Background: The obliquebanded leafroller, Choristoneura rosaceana Harris, and the cabbage looper, Trichoplusia ni Hübner, are serious fruit and vegetable pests requiring multiple insecticide applications per year. To reduce non-target exposure to hazardous insecticides and to curb resistance development, alternative controls are required. Accordingly, a selection of 17 essential oils was screened against both lepidopteran pests, and the influence of azinphos-methyl resistance on essential oil toxicity to C. rosaceana was studied.
Results: Of the 17 essential oils screened, patchouli oil (Pogostemon cablin Benth.) and thyme oil (Thymus vulgaris L.) were selected for further testing against C. rosaceana, whereas patchouli oil, garlic oil (Allium sativum L.) and lemongrass oil (Cymopogon nardus L.) were selected for further testing against T. ni. LC(50) and LD(50) values confirmed that patchouli oil was the most toxic to C. rosaceana larvae, with LC(50) = 2.8 µL mL(-1) and LD(50) = 8.0 µg insect(-1). Garlic oil was the most toxic oil to T. ni larvae with LC(50) = 3.3 µL mL(-1) and LD(50) = 22.7 µg insect(-1), followed by patchouli oil and lemongrass oil. Azinphos-methyl-resistant leafrollers were 1.5-fold more tolerant to patchouli oil and 2.0-fold more tolerant to thyme oil.
Conclusions: Based on these results, patchouli oil and other essential oils have sufficient efficacy to be considered as components of an essential oil-based insecticide that targets these lepidopteran pests.