evaluation of resistance to thrips (thrips tabaci lindeman) in improved sefid-e-khomein genotype and some other iranian onion cultivars

thrips tabaci is one of the most important pests causing serioues damages on onion s in iran despite several chemical sprayings. to assess the population density and damage of the thips on onion, improved sefid-e-khomein genotype along with seven other iranian cultivars of onoin were planted in a rcb design with four replications in two separate experiments (sprayed and non-spray ed ) in arak for three years (2001-2003). the control field was also planted by ten meters distance and sprayed with current insecticide in order to reduce thrips damages. the highest number of thrips were counted on ghermez-e-azarshar, sefid-e-shahroud and ghermez-e-shahroud cultivars (27.6, 25.7 and 25.3 thrips per plant, respectively) and the lowest on numbers were recorded on sefid-e-qom and sefid-e-kordestan (16.01 and 15.8, respectively). amount of damage was highest on ghermez-e-azarshahr and the lowest sefid-e-kordestan and sefid-e-qom cultivars. in non-sprayed plots, the highest yield was recorded for sefid-e-kashan and sefid-e-qom cultivars (79.2 and 75.1 tha-1, respectively) and the lowest yield for sefid-e-kordestan cultivar (61.0 tha-1). based on the average density of thrips, average damages and curling scores and average yield in sprayed and non-sprayed plots, the improved sefid-e-khomein genotype was found to be tolerant to the thrips.