Your feline may find it irresistible, but catnip has rather a different effect on mosquitoes. Indeed, according to findings presented yesterday at a meeting of the American Chemical Society, nepetalactone, the essential oil that lends the plant its characteristic scent, repels the pesky insects about 10 times more effectively than does DEET, the active ingredient in most commercial insect repellents.

Earlier work carried out by Joel Coats of Iowa State University and then¿postdoctoral researcher Chris Peterson had revealed the cockroach's dislike of catnip. The same team thus decided to investigate whether mosquitoes might share that opinion. Placing groups of 20 mosquitoes in a glass tube, half of which was treated with a high dose (1 percent) of nepetalactone, the researchers found that after 10 minutes, only 20 percent of the insects remained on the treated side. When they tested the effects of a lose dose (0.1 percent) of the catnip oil, 25 percent of the mosquitoes stayed in the treated area. In contrast, when the team ran the same experiment with DEET (diethyltoluamide), 40 to 45 percent of the bugs held their ground on the treated side of the tube.

Peterson notes that it takes about one tenth as much catnip oil as DEET to have the same repellent effect on the mosquitoes. Though the team only used the so-called yellow fever mosquito, Aedes aegypti, in their experiments, they say nepetalactone should prove similarly abhorrent to all mosquito species. What the researchers can't explain is why the catnip drives the insects away in the first place. "It might simply be acting as an irritant or they don¿t like the smell," Peterson offers. "But nobody really knows why insect repellents work."

If it proves safe for people, nepetalactone could one day find its way into commercial products. In the meantime, the Iowa State University Research Foundation has submitted a patent application for use of the catnip compounds as insect repellents.