How might monoterpenes and sesquiterpenes affect insect behaviour around plants?

Monoterpenes and sesquiterpenes, which are classes of isoprenoids, play a significant role in mediating the complex interactions between plants and insects. Their effects on insect behavior generally fall into two categories: attraction and defense.

Attraction

Many plants produce volatile terpenes that serve as chemical signals to influence insect movement:
* Host Location: Insects often use the distinct odors of plants—frequently dominated by terpenoids—to locate specific host plants from a distance. Receptor cells on an insect's antennae detect these compounds, guiding their flight toward the plant.1
* Pollination: Flowers release blends of volatile compounds, including terpenes, to attract pollinators. While some responses are innate, many insects learn to associate specific floral odors with the presence of nectar, which improves foraging efficiency and facilitates cross-pollination.1

Defense

Plants also utilize these compounds to protect themselves from herbivory:
* Repellence: It is suggested that certain monoterpenes and sesquiterpenes act to repel specific insects.2
* Developmental Interference: Some plants produce isoprenoids that mimic insect hormones. When ingested, these substances can prevent the maturation of the insect, effectively defending the plant against those that prey upon it.2
* Chemical Barriers: As part of a broader strategy of defensive chemicals, these compounds help plants mitigate the constant pressure of herbivory from various insect species.3