Perception of plant volatile blends by herbivorous insects--finding the right mix.

Volatile plant secondary metabolites are detected by the highly sensitive olfactory system employed by insects to locate suitable plants as hosts and to avoid unsuitable hosts. Perception of these compounds depends on olfactory receptor neurones (ORNs) in sensillae, mostly on the insect antennae, which can recognise individual molecular structures. Perception of blends of plant volatiles plays a pivotal role in host recognition, non-host avoidance and ensuing behavioural responses as different responses can occur to a whole blend compared to individual components. There are emergent properties of blend perception because components of the host blend may not be recognised as host when perceived outside the context of that blend. Often there is redundancy in the composition of blends recognised as host because certain compounds can be substituted by others. Fine spatio-temporal resolution of the synchronous firing of ORNs tuned to specific compounds enables insects to pick out relevant host odour cues against high background noise and with ephemeral exposure to the volatiles at varying concentrations. This task is challenging as they usually rely on ubiquitous plant volatiles and not those taxonomically characteristic of host plants. However, such an odour coding system has the advantage of providing flexibility; it allows for adaptation to changing environments by alterations in signal processing while maintaining the same peripheral olfactory receptors.