# Trepenes and Trepenoids in Plants: Interactions with Environment and Insects
## 2 Trepenoids in Plant-Insect Interactions
### Plant Trepenes with Beneficiaries  
Owing to their diversity, terpene products not only provide plants with adaptative strategies for various environmental conditions but also bring benefits especially to long-term cooperators such as pollinators. The above mentioned 1,8-cineole, which can be used as an insecticide, can be tolerated by orchid bees (Euglossinae, e.g., _Eulaema_, _Eufriesea_, _Euglossa_). Male orchid bees are well known fragrance collectors, sensing undiluted 1,8-cineole directly from the flowers of several neotropical orchid genera and species of _Dalechampia_ during pollination [62; 65]. This monoterpene was reported to be stored in males' hind leg pouches and later exposed during courtship [89]. On the other hand, female bee species (Apidae: Euglossini), _Hypunthidium_ (Megachilidae: Anthidini), and worker _Trigona_ (Apidae: Meliponini) utilize triterpen resins (mixtures of \(\beta\)-amyrin, \(\beta\)-amyrone, dammadienol, and dammadienone) collected from flowers of some _Dalechampia_ species as nest construction materials and pollinate these flowers during collection. [90]. While the same resin compound has an inhibitory effect towards generalists and specialized herbivores [91], it is an example of species-specific reward to beneficiaries. Other than providing terpen as a reward to pollinators, some flowers provide a nursery ground (the flower itself) to the pollinators. Terpene compounds serve as a cue for pollinators to locate their specific host. In the case of the noctoid moths _Hadena bicruris_, these insects are attracted by lilac aldehydes which are the main components of the scent of _Silene latifolia_[67].  
Terpene compounds can be finely adjusted to a certain concentration to repel or attract pollinators. Pollination of male and female _Macrozamia lucida_ is sort artistic. Thrips (_Cycadothrips chadwick_), the pollinators, are retained in the male cone early in the morning. During midday, after the temperature of the male cone changes, volatile emission increases tremendously (female cone emission is 1/5 of that of male cone). Under such a high concentration of volatile compounds which contain three terpenes (\(\beta\)-myrcene, (E)-\(\beta\) ocimene, and allo-ocimene), the thrips are repelled from the cones. Later, the thrips are attracted to the female cone and finish the whole pollination as they are attracted by a low emission of terpen, especially \(\beta\)-myrcene [68].  
Research studies have also revealed that terpenes serve as signals for pollinators to identify floral stages. In the species-specific relationship between fig (_Ficus hispida_) and wasp (_Ceratosolen solmsi marchali_), the pollinator needs to identify the female floral stage because it is the only stage receptive to pollinators. Three monoterpenes, namely, linalool, limonene, and \(\beta\)-pinene, can be found in receptive fig volatiles. Wasps are responsive to both quantity (change in concentration) and quality (substitution with different isomers), but quality does matter regarding locating the host and floral stage differentiation [69].  
_Peperonia macrostachya_ produces seeds that attract arboreal in nutrient-rich formicaries. This species-specific symbiosis is called neotropical ant-gardens (AGs). The seeds emit numerous phenolic and terpenoid volatiles including geranyllinalool. This terpenoid can be perceived by the seed-dispersal partner AG ant species _Camponotus femoratus_; as a result, these ants are attracted to the location of the scent. This scent is rather toxic to most insect species [73]. This explains why this plant is distributed largely around AG ant nests [74].