The leader of the nation’s largest sheriff’s department expects federal drug agents will attempt to step up marijuana enforcement as California moves forward with legalization. But he believes there isn’t the manpower to conduct widespread raids on growers and businesses selling marijuana.

In a wide-ranging interview with The Associated Press on Tuesday, Los Angeles County Sheriff Jim McDonnell also decried California legislation that would make the state a sanctuary for immigrants in the country illegally by limiting how much local law enforcement agencies can work with federal immigration authorities. McDonnell said he doesn’t want his deputies acting as de facto immigration agents, but he believes the bill goes too far and would hamper cooperation on counterterrorism and gang initiatives.

McDonnell said he expects legalization of recreational marijuana to bring additional challenges for his deputies, who patrol nearly 4,000 square miles in Southern California, from an increase in fatal traffic collisions...