Yellow starthistle, Centaurea solstitialis, is one of the most pervasive and damaging weeds in western USA. It has been a target for biological control for a half-century. In 1991, while releasing the approved Chaetorellia australis (Diptera: Tephritidae), a second fly species, Chaetorellia succinea, was also inadvertently released in southwestern Oregon. This second fly established and spread rapidly, and is now the most common natural enemy of yellow starthistle in California. Earlier, in our laboratory tests, we showed that Ch. succinea might also attack a native knapweed, American basketflower, (Centaurea americana). During 2006, we grew ‘trap plants’ of two varieties of American basketflower at three field locations in northern and central California where Ch. succinea was abundant. At all three locations, American basketflower was infested by this fly, but seed reduction was much less than on yellow starthistle.