When a plant is attacked, it emits GLVs into the environment through the air.  How a plant responds depends on the type of damage involved. Plants respond differently to damage from a purely mechanical source and damage from herbivores.  Mechanical damage tends to cause damage-associated molecular patterns (DAMPs) involving plant-derived substances and breakdown products.  Herbivore-associated molecular patterns (HAMPs) involve characteristic molecules left by different types of herbivores when feeding. The oral secretions of herbivores appear to play an essential role in triggering the release of species-specific herbivore-induced plant volatiles.  Wounds from herbivores, and mechanical wounds that have been treated with herbivore oral secretions,  both trigger the release of higher quantities of plant volatiles than mechanical damage.
Volatile blends are proposed to convey a variety of information to insects and plants.  "Each plant species and even each plant genotype releases its own specific blend, and the quantities and ratios in which they are released also vary with the arthropod that is feeding on a plant and may even provide information on the time of day that feeding occurs."  In addition to GLVs, herbivore induced plant volatiles (HIPVs) include terpenes, ethylene, methyl salicylate and other VOCs.
GLVs activate the expression of genes related to the plants' defense mechanisms. 
Different antagonists trigger different expression of genes and the biosynthesis of signaling peptides which mediate systemic defense responses.