One of the important pests of onion which causes reduction in yield as well as in quality is Thrips tabaci Lind. In order to study the population fluctuations in onion thrips as well as its effect on bulbing and yield in onion plants, nine selected cultivars were employed. Seeds were planted in plots as two separate experiments (sprayed and non-sprayed) in a randomized block design with four replications. Samples were taken from each plot weekly with number of thirps, fresh and dry weigth and bulbing ratio of plants being determined. Also, after harvest, the yield and the effect of spraying on the cultivars were determined. Data from each experiment were separatly analysed as split plot in time and then combined. Spraying caused reduction in population density, increased fresh and dry weight, reduced bulbing ratio but did not affect yield. Significant differences were observed between cultivars as regards population density in onion thrips. Maximum population density of thrips was observed in Azarshahr, whereas Ghom, Kashan, Kavar, Abarkooh and Yellow Sweet Spanish cultivars carried a low population of thrips. Also, cultivars were significantly different regarding fresh and dry weigths of plants, bulbing ratio as well as yield. Among cultivars, Kavar bore a minimum of fresh and dry weigth, and yield whereas Abarkooh the maximum. There was a significant interaction observed between spraying and cultivar for all traits. Comparison of traits at different times of sampling indicated a significant difference in population density, fresh and dry weigth, and bulbing ratio, so that, 130 and 158 days after planting, a maximum population density was recorded. Fresh and dry weigth as well as bulbing ratio were shown to increase in the course of time. There was a significant interaction observed between spraying and time of sampling for most traits, i. e. the effect of spraying was different in samples taken at different times.
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