SOME OLD SPICES may turn out to give new relief from annoying insects. Catnip, for instance. The aromatic oil in this herb is ten times better at warding off mosquitoes than DEET, the chemical used in most bug sprays, researchers from Iowa State University say. "It might simply be acting as an irritant or they don't like the smell," says Chris Peterson, who led the study. "Nobody really knows why insect repellents work." Although catnip grows wild and is already used in herbal teas, the oil must still be tested for safety before it can be sold as an insect repellent. But a compound based on piperidine, found in black pepper, may come to market sooner. Federal scientists in Maryland are testing this substance on soldiers and have learned "it is equivalent to or better than DEET," says entomologist Jerome Klun. Not only does it repel mosquitoes, ticks and flies but, unlike DEET, it doesn't melt plastics. Hold the DEET and pass the pepper, please . . .