Most people swat mosquitoes. Walter Leal wants to attract them.

And he does. He makes offers that the mosquitoes can't refuse.

Leal, an entomology professor at the University of California, Davis draws gravid mosquitoes—or females ready to deposit their eggs—to his chemically scented oviposition traps set in Sacramento, Fresno, Shasta and Los Angeles counties, Calif.

Culex mosquitoes, the principal carriers of West Nile virus, buzz excitedly around Leal's traps, drawn by the tantalizing smell, the whirring movement of battery-operated fans and the water-filled tray.

"The smell is offensive to us, but the mosquitoes like it," says Leal, who has been perfecting the "stinky" chemical compound mixture for the past four years.

Once sucked into the traps, the mosquitoes can be tested for the presence of West Nile virus, which last year killed 18 people in California and infected some 900 others throughout the state.

Leal's ovitraps work much better than the host-seeking standard traps, says medical entomologist Anthony Cornel, director of the Mosquito Research and Control Laboratory at the UC Kearney Agricultural Center in Parlier, Calif.

To the outsider, the ovitrap resembles a metal tool box nestled in a water-filled tray. Generally, female mosquitoes consume a blood meal and later-after the.blood is digested and nutrients converted to egg yolk-look for a place to "ovipost" or lay their eggs.

With the onset of mosquito season, "we'll be collecting mosquitoes where the WNV outbreaks occurred last year, and then they will be tested for WNV infections," Leal says.

The ovitraps are also somewhat like the canaries in coal mines that alert the miners of the presence of dangerous underground gases.

Already the ovitrap has proved more efficient, effective and user friendly than the infusion traps that are standard in the industry, Leal said. "With these ovitraps, we attract more gravid mosquitoes, the traps are easier to operate and they don't stink as much." The infusion traps smell like a plugged-up toilet.

The ovitraps also serve another purpose: they indicate the size of mosquito populations.