Preliminary data in 2004 suggested that wheat straw mulch placed around onion plants significantly depressed thrips populations on the onion plants. Subsequently this was further explored in 2005 in four studies. Also explored were the effects of different mulches on Iris Yellow Spot Virus (IYSV) and onion crop yield. Two trials were conducted at the Agricultural Research, Development and Education Center (ARDEC) of Colorado State University in Larimer County, Colorado, and involved both a field established by transplants and a seeded field. Two other studies were conducted on transplant grower fields within the county. There was a consistent trend in all four 2005 trials towards reduced thrips populations and IYSV incidence. The 2006 data will be included in the presentation.

Key Words: Mulches, Thrips Populations, Iris Yellow Spot Virus, Onion Yield