Effect of plant essential oils on the efficacy of insecticides against diamondaback moth Plutella xylostella (L.)

Plant-derived products can play an important role in sustainable pest management programs. Essential oils from Lavender Lavandula angustifolia (Lamiales:Lamiaceae), rosemary Rosmarinus officinalis (Lamiales:Lamiaceae), and eucalyptus, Eucalyptus globulus (Myrtales:Myrtaceaa) were evaluated for their insecticidal activity and synergistic potential in combination with synthetic insecticides against the diamondback moth, Plutella xylostella (L.). When applied alone at 1%, 2%, and 3% concentrations using the leaf-dip method, the essential oils exhibited negligible insecticidal activity against larval stages. However, in synergism assays, the toxicity of fipronil, imidacloprid, and flubendiamide was markedly enhanced by L. angustifolia and E. globulus. In contrast, while R. officinalis produced the strongest synergistic effect with flubendiamide, it antagonised the insecticidal activity of fipronil. Notably, E. globulus reduced the efficacy of imidacloprid at lower concentration but exhibited synergistic effects at higher concentrations, indicating a non-linear concentration-dependent interaction. Overall, these results demonstrate that some plant essential oils can function as effective insecticide synergists,