Insecticidal Activities of Commercial Rosemary Oils (<i>Rosmarinus officinalis</i>.) Against Larvae of<i>Pseudaletia unipuncta</i>. and<i>Trichoplusia ni</i>. in Relation to Their Chemical Compositions

Rosemary [Rosmarinus officinalis. L. (Lamiaceae)] essential oil has insecticidal properties and is the active ingredient in a number of commercial insecticides. Like other plant essential oils, the chemical composition of rosemary oil can vary based on genotype, geography, climate, and method of preparation. This study explored the relationship between chemical composition and insecticidal activity of 10 commercial samples of rosemary oil, based on laboratory bioassays with two agricultural pests, the armyworm Pseudaletia unipuncta. Haworth (Noctuidae) and the cabbage looper Trichoplusia ni. Hübner (Noctuidae). Nine major terpenoid constituents of rosemary oil were quantified in the samples by gas chromatography–mass spectrometry (GC-MS). The major constituents were 1,8-cineole, α-pinene, β-pinene, and camphor; on average 1,8-cineole made up 52% of the oil by weight. Bioassayed individually, camphor was the most toxic compound to the armyworm (LD50 = 189.4 μ g larva− 1) whereas μ-terpineol was the most toxic to the looper (LD50 = 128.5 μ g larva− 1). LD50 values for the ten rosemary oils ranged from 167.1 to 372.1 μ g larva− 1in the armyworm and from 58.9 to 335.9 μ g larva− 1 in the looper. Correlation analysis comparing rosemary oil toxicity to chemical composition revealed slight but significant correlation for d.-limonene and α-terpineol in bioassays with the looper but no correlations between constituents and toxicity with the armyworm. An “artificial” rosemary oil was prepared by mixing the nine major constituents in proportions reflecting the average proportion of each constituent in the 10 commercial oils. In the armyworm, this “artificial” oil was significantly less toxic than five of the intact oils; in the looper, the artificial oil was less toxic than three of the intact oils. Our results suggest that toxicity of rosemary oil, at least to lepidopteran larvae, is a consequence of the combined (and possibly synergistic) effects of several chemical constituents, with no individual compound making a dominating contribution.