Yellow star thistle is native to Eurasia and has spread from west coast ballast areas to sea ports throughout California and the Pacific Northwest. California has about 800 million acres infested; Idaho 1,128,800 acres; Oregon 3,950 acres; Washington 133,380 acres. In California the area infested has increased more that six times since 1958.  Contaminated seed and feed are important sources of spread. Livestock, farm equipment, and vehicles, wind, animals, birds, and man may also be responsible for the spread.  
Yellow starthistle is among the top 10 worst weeds in several western states and covers tens of millions of acres. It can kill horses that eat it, and other livestock will eat only small portions of very young plants. The weed displaces other plants as it spreads, and turns prime grazing land into worthless weed pastures.
But a small beetle named Eustenopus villosus is beginning to tame yellow starthistle. Female beetles lay eggs in older buds and the larvae eat most of the maturing seeds before they can disperse. 
The beetle is the latest of five natural enemies imported by the Agricultural Research Service from the weed’s homeland in Eurasia. The insects were released after years of safety and efficacy testing by several ARS scientists. Cooperators, such as University of Idaho researchers, monitor and redistribute insects from the initial release sites. 
Biological control is viewed as the best long-term strategy for managing the weed. Once it takes over, physical controls like cutting or pulling it can’t keep up. Chemical control is too expensive for widespread use on rangeland and may be environmentally undesirable.  Courtesy of Washington State Department of Agriculture 
and 
Kathryn Barry Stelljes of Agricultural Research Service of the USDA