Chemical constituents of essential oils from Thymus vulgaris and Cymbopogon citratus and their insecticidal potential against the tomato borer, Tuta absoluta (Lepidoptera: Gelechiidae)

This study aimed to determine the chemical composition and insecticidal activity of essential oils from Thymus vulgaris (thyme) and Cymbopogon citratus (lemongrass) against the devastating pest, Tuta absoluta. Essential oils were extracted by hydrodistillation and chemically analyzed by Gas chromatography coupled to mass spectrometry. Their knockdown (KD50) and larvicidal (LD50) activities were assessed through direct contact and fumigation toxicity assays. The major components of thyme oil were Thymol (21.53%), α-Pinene (Dextro) (17.43%) and o-Cymene (15.37%), while, Neral (34.48%), Geranial (34.37%) and β-Myrcene (12.84%) were prominent in lemongrass oil. Statistical analysis indicated that both oils exhibited similar knockdown and insecticidal efficiencies through direct contact and fumigation routes. The resultant biological parameters for lemongrass and thyme oils were KD50 values of 0.193 µL/mL and 0.592 µL/mL and LD50 values of 0.328 µL/mL and 0.608 µL/mL respectively for contact toxicity and KD50 of 1.290 µL/mL and 2.565 µL/mL and LD50 of 1.479 µL/mL and 3.046 µL/mL for lemongrass and thyme oils respectively for fumigant toxicity. The reference insecticide, Lynx (λ-cyhalothrin 15 g/L + acetamiprid 20 g/L) elicited no mortality through both routes after 4 hours of exposure. Both essential oils prolonged life cycle of insect pest through both routes as compared to the reference insecticide. However, greenhouse and open field trials are required to validate this approach prior to its implementation for T. absoluta control in tomato farms. To our knowledge, this study reports for the first time the insecticidal potential of lemongrass essential oil against T. absoluta.