# Biochemistry of Terpenes and Recent Advances in Plant Protection
## 3 Role of terpenes in Plant Protection
### Role of Terpenes in Insect Modulation  
Host identification of feeding insects largely depends on either visualization, olfactory cues, or both for chemotactic and landing efficiency. The embodiments of olfactory cues are mostly C\({}_{5}\), C\({}_{10}\), and C\({}_{15}\) isoprene chain terpenes produced in the host plants' glandular trichomes or epidermis for insect defense. These compounds confer antixenosis, a mechanism that disrupts arthropods' mode of life or indirectly attracts their natural enemies [73]. Besides gibberells and brassinosteroids having primary roles in growth and development, most terpenes are defense compounds. Terpenes can be alarm substances, defensive emissions, trail markers, and deterrent antifeedant substances. Terpenes contain over 25,000 VOCs with varying concentrations and toxicity levels used for defense against insects and pathogenic microbes. Plants elicit VOCs under stress conditions. For example, the monoterpene-derived compound, cyclopentanoid (iridoid) is incredibly bitter and a robust defensive agent against insect pests. Iridoid glycosides covalently bond to nucleophiles to form amino acids, proteins, and nucleic acids. This bond causes the denaturation of proteins, amino acids, and nucleic acids. This phenomenon reduces plant nutrients, depriving the insects of proteins and nucleic acids. Iridoids can also inhibit the synthesis of prostaglandandins and leukotrienes in insects, thereby stifling their growth and development. Under in vitro conditions, iridoid prolongs the larval stage and reduces insects' growth and survival rates [74; 75; 76]. Flowers and leaves elicit neurotoxin-modulating monoterpene esters called pyrethroids. This terpene harms wasps, bees, beetles, and moths. Studies have shown that pyrethroid is harmless to the environment and, therefore, used as active ingredients in most commercial insecticides. Methylcyclopentanoid monoterpenes produced by _Teurium marun_ (cat thyme or kitty crack) is a repellent to ants and cockroaches. \(\alpha\)-pinene, \(\beta\)-pinene, limonene, and myrcene accumulate in the spindle twigs of conifers are toxic to beetles and most pests of conifers [77; 78; 79]. Essential oils from Gaultheria (Ericaceae) and Eucalyptus (Myrtaceae) also repel several insects, including the housefly, drugstore beetle, and rice and bean weevils [80]. Moreover, momilactone A, a rice diterpenoid, also prevented white-backed planthopper infestation [81; 82]. A sesquiterpene (\(E\))-\(\beta\)-caryophyllene accumulated following herbivore's attack on maize plants. Although (\(E\))-\(\beta\)-caryophyllene was not directly involved in the defense against the herbivores, it attracted natural enemies to fight them. (\(E\))-\(\beta\)-caryophyllene was synthesized as a decoy to attract entomopathogenic nematodes to fight off the attack from _Diabrotica virgifera virgifera_ (corn rootworm) [83]. (E)-\(\beta\)-caryophyllene, in a related study, attracted _Cotesia sesamiae_ (larval parasitoid) of _Chilo patellus_ (spotted stalk borer). These lepidopteran stem borers are a threat to cereal production as studies showed they could reduce about 80% yield [84]. OsTPS3 and OsTPS13 genes encode (E)-beta-caryophyllene synthase and (E, E)-farnesol synthase, respectively, in rice plants. OsTPS3 protein under in vitro conditions catalyzed the synthesis of (E)-beta-caryophyllene, alpha-humulene, and beta-elements in rice plants. The transgenic lines of OsTPS3 in _Oryza sativa_ induced high production of (E)-beta-caryophyllene upon methyl jasmonate (MeJA) treatment. This consequently attracted parasitoid wasps of _Anagrus nilaparvatae_ (Hymenoptera: Mymaridae), implicating OsTPS3 in signaling volatile sesquiterpenes [85]. Furthermore, the overexpression of FPS2 in Arabidopsis with chloroplast as a target stimulated the production of E-\(\beta\)-farnesene and other sesquiterpenes that subsequently induced resistance against aphids [86]. The tea green leafhopper is a harmful pest of _Camellia sinensis_ (Green tea).