Chemical Profiling, Insecticidal and Enzyme Inhibition Activities of <i>Rosmarinus officinalis</i> and <i>Ocimum sanctum</i> Against Aphid, Mealy Bug and Diamondback Moth

ABSTRACT Aphis craccivora Koch (Hemiptera: Aphididae), Planococcus lilacinus Cockrell (Hemiptera: Pseudococcidae) and Plutella xylostella L. (Lepidoptera: Plutellidae) are key pests of agricultural and horticultural crops. Indiscriminate use of insecticides led to resistance, harmful to beneficial insects and environment. Essential oils (EOs) are plant‐derived secondary metabolites that can be used to combat insect pests. In this work, chemical composition and insecticidal activities of EOs were screened against target pests for identification of lead(s). GC and GC–mass spectra (MS) analysis of EOs revealed that ocimene (36.4%) in Rosmarinus officinalis L. (Lamiaceae) and β‐caryophyllene (58.77%) in Ocimum sanctum L. (Lamiaceae) were major constituents. Combination of R. officinalis + O. sanctum (RO + OS) at 1:1, 3:1 and 1:3 ratios comprised eugenol (36.95%, 42.02% and 19.01%, respectively) as the main constituent. R. officinalis is most effective against A. craccivora and P. lilacinus (LD 50 = 0.97 and 2.02 µL/insect, respectively) after 96 h. O. sanctum was more effective against P . xylostella (LD 50 = 3.92 µL/insect). All the combinations of EOs displayed toxicity (LD 50 = 0.11–0.14 µL/insect), synergistic activity for A. craccivora and P. lilacinus , whereas P. xylostella showed antagonistic activity. Further RO + OS at 1:3 and 3:1 ratios also displayed high toxicity to P. lilacinus (LD 50 = 0.36–0.40 µL/insect) and P. xylostella (LD 50 = 1.36–1.49 µL/insect). EO of R. officinalis showed repellent (RC 50 = 2366.62 µL/L) against P. xylostella , deterrent activity of O. sanctum against A. craccivora (88%) and synergistic action against A. craccivora (Cotoxicity coefficient (CTC) = 640.67–1366.67) and P. lilacinus (CTC = 404–561.11). Both EOs showed inhibition of acetylcholinesterase (AChE), glutathione‐ S ‐transferase (GST), CES1 and mixed‐function oxidase (MFO) in test insects. The EO of RO and OS alone or in combination may be recommended for the control of target pests based on field bio‐efficacy studies.