The department is mandated to eradicate certain exotic pests. The location of the first find of Whitestem distaff thistle in the Western Hemisphere was in Sonoma County. This broadleaf weed has been eradicated. Tulip tree scale, a serious pest on tulip trees, deciduous magnolia and walnut, has been eliminated from the County. Iberian star thistle has been reduced to a small fraction of its original numbers. Hydrilla, a pest of waterways was found in and eradicated from Spring Lake.