Straw Mulch and Reduced-Risk Pesticide Impacts on Thrips and Iris Yellow Spot Virus on Western-Grown Onions

Iris yellow spot virus and its vector the onion thrips, Thrips tabaci Lindeman, are yield-limiting pests of onion, Allium cepa L., throughout the western U.S. In experiments in Colorado during 2005 to 2007, straw mulch applied to the center of onion beds at the early to mid-bulb growth stage reduced abundance of thrips as much as 33% when compared to nontreated plots of transplanted onions. Cumulative thrips-days indicated that straw mulch significantly reduced season-long abundance by 10 to 20% compared with check plots in bare soil. The addition of conventional insecticides (methomyl alternated with lambda-cyhalothrin) was associated with 12 to 27% greater cumulative thrips-days compared to the nontreated check in two experiments. In contrast, a reduced-risk insecticide program (spinosad alternated with azadirachtin) had fewer cumulative thrips-days on both bare soil (15%) and straw mulch (36%) compared to nontreated checks. Enhanced thrips control generally persisted during mid-season and may have contributed to reduced stress from damage by thrips feeding and reduced incidence and/or severity by Iris yellow spot virus during the early to mid-bulb stages of plant growth. Total yield and yield of jumbo-sized onions were increased as much as 13 and 18% by straw mulch compared to bare soil treatments among the individual experiments. Peak abundance of thrips on commercial red onion plants evaluated during 2004 was positively correlated with the incidence of iris yellow spot 40 days (R2 = 0.5864, P = 0.0060) and 54 days (R2 = 0.6086, P = 0.0046) later, indicating that suppressing thrips might provide some control of the disease. Effective long-term management of thrips and iris yellow spot in onion crop systems will depend on a multi-faceted approach that integrates host resistance, modified cultural practices such as straw mulching and irrigation scheduling, and judicious use of reduced-risk insecticides.