Objective:To evaluate toxic activities of essential oils obtained fromRosmarinus officinalis andLavandula stoechas against the fourth larval instars of Orgyia trigotephras.Methods:A total of1200 larvae were divided into three groups 栺,栻,栿.Group 栺 was to investigate the effect of extracted essential oils from these aromatic plants as gastric disturbance.Bacillus thuringiensis and ethanol were used as control group.Group 栻 was used as contact action and Group 栿 was used as fumigant action.Decis and ethanol were used as control group.During the three experiments, the effect of essential oils on larvae was assessed.Results:The chemical composition of essential oils from two medicinal plants was determined and, their insecticidal effects on the fourth larval state ofOrgyia trigotephras were assessed.The two simples presented an insecticidal activity, neverthelessRosmarinus officinalis essential oil was less efficient compared toLavandula stoechasone are discussed.Conclusions:The relationship between the chemical composition and the biological activities is confirmed by the present findings.Therefore the potential uses of these essential oils as bioinsecticides can be considered as an altative to the use of synthetic products.