Olfactory and gustatory contributions to insect repellent detection in Drosophila melanogaster

Insect repellents reduce contact between insects and humans, animals, crops, and treated surfaces, yet the sensory mechanisms that support repellent avoidance remain unresolved. Repellent detection may occur before contact through olfaction, after contact through gustation, or through a combination of both modalities. This thesis uses Drosophila melanogaster to define how olfactory and gustatory pathways contribute to avoidance of three chemically distinct repellents: DEET, picaridin, and eucalyptol. Binary behavioural assays showed that the three repellents differ in sensory dependence: DEET avoidance required both olfactory and gustatory input, picaridin avoidance was primarily olfactory with a smaller gustatory component, and eucalyptol avoidance was strongly dependent on olfaction. Receptor family analysis further separated these mechanisms. Picaridin avoidance required the Ionotropic Receptor (IR) pathways, eucalyptol avoidance required Odorant Receptor (OR) pathways, and DEET avoidance involved both receptor families. DEET avoidance did not resolve to a single tuning receptor. For picaridin avoidance the coreceptor Ir8a was required, and the tuning receptor Ir64a modulates the balance of IR pathway output rather than directly driving avoidance. Or10a was identified as a major receptor required for eucalyptol avoidance. Together, these findings show that repellent avoidance is compound specific across sensory modality and receptor families, providing a framework for linking behavioural repellency to peripheral chemosensory mechanisms.