# Trepenes and Trepenoids in Plants: Interactions with Environment and Insects
## 2 Trepenoids in Plant-Insect Interactions  
### Terpenoids with Toxic and/or Repellent Effects on Insects  
Terpenoids are also known to mediate plant-insect (e.g., pollinators, predators, parasitoids, and herbivores) interactions. Generally, the interactions work to the benefit of the plants and to the detriment of herbivores. These interactions are assumed to be mediated by the emission of volatile organic compounds (VOC) including, but not limited to, terpenoids, albeit the fact that a substantial fraction of the VOC consists of terpenoids. The characteristics of VOC mixture are believed depend on the identity and proportion of individual components, and attacking herbivores are sensitive to a particular blends of VOC. The emitted VOC may include one or few with toxic, deterring/repellent, or attractive properties.  
Cinnamon and clove essential oils are toxic and repellent to _Sitophilus granaries_, an important grain pest. In addition to these two essential oils, five toxic terpenoids were proposed, i.e., eugenol (considered a terpenoid in that study but not in other works), caryophyllene oxide, \(\alpha\)-pinene, \(\alpha\)-humulene, and \(\alpha\)-phellandrene. Eugenol shows the strongest contact toxicity [58]. It was found that less damage was caused by _Paropsisterna tigrina_ adults in _Melalecua alternifolia_, in the presence of a high concentration of terpinolene [59]. In addition, volatiles containing the same terpenoid's present in intact and infested rice plants attract or repel the rice pest _Orseolia oryzivora_ depending on their concentrations [60] Table 2 summarizes examples from relevant studies.  
Unlike terpenoids directly targeting insect pests, terpenoids can act as elicitors to trigger defense mechanisms to pests in plants. Overexpression \(\beta\)-ocimene from tobacco triggered a high release of methyl salicate and cis-3-hexen-1-ol in tomato, both of which are proposed to cause impairment of aphid development and reproduction, increasing the defense ability of tomato against aphids [61].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Plant Species** & **Terpenes** & **Targets** & **Effects** & **References** \\ \hline Tropical orchids & monoterpene: 1, 8-cineole & Male euglossine bees & an attractant and reward to pollinator & [62] \\ \hline Dalechampia (Euphoriaceae) & terpenoid resins (oxygenated triterpenes) & Female euglossine (Apidae), female anthidine (Megachilidae), or worker & reward to pollinators for use in nest construction & [63; 64; 65] \\ epiphytes (_Clusia_) & & & & \\ \hline Kiwifruit (_Actilitia delicious_) & Sesquietpene \(\alpha\)-farnesene, germetence D, Monoterpene: (E)-\(\beta\)-ocimene, (Z,E)-\(\alpha\)-farnesene & Mainly honeybees (Apidae) & attract a variety of pollinators & [66] \\ \hline White Campion (_Silene tifolia_) & & & & \\ \hline Pineapple zamia (_Macozanita hcida_) & \(\beta\)-myrcene, (E)-\(\beta\) cotimene and allo-ocimene & Thrips (_Cyarolettrips_) & repel or attract pollinators to complete the pollination from male to female & [68] \\ \hline Fig (_Ficus hispida_) & Monoterpene: linalool, limonene and \(\beta\)-pinene & Wasp (_Creatosollen solusi_) & signals for pollinators to identify floral stages & [69] \\ \hline Monkeyflower (_Mimulus levissii_) & D-limonene, \(\beta\)-myrcene and E-\(\beta\)-ocimene. & Bumblebee (Bombus vosnesenskii) & attract specific pollinators & [70] \\ \hline Sweet rocket (_Hesperis Matronalis_) & linalool, \(\beta\)-ocimene. & Mainly synbrid lines (Syrophidae) & attract a variety of pollinators & [71; 72] \\ \hline Radiator plant (_Premonia mcrostacharya_) & & & & \\ \hline Cabbage plants (_Brussica_) & monoterpene: 1.8-cineole & Parasitic wasps (_Cotesia glomerta_) & attracts parasitoids that lay eggs in the caterpillars of specific herbivores & [75; 76] \\ \hline \hline \end{tabular}
\end{table}
Table 2: Plant terpen and their observed roles and effects against insects.