The female Anopheles gambiae mosquito pictured at the right, shows her weapons researchers are trying to combat in this ancient war of man against mosquito. It is only the female mosquito that bites and can spread disease using these parts of olfactory (smelling) appendages (antennae, maxillary palps and proboscis) as so graphically seen in this electron micrograph image.
Dr. Leslie Vosshall and two colleagues at Rockefeller University published a series of experiments that seemed to settle the 50-year-old question of how the insect repellent DEET kept mosquitoes at bay (Science, 319:1838-42, 2008).
Vosshal explained their findings “It doesn’t smell bad to insects. It masks or inhibits their ability to smell you.”
The Bill & Melinda Gates Foundation funded the research to understand how and why DEET works. This is critical to creating the next generation of chemicals, which may head off insect-borne diseases such as malaria and dengue fever.
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Laurence Zwiebel of Vanderbilt University (also a Gates’ grantee) and Ulrich Bernier of the US Department of Agriculture are not sure the findings just didn’t make sense, given everything they knew about this system
In Vosshall experiment, the response of the mosquito’s olfactory neurons to two separate, attractive odors in human breath. Then, she combined each odorant with DEET in a single odor cartridge and noticed a smaller neural response. Vosshall believes DEET was blocking the mosquito’s olfactory co-receptor.
Another teams experiment another interpretation
Using gas chromatography, Leal confirmed his suspicions this year. When he repeated Vosshall’s experiment using separate odor cartridges that blended DEET and each attractive odor only at their tips, the mosquito’s neural response was no longer diminished. Then, Leal identified a 
DEET-sensitive odor receptor neuron and showed that mosquitoes avoid passing through a “curtain” of DEET vapors.
Leal’s paper surprised Vosshall, but is unconvinced by Leal’s results, and has been trying to reproduce the effect in her own lab. “Competition in science is good,” she says, “It can be difficult when it’s a small field, and this is a very small field.”
Genomic studies in 2005 have since shown that this co-receptor is found in insects ranging from mosquitoes to moths, making humans invisible to insects. Using tissue cultures, she uses targeted drug discovery to screen 91,520 compounds from a chemical library, short-listing about 150 that she believes have the potential to be insect “confusants.”
Even Vosshall’s skeptics admit the confusant strategy is fundamentally sound. Zwiebel says his unpublished molecular work confirms the existence of confusants, but when it comes to DEET, he and Vosshall aren’t willing to budge. “We have agreed to disagree on the DEET story,” he says.
Resources
Smells funny? – Brendan Borrell The Scientist.com Volume 23 | Issue 1 | Page 19.
http://www.the-scientist.com/2009/01/1/19/1/
Mosquitoes smell and avoid the insect repellent DEET – Leal and Zainulabeuddin Syed, PNAS 105:13598-603, 2008 September 2008.
Image courtesy of LJ Zwiebel, colorization by Dominic Doyle / Vanderbilt University
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