Comparison of Plant derived Oils as Insecticides against Lepidopteran Insect Pests

Supervisors: Dr Julia Bally & Dr Samanta Bolzan De Campos

The fall armyworm (FAW), Spodoptera frugiperda, and cotton bollworm (CBW), Helicoverpa armigera, are two polyphagous lepidopteran insect pests that cause millions of dollars of agricultural losses globally. Both pest species are invasive in Australia, CBW is widespread, having been introduced pre-1991, whereas FAW is less so, only being detected since 2020. Unsustainable pesticide practices have led to resistance to many synthetic insecticides in FAW and CBW.

Plants naturally develop chemicals which can have insecticidal effects, this project aims to investigate soy plant derived oils efficacy as pesticides against FAW and CBW on lettuce (Lactuca sativa var. longifolia). The oils’ insecticidal behaviour was measured using a feeding assay with lettuce leaves sprayed with different concentrations of diluted oils, (steposol and palmitic acid). Caterpillars were counted and weighed at intervals of two to three days to measure their growth and mortality. Caterpillars collected on days 4 and 9 had RNA extracted to measure whether the expression of stress related genes changed due to the oils in the caterpillars via qPCR.

The mortality rate at the highest concentration of steposol with the FAW was 74%, however in CBW the mortality was only 51%. With palmitic acid, similar differences between CBW and FAW were observed, with the mortality rate of FAW at 38% and the mortality rate of CBW at 26%. No significant trends were observed in the weight data collected; all standard deviations fell between each other. No trends were observed in the qPCR data due to a lack of detection in many of the wells.

In conclusion, steposol is more promising of the two as an insecticide against FAW and CBW, however more research should be dedicated to determining the optimal concentration and application.