A better way of keeping mosquitoes at bay is under development


minsect repellents have come a long way. Market leader for decades DEET, which successfully repels the pests, but only for an hour or two. Icaridin has recently become available. This lasts up to eight hours and is just as effective. Yet both are mildly toxic to cells grown in culture and their toxicity (if any) to human users is under constant debate. So the search is on for something that is undoubtedly not poisonous at all.

Francesca Dani of the University of Florence, in Italy, thinks she has the answer. As she and her colleagues describe in the Journal of Agricultural and Food Chemistrythey looked at a range of chemicals called aldehydes and ketones and, with a little tweaking, produced something that seems like a good, long-lasting, safe mosquito repellent.

Dr. Dani knew from the scientific literature that some aldehydes and ketones have insect repellent properties. However, such chemicals evaporate faster than DEET. But she also knew from her own work that molecules called hydroxylated cyclic acetals, which form from certain aldehydes and ketones when exposed to alcohol, are much less likely to vaporize. That, she thought, might be the key to the puzzle.

So she and her colleagues made about twenty hydroxylated cyclic acetals and tested their mosquito-repelling properties against those of DEET and Icaridin. The standard way to do this is to spray some on the back of a volunteer’s hand and then have them place both hands in a mosquito cage. The unsprayed hand acts as a control, and thus it is possible to decide, by comparing the number of insects that land on it, how effective a repellent is for a particular chemical.

As expected, DEET scored 95% protection efficiency (calculated by subtracting the number of mosquito probes on the treated hand from the number on the untreated hand, dividing by the number on the untreated hand, then multiplying by 100). Such protection was provided by applying 8.3 micrograms of the stuff per square centimeter of skin and lasted for two hours. For Icaridin, one-fifth of that dose produces equal rejection for eight hours.

To the delight of the researchers, two of their hydroxylated cyclic acetals performed as well as Icaridin. And when they tested them on cell cultures, they found that the one, called 12a in the paper for simplicity, didn’t kill any of the cells that had been exposed to it.

DEET and Icaridin have had good runs. DEET was developed in the 1940s to help protect American soldiers against mosquito-borne diseases during campaigns. Icaridin, with its extended protection period, arrived in the 2000s. However, if 12a succeeds, both may soon be reaching the end of their useful lives.

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