The spread of the Mediterranean fruit fly throughout Southern California is following a pattern predicted by the principles of invasion biology, according to a UC Davis entomologist, whose claim that the Medfly is established -- rather than being repeatedly reintroduced into the state -- has sparked controversy over how to deal with the pest. "Some people think that the invasion model is based on the ink-in-the-swimming pool concept," says James Carey, an authority on insect demographics. "I don't think it works that way; rather you get linear movement channeled by topography." In this case, the spread of the Medfly appears to be channeled by the mountains rimming the Los Angeles basin, suggests Carey. He predicts the pest is spreading northward along the California coast, as well as inland, and will enter the state's agriculturally rich Central Valley by the end of the decade. Nearly 400 Medflies have been captured in Southern California this year, leaving approximately 1,000 square miles quarantined and preventing the shipment of fresh fruit.