The role of dilution mediums in studies of fumigant insecticidal activity of essential oils

The essential oils (EOs) are volatile plant extracts that are commonly considered as potential biopesticides. They present promising properties as relatively safe and potent insecticides, primarily delivered via fumigation. Such specificity of the EOs has led to the growing interest of researchers and, subsequently, to an increasing body of publications. The studies focusing on assessing the insecticidal action of EOs frequently require testing the effects of several dilutions of a given EO. To this end, researchers employ a range of solvents differing widely between the studies. While informative, the standardization concerning either tested oil or pest species is lacking in these studies. The presented study herein aimed to test whether the solvent used could affect the observed insecticidal activity of tested EO. As the model setup for studying the possible influence of solvents on the insecticidal effect of EOs, two EOs: mint and rosemary were chosen. The EOs were applied in fumigation assay against three economically important species of stored product pests: Callosobruchus maculatus , Sitophilus oryzae , and Tribolium castaneum . Each EO was tested in three concentrations—diluted with a range of solvents; dimethyl sulfoxide, acetone, methanol, ethanol, paraffin (mineral) oil, ultrapure water with 0.1% polysorbate 20 (TWEEN ® 20). The results confirm the hypothesis that the applied solvent could significantly alter the observed efficiency of tested EO. Such a result is of crucial value for conducting credible meta-analysis as well as for establishing a high standard of reproducibility.