Effects of local landscapes on thrips populations and iris yellow spot virus incidence 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. Ten onion fields in Utah were sampled in 2011 and 2012 to evaluate the influence of distance from the field edge and presence of adjacent crops and weeds on thrips densities and IYSV incidence. Thrips densities varied among fields but showed a general increase from early season to peak in mid-July to early August. There was a significant trend for more adult onion thrips on onions at 3 m than at 101 m from field edges across all fields. Similar trends were seen in thrips egg and larval densities, both those present on onion at the time of sampling and those that hatched from incubated onion leaves. Of the other nearby plants, thrips numbers were highest on alfalfa and weeds, and lowest on wheat and corn. Onion plants tested positive for IYSV, using ELISA (enzyme linked immunosorbent assay), starting late May (2012) and early June (2011) and incidence increased in July and August with noticeable disease symptoms becoming evident in late August and September.  IYSV incidence was highest in plots closer to onion field edges correlating with highest thrips densities. Several weed species such as Malva neglecta,  Lactuca serriola, Taraxacum officinale, and Convolvulus arvensis, tested positive for IYSV and/or were shown to be reproductive hosts of thrips.