We tested DEET, IR3535, and picaridin, and found these synthetic repellents did not activate olfactory neurons. This was surprising, as research in Aedes and Culex mosquitoes suggested DEET would activate olfactory neurons. We reasoned that under normal conditions these repellents are applied to the skin and mixed with human-odors. We mixed DEET, IR3535, and picaridin with the stimulating odorants, and then repeated the calcium imaging. In this situation, the human-derived odorants no longer stimulated olfactory neurons. We concluded that commonly used synthetic repellents function to ‘mask’ (suppress) the activity of olfactory neurons in response to host odors.
We further found that synthetic repellents reduce the volatility of other odorants (Afify, 2019). Therefore, synthetic repellents function to hide human odors from host-seeking mosquitoes. We also examined the olfactory response to natural repellents (e.g., those derived from plants such as lemongrass oil). In contrast to synthetic repellents, all natural repellents stimulated small reproducible subsets of olfactory neurons. Our analyses further suggest that different natural repellents might be activating the same olfactory neurons. This suggests a small number of olfactory neurons might be driving most odor-induced repellent behaviors.