Yellow star-thistle (YST) is a widespread, non-native weed in the western U.S. and currently expanding in southern California, including Ventura Co. where it infests drier agricultural fields and grasslands, interferes with biodiversity in floodplains and promotes wildfire in upland landscapes. Host-specific, seedhead-feeding insects have been released for biological control of YST and closely related Malta star-thistle (MST), but with only minor impact. A root crown-feeding insect, the rosette weevil, Ceratapion basicorne, was recently approved for use in California and is anticipated to improve weed suppression, but has only been tested in two locations in northern California. We propose to conduct field trials with C.basicorne in southern California to determine if it can establish and suppress YST under our environmental conditions, and further to evaluate its potential to control MST. If so, we would subsequently develop a public relations exercise to explain the biology and impacts of YST and MST, and how biocontrol using the rosette weevil can be implemented Region-wide.