Terpenoids facilitate communication between plants and insects, mammals, fungi, microorganisms, and other plants. Terpenoids may act as both attractants and repellants for various insects. For example, pine shoot beetles (Tomicus piniperda) are attracted to certain monoterpenes ( (+/-)-a-pinene, (+)-3-carene and terpinolene) produced by Scots pines (Pinus sylvestris), while being repelled by others (such as verbenone).
Terpenoids are a large family of biological molecules with over 22,000 compounds. Terpenoids are similar to terpenes in their carbon skeleton but unlike terpenes contain functional groups. The structure  of terpenoids is described by the biogenetic isoprene rule which states that terpenoids can be thought of being made of isoprenoid subunits, arranged either regularly or irregularly. The biosynthesis of terpenoids occurs via the methylerythritol phosphate (MEP) and  mevalonic acid(MVA) pathways both of which include isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) as key components. The MEP pathway produces hemiterpenes, monoterpenes, diterpenes, and volatile carotenoid derivatives while the MVA pathway produces sesquiterpenes.