The active component in citronella oil is citronellal. However, the molecular pathways through which insects sense this botanical repellent are unknown.
Now Johns Hopkins scientists led by Craig Montell, PhD, have discovered what it is in the insects’ molecular makeup that enables citronellal to deter insects from landing and feeding on you.
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Taste receptors on the insects' tongue, legs and wing margins are needed to detect citronellal. When these molecular receptors are activated by exposure to citronellal, they send chemical messages to the insect brain, resulting in "an aversion response.
Fruit flies were used in the study because they are genetically easier to manipulate than mosquitoes. To measure repulsion to the vapors it emits, they applied the citronellal to the inside bottom of one of the two connected test tubes, and introduced about 100 flies into the tubes. After a while, the team counted the flies in the two tubes. As expected, the flies avoided citronellal.
The researchers identified two distinct types of cell surface channels that are required in olfactory neurons for avoiding citronellal vapor. The channels let calcium and other small, charged molecules into cells in response to citronellal. One type of channel is called Or83b, and the other is a TRP channel.
Citronellal detection is enabled by pore-like proteins known as Or83b and TRP channels. The channels let calcium and other small, charged molecules into cells in response to citronellal and turn the insect away.
The team then "mosquito-ized" the fruit flies by putting into them the gene that makes the mosquito TRP channel (TRPA1) and found that the mosquito-version of TRPA1 was directly activated by citronellal.
Young Kwon, Sang Hoon Kim, David S. Ronderos, Youngseok Lee, Bradley Akitake, Owen M. Woodward, William B. Guggino, Dean P. Smith, Craig Montell. Drosophila TRPA1 Channel Is Required to Avoid the Naturally Occurring Insect Repellent Citronellal. Current Biology, published ahead of print 25 August 2010