Olfactory Mechanisms of Host Selection in Phytophagous Insects

The most challenging tasks for phytophagous insects are the location and selection of mates, food sources, and oviposition sites, all crucial for survival and reproduction. To perform these tasks insects rely largely on their sense of smell (olfaction). I address how the insect olfactory system discriminates between components of complex odor mixtures, modulating behavior and fitness. I have studied modulation of attraction in the moth Spodoptera littoralis and the bark beetle Ips typographus by separation of pheromone (Ph) and anti-attractants, and of Ph components alone. An antagonist reduced male moth attraction towards the female sex Ph, and a blend of non-host volatiles (NHV) reduced attraction of both sexes of I. typographus towards their Ph, insect catches decreased with decreasing odor-source distance. Conversely, increasing distance between Ph components decreased attraction in both insect species. However, moths were more sensitive to small-scale spacing. Reproductive behaviors as well as fecundity and longevity of S. littoralis moths were negatively affected in the presence of volatiles from leaves of non-host plants, Picea abies or Adhatoda vasica. The presence of non-host plants strongly modulated male moths’ behavior, reducing their attraction towards the Ph source in flight assays. Gas chromatography-electroantennographic detection (GC-EAD) by female S. littoralis antennae with headspace volatile collections from P. abies and A. vasica revealed eight active compounds, with seven new actives. Single sensillum recordings (SSR) created a functional-morphological map of 49 olfactory sensory neuron (OSN) functional types in six morphological sensillum types in female S. littoralis. Proximally located OSNs showed a higher sensitivity, shorter latency, and displayed more phasic responses than distally located OSNs of the same class. GC-SSRs with volatiles from a larval host, cotton plants, and the adult nectar source, lilac flowers, revealed 38 active compounds for female OSNs, including 12 new actives. The odor response specificities of four olfactory receptor (OR) genes of S. littoralis were deorphanized by expression in the Empty Neuron System (ENS) of Drosophila melanogaster using SSR and GC-SSR (GC-SSR-ENS). Two of the ORs responded specifically to single odorants, while the other two responded similarly to the same 9 compounds, but dose-response experiments with new compounds, identified by GC-SSR, revealed specific odor-response profiles.