Biological control of yellow starthistle in western rangelands: Implications for grazing

In the semi-arid western United States range and grasslands are among the most abundant agro-ecosystems. The herbaceous annual yellow starthistle (Centaurea solstitialis L., YST), introduced as contaminant of hay from Europe in the mid-1800s, is a common and economically important invader of these agro-ecosystems. Five seed consuming insect biological control agents have been introduced in the United States to control YST, which reproduces solely through the production of large numbers of seeds. We assessed and assigned the net seed reduction of biological control agents at field sites in northcentral Idaho during 2005 and 2006. We also followed YST densities and vegetation community cover at field sites between 2005 and 2010 in a set design experiment in which different clipping and biocontrol exclosure or augmentation treatments were applied. We finally compared for 5 field sites historic YST density data from 1997 with YST densities and biocontrol parameters in 2009 and 2010, respectively.  While there are conflicting accounts about the efficacy of YST biocontrol, we conclude that despite a high average annual seed reduction of approximately 80%, the weed’s population biology may not be impaired in the short term by that seed loss. Cattle typically avoid YST and deer and elk preferentially graze on native forbs, both of which facilitate YST. We discuss how grazing goals of ranchers and biocontrol may be combined to maximize the management of YST.