Feline stimulant fends off mosquitoes.

Catnip may be ambrosia to cats, but for some creatures it's the opposite. Preliminary results now suggest that the oil in the herb rebuffs mosquitoes more effectively than the widely used repellent diethyl-m-toluamide, or DEET.

Chris Peterson and his colleagues from Iowa State University in Ames had previously found that catnip--a plant in the mint family--repels cockroaches (SN: 9/4/99, p. 158).

For each trial in the new mosquito experiment, Peterson placed 20 mosquitoes in the middle of a closed, 2-foot-long tube that he'd treated on one end with DEET, catnip oil, or one of the oil's two major chemical components. The other end of the see-through tube received no treatment.

After 10 minutes, Peterson counted the mosquitoes in each half of the tube as a measure of the repelling power of the chemical applied to its end. The researchers needed to apply DEET at about 10 times the concentration of catnip oil or either oil component to repel the same number of mosquitoes. "We suspect catnip is acting as a general irritant, but we just don't know," he says.

Peterson, now with the U.S. Department of Agriculture Forest Service in Starkville, Miss., reported the new work on Aug. 27 in Chicago at a meeting of the American Chemical Society.

Catnip oil is years away from any potential commercial insect-repelling application, Peterson admits. For one thing, the substance hasn't undergone safety trials or sufficient testing of the endurance of its antimosquito activity.

Experimentation on catnip as a repellant is in its early phases, concurs entomologist Florence V. Dunkel of Montana State University in Bozeman. Her group has patented natural mosquito-repellent compounds, also from mint plants, which she says are further along in development than catnip.

To move the catnip along, Dunkel says that Peterson's group should test how the oil and its components behave under conditions resembling the moisture and temperature of human skin. A commercially viable catnip-based repellent would have to remain effective at least 4 hours after application, she says. Moreover, the researchers might want to compare the power of catnip oil with other natural repellents, including those being studied by other research groups.

If catnip delivers on its promise, Peterson notes, the plant could be grown on a large scale--and extracting the oil requires no equipment beyond that found in a high school chemistry lab.