## 3 Discussion
Therefore, similar synergistic effects can explain the lower toxicity of individual compounds with respect to \(O\). _vulgare_ leaves. Despite the known effects of thymol as an insecticide [49; 50; 51], no significant effects were observed in our study, probably because of the very low amount of thymol produced by the \(O\). _vulgare_ chemotype used in this study. The inhibition of larval growth was mainly due to the presence of \(\gamma\)-terpinene, the biosynthetic precursor of carvacrol [8; 52], whereas carvacrol and \(p\)-cymene did not affect significantly the larval growth. Although the topical application of a sub-lethal concentrations of carvacrol to \(S\). _littoralis_ has been demonstrated to exert a significant delay of larval development [16], feeding on carvacrol was found to be more tolerated by the insect, possibly by a different biodegradation/detoxification of this compound in the insect's gut. Since the growth rate, development lifespan, final body weightand survival percentages are strongly affected by the food intake and by the nutritional value of ingested food [53], it is conceivable that the larvae growth delay and reduced survival observed upon feeding on _O. vulgare_ leaves might be correlated to both the deterrent and anti-nutritional effect of oregano terpenoids. Although the naive larvae preferred the ACD during the choice tests, the presence of leaves as alternative food source appears to stimulate a higher feeding rate. In fact, larvae that experienced feeding on leaves selectively preferred _O. vulgare_ leaves as nutritive source during the choice test. Our results also indicate a possible attractant effect mainly mediated by carvacrol and \(\gamma\)-terpinene [54; 55]. Our data suggest that the continuous feeding on _O. vulgare_ leaves significantly impairs the larvae growth (weight and size) and that this anti-nutritional effect is mainly mediated by \(\gamma\)-terpinene. _S. litura_ larvae are able to detoxify terpen, including \(\gamma\)-terpinene [56; 57; 58]. The absence of chemical derivatives of _O. vulgare_ terpen and the assimilation of more than 50% of ingested terpen by _S. littoralis_, as evidenced by our insect's frass analyses, show that _S. littoralis_ catabolizes the plant terpenes and shows a limited ability to biottransform these molecules, in contrast to the typical behavior of specialist herbivores [59]. However, the catabolism of _O. vulgare_ terpen can also produce non-volatile derivatives which might have not been detected by our GC-MS and GC-FID analyses. Despite the absence of catabolic products, the insect response to ingestion of _O. vulgare_ leaves was a significant increase of all tested enzyme activities. This increase was correlated to the presence of some _O. vulgare_ monoterpenes only for CAT and SOD, whereas the increased GST activity was probably dependent on other leaf constituents or by the synergistic effect of the plant terpenoid blend. Increased oxidative stress is common in herbivore insects feeding on plants producing toxic allelochemicals. Insects like _S. littoralis_ possess a suite of antioxidant enzymes such as CAT and SOD for protection against oxidative stress [60]. Insects also possess a GST which is effective in targeting hydroperoxides [61] and some terpenoid have the ability to modulate the activity of this enzyme [21]. For all tested enzymes and genes, there was a positive correlation between insect enzyme activity and gene expression upon feeding on leaves. In _S. littoralis_, a significant up-regulation of _SOD_, _CAT_ and _GST_ was shown upon feeding on a diet containing potato extracts showing that increased concentrations of antioxidants represent an herbivore defense against exogenous oxidative stress [62]. In agreement with the general hypothesis that increased oxidative stress may lead to a modulation of genes coding for antioxidant enzymes [62], we also found a positive correlation between SOD and CAT activity and gene expression upon feeding on selected monoterpenes.