Yellow starthistle (Centaurea solstitialis L.) is an invasive weed of significant importance on rangelands in the western United States. Field experiments were conducted in 2003 and 2004 to determine the effect of targeted grazing on yellow starthistle growth and bud production, and on the efficacy of four established biological control seed-head–feeding insects, which included three species of weevils and one fly species. We tested sheep and cattle grazing at three yellow starthistle growth stages—rosette, bolting, and late bud—at a site where all four biocontrol agents were established. The timing of grazing had a greater impact on yellow starthistle growth and bud production than the type of grazing animal. In comparison to the control, grazing at the rosette and bolting stage resulted in shorter plants both years of the study, but increased the number of buds following grazing at the bolting stage and at the rosette stage in 2003. Negligible seed production across treatments, in 2003, precluded detection of treatment effects. However, in 2004, grazing at the rosette and bolting stages resulted in a greater number of seeds per plant compared to the control and the late bud stage, which were similar. Results indicated that the timing of grazing did not negatively impact biocontrol efficacy. Eustenopus villosus adult injury and total insect larval damage were similar to control plants following each grazing treatment both years, indicating potential compatibility between targeted grazing and biocontrol for integrated management of yellow starthistle.
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