Characterization of resistance to onion thrips (Thrips tabaci Lindeman) and incidence of Iris yellow spot virus in onion cultivars

Onion thrips (OT), Thrips tabaci Lindeman, is the most important insect pest worldwide of onions, Allium cepa L. OT can cause yield losses (>50%) but can be even more problematic when it occurs with Iris yellow spot virus (IYSV). In field studies on onion resistance conducted in 2007 and 2008 using 49 cultivars, 11 showed the lowest leaf damage by OT. Five of them may possess antibiosis/antixenosis to OT because of the lower numbers of OT they sustained. In 2009, 16 of the cultivars were evaluated further to understand the relationship between OT and IYSV infection in more detail. Four plants per cultivar were transplanted into a single pot (20 pots/cultivar), and 4 wks later moved to a field where IYSV was present. After 7 and 14 d, larvae were counted and plants tested for IYSV using the DAS-ELISA test. Complementary experiments to assess transmissibility were conducted in the greenhouse by infesting each pot with 10 OT from IYSV-infected fields. Additionally, categories of resistance to OT were examined by performing non-choice tests in which plants were grown in cages and OT adults were introduced and the progeny counted. Choice tests were performed by planting all the cultivars together in a single pot and releasing OT adults on a filter paper placed at the center of the circle of plants and assessing their preference. Results from these studies will be discussed in relation to the categories of resistance observed in the laboratory and to the field results from 2007 and 2008.