Comparison of the Toxicity of Mexican Oregano Oil from Two Locations on Epilachna varivestis Mulsant

Toxicity of essential oil of Mexican oregano (Lippia graveolens H.B.K), obtained by steam distillation, from the states of Chihuahua and Durango, Mexico was bioassayed against larvae of third and fourth instars of Epilachna varivestis Mulsant. Different concentrations (8, 30, 50, 70, and 100 mu g mu(-1)) of essential oil were tested using topical application administered by a microsyringe. Mortality was analyzed by Probit to obtain LD50 values. LD50 values obtained for essential oil were 38.5 mu g mu(-1) from Durango State and 26.15 mu g mu(-1) from Chihuahua State. Additionally, Homogeneity Regression coefficients were tested to determine significant differences among regression equations and confidence intervals obtained. Oregano oils from Chihuahua and Durango had no statistically significant differences, and there was a synergistic effect between the metabolites of the oils that increased the insecticidal effect.