Infestation by onion thrips, Thrips tabaci Lindeman, was determined on transplants of onion (Allium cepa L.) received in Colorado during March and April from out-of-state sources (Imperial Valley, CA; near Phoenix, AZ; and southern Texas) during 2004 to 2008. In the 5 years of the study, 50 to 100% of the transplant lots sampled arrived infested with thrips. Among infested transplant lots, the overall number of thrips averaged 0.15 to 0.63 per plant, with as many as four per plant in some lots. T. tabaci was the dominant thrips species in all seasons and locations of transplant origin. In addition, 19 of 83 (23%) tested lots had plants positive for Iris yellow spot virus. Iris yellow spot virus and T. tabaci were detected in volunteer onion plants as early as 1 May, a few weeks after the summer onion crop was planted, suggesting a possible role of infected volunteer plants in perennation of the virus between onion crops. Iris yellow spot virus and T. tabaci were detected in many common weeds including blue mustard (Chorispora tenella (Pall.) DC), common purslane (Portulaca oleracea L.), field bindweed (Convolvulus arvensis L.), flixweed (Descurainia sophia Webb & Berth.), prickly lettuce (Lactuca serriola L.), and redroot pigweed (Amaranthus retroflexus L.) in early spring near onion fields in Colorado during 2006 to 2009. Confirmation that Iris yellow spot virus and Iris yellow spot virus-infective thrips overwintered in volunteer onions and some common winter annual and perennial weeds emphasizes that managing volunteer onions and weeds is important for management of iris yellow spot, in addition to planting transplants free of thrips and the pathogen.