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.