US scientists have identified the underlying chemistry that makes insect-repellants containing DEET and citronellal so repellant to mosquitoes. They have worked out how the disease-carrying insects “taste” DEET, N,N-diethyl-meta-toluamide, and “smell” citronellal, a monoterpenoid aldehyde, 3,7-dimethyloct-6-en-1-al.

Three taste receptors on the insects’ tongue and elsewhere are required for DEET detection while smelling citronellal is enabled by TRP protein channels. Activation of these molecular receptors sends a chemical message to the mosquito brain triggering an aversion response.

• Avoiding DEET through Insect Gustatory Receptors
  Y. Lee, S. H. Kim, C. Montell,
  Neuron, 2010, 67(4), 555-561.
  DOI: http://www.cell.com/neuron/abstract/S0896-6273(10)00544-1
• Drosophila TRPA1 Channel Is Required to Avoid the Naturally Occurring Insect Repellent Citronellal
  Y. Kwon, S. H. Kim, D. S. Ronderos, Y. Lee, B. Akitake, O. M. Woodward, W. B. Guggino, D. P. Smith, C. Montell,
  Curr. Biol. 2010, 20.
  DOI: http://www.cell.com/current-biology/abstract/S0960-9822(10)01006-7