Mosquitoes smell and avoid the insect repellent DEET

Area of Science:

• Neuroethology
• Chemical Ecology
• Insect Physiology

Background:

• The precise mechanism by which N,N-Diethyl-meta-toluamide (DEET) repels insects remains debated.
• Previous hypotheses suggested DEET interferes with lactic acid detection or masks olfactory responses to compounds like 1-octen-3-ol.
• Evidence of DEET repellency independent of lactic acid challenged existing theories.

Purpose of the Study:

• To investigate the direct olfactory detection of DEET by mosquitoes.
• To elucidate the primary mode of action for DEET-induced insect repellency.
• To identify specific olfactory receptor neurons (ORNs) involved in DEET detection.

Main Methods:

• Single-unit electrophysiological recordings from olfactory receptor neurons (ORNs) in the antenna and maxillary palps of Culex quinquefasciatus.
• Gas Chromatography-Mass Spectrometry (GC-MS) and Solid-Phase Microextraction (SPME) analysis of odorant trapping in stimulus cartridges.
• Behavioral bioassays, including sugar-feeding assays and assays for physically stimulated mosquitoes, to assess DEET avoidance.

Main Results:

• Identified a specific ORN in Culex quinquefasciatus that responds directly to DEET in a dose-dependent manner.
• Demonstrated that reduced electrophysiological responses in previous studies were due to odorant trapping in stimulus cartridges, not DEET interference.
• Behavioral assays confirmed that mosquitoes actively avoid DEET, even when only responding to physical stimuli.

Conclusions:

• Mosquitoes possess dedicated DEET-detecting ORNs, indicating direct detection is the primary mode of action.
• DEET's "fixative" effect on skin emanations may contribute to repellency but is secondary to direct detection.
• The findings resolve the debate by confirming direct DEET detection as the principal repellent mechanism.