This study was conducted in kewet district of Amhara region, Ethiopia. The experiment on seasonal thrips population dynamics and onion yield loss was conducted in 2003 and 2004. The treatments were five transplanting dates of two month interval with and without spray of insecticide in five replications. Thrips count per plant was low during the rainy and cool months of July to November and in April. Thrips population started increasing in December and reached the pick value of 159 thrips per plant in March. Thrips population on onion is high from six to ten leaves growth stages. Damage caused by thrips on onion significantly varies with growing season. More yield loss was observed in onion transplanted during dry seasons (October to February). The highest total bulb and marketable bulb yield loss of 16.9 and 17.7 tons/ha occurred on December transplanted onion. Onion from six to ten leaves growth stage is the most critical period for thrips control. These results are important for the management of onion thrips by growers.
Key word: Onion thrips, population pick, bulb yield loss, insecticide, season.