against larvae (LC50 =7.2 µL/cm2) after 72 h of exposure. At the same assaying conditions, EO (LC50 = 14.1 µL/cm2) and camphor (19.3 µL/cm2) exhibited promising bioactivities. For adults, the LC50's of NE, EO and camphor after 72 h of exposure were 5.8, 9.2, and 19.8 µL/cm2, respectively. Upon fumigation, the NE, EO and camphor were the most effective as fumigants, where LC50 values after 72 h larval exposure were 8.1, 11.0, and 14.0 µL/L, respectively, and for adult were 5.4, 9.6, and 12.1 µL/L, respectively. The phytochemicals effectively repelled the weevils at low concentrations, and considerably inhibited the activity of acetylcholinesterase (AChE). None-target evaluations proved that the phytochemicals and NE were safe for earthworm, Eisenia fetida (Savigny, 1826) at a limit concentration of 200 mg/kg soil, and the albino rat at an oral limit dose of 4500 mg/kg body weight, except for α-thujone. The test phytochemicals were non-phytotoxic to wheat grains at concentrations reached 150 µL/ml. Results provide evidence for a potentiality for using sage EO, particularly at its nanoscale, as ecofriendly grain protectant against the granary weevil.