California does not recognize international driving permits (IDPs) as valid driver’s licenses. They do, however, recognize a foreign license issued in another country, state, or territory where the license holder resides.
In California, an IDP is only used for translating information on a foreign driver’s license. So it can be helpful in that sense to bring along if you already have one.
The bottom line: Your Italian license is sufficient to drive in California, but you can carry an IDP for translation purposes.