John Clark is trying to solve a problem that has left hundreds of parents -- not to mention their children -- scratching their heads in search of answers.

Over-the-counter shampoos are losing their effectiveness against head lice, parasites that plague as many as 12 million schoolchildren each year. Clark and researchers at the University of California have been trying to find out why some head lice don't die when doused and scrubbed with pesticide-based shampoos.

"The lice are becoming resistant and the resistant lice are taking over," said Clark, an entomologist at the University of Massachusetts.

The reason, he said, lies in the shampoos' active ingredient that attacks the insects' central nervous systems and causes the critters to suffocate. The poison acts in the same way as DDT, a pesticide that was widely used to kill lice before it was banned in the United States in 1973.

After three decades of exposure to DDT, lice began developing a genetic immunity to it. The pesticide-laced shampoos, which were made available without a prescription in the 1980s, helped the resistant strain of lice evolve, public health experts said.

"We get calls from child care providers and schools who say lice are increasingly more difficult to get rid of," said Steven Shuman, the Massachusetts Department of Public Health's deputy director for maternal, child and family health. "When people get lice, they want to get rid of them as fast as possible. And that leads to a misuse of the products designed to kill them."

In samples of head lice sent to him by school nurses across the country, Clark said he has no problem finding bugs with the genetic makeup that makes them stronger than the pesticides.

"We started hearing about resistance in 1994," Clark said. "I would say that between five and 10 years at the most, they won't be effective at all."

But studying dead head lice tells only so much. To get a sense of how the parasites behave and exactly how they react to pesticides, researchers had to design an environment in which the lice -- which can survive only on a human scalp -- can live.

"Nobody wants these things living on them, so we had to create an artificial scalp to study them while they're alive," Clark said.

Clark and the other researchers, funded by a $500,000 grant from the National Institutes of Health, have been trying to replicate the conditions of a human scalp, where lice can feed on blood and lay eggs.

In a setup that looks nothing like a human head, the lice are placed inside tubes with a tuft of human hair. A thin plastic membrane stretches over the bottom of the tube, fooling the lice into believing it is a scalp. The tube is then lowered into a container of blood, which a louse can feed on through the membrane.

"They lay their eggs there, they raise their kids there and they eat there," Clark said. "They're happy."

Miwa Takano-Lee, a researcher at the University of California at Riverside who helped design the artificial scalp, said the device will help scientists study why lice spread so rapidly among people.

"Everybody assumes they're transported from head-to-head contact," Takano-Lee said. "But they do not jump. . . . They have everything they need when they're on a person's scalp, so it doesn't make sense why they would want to leave."

But the research goes beyond the interest of science.

When students are diagnosed with head lice, some are not allowed to attend class.

In a rush to relieve their children's itchy scalps and anxious to save the hours it could take to comb lice out of their hair, parents often turn to insecticide-based shampoos.

"I speak to so many frustrated parents," said Debrah Altschuler, president of the National Pediculosis Association, a Needham, Mass.-based nonprofit group that advocates against the misuse of pesticidal lice treatments. "They've spent all this money on products that didn't do anything, and their kids still have lice."

Altschuler, who said using chemical lice treatments can cause cancer and other health hazards if used incorrectly, said picking lice and their eggs off a scalp is the best way to get rid of them.

"We are not entirely anti-pesticide or anti-chemical," she said. "But we need to reserve those treatments in a way that they'll be safe and effective."

A convenient solution to lice control might be found in the development of new products that mix different active ingredients, Clark said. By mixing compounds that kill lice differently, it would take the insects longer to develop a resistance than if they were subjected to just a single poison.

"If we're going to get a handle on control, it's going to have to be done through a very controlled prescription use of pesticides," Clark said. "When these things are sold over-the-counter, they're not used properly. And that leads to the problems we're seeing now."

Lice and their eggs are seen on a shock of hair in this microscopic image taken in a laboratory. Entomology professor John Clark and student Kyong Sup Yoon are studying ways to eradicate head lice, parasites that plague millions of schoolchildren.