Acute, sublethal and synergistic effects of some essential oil constituents against the Asian armyworm, spodoptera litura (lepidoptera: noctuidae) |

Public concerns over synthetic pesticides have stimulated the search for alternative methods of control. Plant essential oils are the odorous components and secondary metabolites that can be separated from other plant tissues through steam distillation. Generally regarded as plant defenses against herbivory, they have demonstrated acute toxicity to various insect species and are of interest for the development of 'ecologically friendly' pesticides. After identifying three main active ingredients in several samples of commercial fragrance oils against the Asian armyworm, Spodoptera litura, a number of related compounds were also tested for acute toxicity (measured as LD5 0). The most toxic was thymol (LD50=25.4 jj.g/larva). Compounds were then tested for sublethal effects. Growth inhibition after topical application and feeding deterrence using leaf-disc choice experiments were determined. An LD1 0 dose reduces growth by 20% on average three days after topical application, and the most deterrent compound to feeding was thymol, with a DC5 0 of 85.6 u.g/cm2 leaf disc area. Since minor constituents in complex essential oils are thought to act as synergists, binary mixtures of the compounds were tested for synergy vis a vis acute toxicity and feeding deterrence. Trans-anethole acts synergistically with thymol, citronellal and a-terpineol. Based on these findings, several complex mixtures were developed and tested as leads for effective control agents. Candidate mixtures demonstrated good synergistic effects. The observed LD5 0 of Mixture 3 was 40.6 u.g/larvae compared to an expected value of 74.6 (.ig/larvae. The result of this research is a proprietary product soon to enter commercial production.Land and Food Systems, Faculty ofGraduat