Onion production in the United States is seriously affected by the tospovirus Iris Yellow Spot (IYSV), whose symptoms include lenticular-shaped lesions that reduce photosynthesis and bulb yield. Thrips tabacai Lindeman, onion thrips (OT), is the only known vector of the disease and a primary arthropod pest of onion. Frequent insecticide applications, increasing resistance in OT populations to insecticides, high nitrogen (N) fertilization rates and loss of yield to disease and insect pressure threaten sustainable onion production. The objectives of this study were to identify crop management strategies to enhance onion productivity while suppressing OT and IYSV. Three fertilizer rates and two crop rotations were assigned to replicated plots to assess effects on onion growth, yield, bulb storage quality, soil quality, thrips populations and IYSV incidence. Trap crops of carrot, buckwheat and lacey phacelia were established in commercial fields to evaluate impact on thrips populations and IYSV occurence. Reduced nitrogen (N) rates, one-third the standard grower rate (133.8 kg N ha-1), resulted in no yield loss as compared with the standard N rate, despite slower crop maturation. Onions treated with a standard N rate(401.8 kg N ha-1) had greater numbers of adult and immature OT than other treatments. Soil nitrate levels were lower and microbial activity measured as dehydrogenase and biomass were greater in reduced N treatments. Plots with buckwheat and phacelia had greater numbers of both adult and immature OT when trap crop apparancy was high (i.e. when onion plants were relatively smaller). There was no observed effect of trap crops on IYSV levels. Results suggest that reduced rate N applications lower numbers of OT while enhancing the microbial population, reducing potential for nitrate leaching while still maintaining yields. Potential for trap crops of buckwheat and lacey phacelia to attract onion thrips from onions exists with successive stands of highly apparent trap crops.

Onion thrips (Thrips tabaci) and Iris Yellow Spot Virus (IYSV) constitute a pest complex of global concern for allium growers. Yield losses due to onion thrips alone can be heavy, and in tandem with IYSV, these losses can be complete. This study was one phase of a multi-phase research project intent on exploring the potential drivers of thrips and virus infestation in onion. Preliminary observations in participating growers’ fields in northern Utah pointed towards a potential link between weedy field borders and thrips/IYSV infestation in onion. Field work was carried out at the Kaysville experiment station examining the possible relationships between thrips and IYSV incidence in onion and the presence of adjacent weed populations. This research was conducted with the intent of guiding growers’ decisions regarding field border management, and to provide additional insights into onion thrips behavior. While implications for growers remain unclear, thrips preferences regarding host plant utilization were identified, as well as preliminary evidence of additional plant species capable of hosting IYSV.