Assessment of Economic Threshold and Economic Injury Levels of Thrips tabaci on Onion Plants

: These experiments were conducted in onion fields during 2016/2017 and 2017/2018 seasons for determining the economic damage threshold and economic injury levels of T. tabaci using marking plants technique as well as pesticide application technique. In regard to the results of marking plants, the values of economic damage threshold were ranged between 3-4 individuals/plant, during the 1 st season and 3-5 during the 2 nd season. The relatively low values were noticed during the vegetative and bulbs growth stages of onion plants. So, the Economic Injury Level (EIL) of T. tabaci on onion plants can be determined as the number of thrips follows the value of ET levels for the detected peaks ranged between 4-5 individuals/leaf during the two study seasons. As the results of pesticide application technique, the thrips mean numbers and mean yield of onion plants were correlated significantly during the two study seasons. Although, the mean yield of onion plants differed significantly according to the variation of thrips mean numbers resulted in relatively high effect (R2 = 67.367 and 67.094) for the two respective seasons. Results of chi-square analysis (rx2) indicated that the values of economic damage threshold of T. tabaci infested onion plants ranged between 6 - 8 thrips/plant, while the economic injury level ranged between 8-13 thrips/plant during the two respective seasons.