Effects of Local Landscape on Population Dynamics of Onion Thrips and Iris Yellow Spot Virus in Onion

Onion thrips, Thrips tabaci Lindeman (Thysanoptera), is a worldwide pest of onion and other crops, and vectors a devastating plant pathogen, Iris yellow spot virus (IYSV). Other plant species in the local landscape may support thrips and IYSV confounding pest management in onions. Five onion fields in Utah were sampled in 2011 to evaluate the effects of distance from the field edge and influence of adjacent crops and weeds on thrips densities and IYSV incidence. The general pattern of thrips populations was an increase from June through early August, and then a decline in mid to late August. Thrips densities varied among fields and over time, however, there was a significant trend (p=0.03) for more adult onion thrips on onions at 3 m than at 63 and 101 m from field edges across all fields. Similar trends were seen in larval densities, both those present on onion at the time of sampling and those that hatched from incubated onion leaves (25˚C). Of the other nearby plants, thrips numbers were highest on alfalfa and weeds (late July), and lowest on wheat and corn. Onion plants tested positive for IYSV, using ELISA (enzyme linked immunosorbent assay), as early as June and incidence increased in July and August with noticeable disease symptoms not becoming evident until late August and early September.  IYSV incidence was highest in plots closer to onion field edges correlating with highest thrips densities. Several weed species tested positive for IYSV and/or were shown to be reproductive hosts of thrips.