LOS ANGELES - Wildlife officials pledged this week to be aggressive in their hunt for a destructive species of non-native mussel that they fear could clog pipes and wreak havoc with water lines supplying Southern California. Quagga mussels were found this month at Lake Mead in Nevada and Lake Havasu in Arizona, alarming California officials who worried that the mollusks could damage the system that brings water across the desert to 18 million people in Los Angeles and San Diego.Those discoveries prompted a wider search for the mussels in California reservoirs and pipelines that are connected to the Colorado River Aqueduct, but divers who combed three major lakes did not find any of the mollusks.Bob Muir, a spokesman for the Metropolitan Water District of Southern California, said those results were encouraging. But he cautioned that even if adult quaggas were not in the reservoirs, their larvae could still be present.In March, officials plan to shut down the entire 242-mile aqueduct for three weeks to dry out the canals and use chlorine to kill the mussels. Even so, because the freshwater mollusks multiply rapidly — with a single female laying as many as 1 million eggs — it is unlikely doing so will rid the state of the hardy mollusks."They're extremely difficult to eradicate," Muir said. "It's more a matter of trying to control them."The mussels likely hitched a ride on a boat from the Great Lakes. Because the mussels are young, officials hope to curb the population before it grows.The freshwater mollusks were accidentally introduced to the Great Lakes in the ballast of ships from eastern Europe and the Ukraine. They can plug pipes up to 12 inches in diameter and restrict flow in larger pipes. Their colonies can also cause corrosion in pipes and other underwater structures.Until the quagga mussels were found in Lake Mead, they had not been spotted in the western United States.