Insecticidal effects of essential oils against cabbage looper, Trichoplusia ni (Lepidoptera: Noctuidae)

The cabbage looper, Trichoplusia ni is an important field and greenhouse pest in North America. It is pest of cruciferous plants but also attacks several other crops and causes a significant economic damage  in its widespread native range from Canada to Mexico. Owing to the fact that the cabbage looper has evolved resistance against many synthetic and microbial insecticides, there is a need to develop new insect control methods for this pest. We have investigated the insecticidal effects of four essential oils including black mustard - Brassica nigra (Brassicaceae), almond - Prunus dulcis (Rosaceae), sesame - Sesamum indicum (Pedaliaceae), narrow-leaved tea tree - Melaleuca alternifolia (Myrtaceae) against the cabbage looper. Cinnamon oil - Cinnamomum zeylandicum (Lauraceae) was used as a positive control. Contact toxicity effects were determined via topical application of different doses of essential oil solutions to the dorsum of third instar larvae. Mortality caused by the oils varied from 20-30% at 10 µg/larva. Growth inhibitory effects of the oils were determined by incorporating different concentrations of the oils in the artificial diet and allowing larvae to feed on it for ~ 6 days. Based on the EC₅₀ values (effective concentration causing 50% growth inhibition), mustard oil  (EC₅₀ = 371.9 ppm)  was twice more active than the positive control. Growth reduction demonstrated by some of these oils and increased development time may expose herbivores to increased mortality in the field as a result of biotic and abiotic factors. Some of these oils have potential to be used as part of the integrated  pest management plan.