Essential oil-based biopesticides from low-tech to high-tech formulations

Due to their biocidal properties, essential oils are increasingly used as biopesticides. The applications are numerous: insecticides, repellents, fungicides, bactericides, virucides, herbicides, nematicides, etc. Essential oils are obtained by hydrodistillation or steam distillation (except for citrus ones) of plant material (leaves, flowers, roots, bark, etc.). They are therefore made up of a very complex mixture, several thousand of compounds, of hydrophobic volatile compounds, mainly monoterpenes, sesquiterpenes and phenylpropanoids. Two major obstacles to the widespread use of essential oils as biopesticides are composition variability and volatility. While volatility offers the possibility of considerably reducing residues and thus reduce the impact on non-target organisms or human health, it can also be a major constraint to the development of essential oil biopesticides. It is in this context that we propose to outline different strategies for the encapsulation of essential oils in a view to protect essential oil from oxidation and allow a controlled and slow release. Emulsification, coacervation, spray drying, complexation, ionic gelation, nanoprecipitation, film hydration methods are among the techniques that can be used. From low-tech solutions adapted to southern countries to the latest biopolymer developments, we will outline the formulatons adapted to essential oils. Montmorillonite nanoclay based formulation for controlled and selective release of volatile essential oil compounds, development and characterization of alginate/montmorillonite hybrid microcapsules for encapsulation, calcium alginate and calcium alginate/montmorillonite hybrid microcapsules to encapsulate rosemary essential oil, cinnamon essential oil emulsion incorporated into chitosan gelatin to form complex films, Cinnamomum zeylanicum essential formulation with poly(propylene imine) dendrimers with Surface-Grafted Glycerol, essential oil-biopesticide direct trunk injection into trees can illustrate the various developments in the field of essential-oil based formulations for applications in agronomy. Finally, a general conclusion will present advantages and constraints linked to the use of essential-oil based biopesticides.