## 3 Discussion  
Terpenoids stored in the secretory structures of aromatic plants act as direct defense against herbivores, particularly against generalist insects [13; 28] and despite their constitutive nature, terpenoids can be modulated by herbivory [29; 30; 31; 32]. In this work, we showed that feeding \(S\). _littoralis_ on \(O\). _vulgare_ leaves caused both the chemical and genetic modulation of the plant terpenoids. However, this modulation was mainly quantitative, because no qualitative changes were found in the terpenoids, with respect to control plants. In the interaction between the specialist _Chrisolina herbacea_ and its host plant _Mentha acquatica_, terpenoid qualitative changes were observed upon herbivory [29]. Therefore, our results confirm the hypothesis that generalist and specialist herbivores may activate different plant responses [27]. The feeding activity of the generalist \(S\). _littoralis_ increased the expression of \(O\). _vulgare_\(DXS\), a gene involved in the early steps of terpenoid biosynthesis for the mevalonate-independent (MEP)-pathway gene, the product of which is considered to catalyze one of the rate-limiting steps of this pathway [33] as well as _GPPS_, whose overproduction is expected to result in increased production of monoterene end products [34]. Upregulation of both \(DXS\) and _GPPS_ correlated with the increased content of terpenoids upon herbivory. It is interesting to note that the specialist \(C\). _herthaca_ was unable to regulate the expression of \(M\). _aquatica_\(DXS\)[29]. Of particular relevance was the herbivore-induced upregulation of \(\gamma\)-terpinene synthase (\(TPS2\)) and _CYP71D180_. CYP71D180 belongs to the cytochrome P450 (CYP) monooxygenases and is involved in further modifications of \(\gamma\)-terpinene backbone to yield carvacrol [8]. The gene is modulated by methyl jasmonate and salicylic acid as well as by feeding of insects [10] and its modulation is associated with a corresponding expression of early terpenoid genes, like _DXP_[35]. \(TPS2\) increased expression has been recently reported upon interaction of \(O\). _vulgare_ with a _Myrmica_ ant [10] and confirms the modulation of this gene upon biotic attack. Besides terpenoids, qualitative differences were found in the production of some GLVs, which are considered typical wound-related VOCs [13; 36; 37; 38; 39]. Upon herbivory, a significant release of GLVs has been observed in several non-aromatic plants, such as lima bean [40], as well as in plants producing glandular trichomes like _Monarda fistulosa_[41] and tomato [39]. The toxicity of terpenoids toward herbivory has been widely documented [42] and the insecticidal effect of several monoterenes has been demonstrated in many species belonging to the Lamiaceae family [43; 44; 45; 46]. Several studies have evaluated the toxicity of plant terpenes by the use of essential oils. However, the essential oil does not reflect the natural composition of accumulated terpen in the glandular trichomes since partial terpene recovery and thermal degradation can occur [47]. In our study we used intact leaves and terpenes extracted from \(O\). _vulgare_ leaves to perform toxicological tests and we exposed the caterpillars to physiological concentrations based on the quantity detected in \(O\). _vulgare_ leaves. Carvacrol and \(p\)-cymene were the most toxic terpen among the tested monoterenes. These results are in agreement with the reported toxicity of carvacrol [48]. In a study on the toxicity of carvacrol and \(p\)-cymene towards \(S\). _littoralis_, carvacrol resulted more active (LD\({}_{25}\) = 7 \(\upmu\)g-larva\({}^{-1}\)) with respect to \(p\)-cymene (LD\({}_{25}\) = 25 \(\upmu\)g-larva\({}^{-1}\)) [17]. We also observed that the survival percentage of larvae upon carvacrol and \(p\)-cymene feeding was higher than larvae fed on leaves at the end of 3 rd stage. Toxicology studies on \(S\). _littoralis_ showed that the binary mixture of carvacrol and \(p\)-cymene have a synergic toxic effect if compared with the single compounds [17].