Toxicity of essential oil compounds against Exorista sorbillans (Diptera: Tachinidae), a parasitoid of silkworm
Essential oils of Ageratum conyzoides and Ocimum species are potential candidates for management of Exorista sorbillans (Wiedemann) (Diptera: Culicidae), a serious pest of silkworm. Considering that the pure compounds in essential oil may exhibit efficacy against the parasitoid, contact and topical toxicity of 22 essential oil compounds found in plants like A. conyzoides, different chemotypes of Ocimum species, Cymbopogon citratus and Eucalyptus globulus were assessed against Exorista sorbillans. The compounds under study were terpenes and coumarin. Most of the compounds were toxic to E. sorbillans but their toxicity varied with the bioassay method. The LC50 values of 11 compounds effective as contact fumigant, namely citral, geraniol, nerol, carvacrol, eugenol, isoeugenol, l-citronellol, carvone, coumarin, α-caryophyllene and terpinen-4-ol were 110.23, 124.11, 135.75, 194.53, 207.25, 266.40, 267.24, 299.23, 364.70, 533.02 and 1213.43 ppm, respectively. Topical application showed a different order of toxicity of the compounds. The compounds, α-pinene, 1,8-cineole, γ-terpinene, β-pinene, S-limonene and D-limonene which did not possess much efficacy as contact-fumigant were highly effective on topical application. We recommend the compounds found effective in this study for further studies to develop viable control agent against E. sorbillans, which is lacking at present.
Key words: Essential oil, monoterpenes, coumarin, Exorista sorbillans, silkworm.