The effect of different percentages of infested sugar beet leaves by beet fly, Pegomya mixta ViII., (% ISLP) starting from 0 till 72.73% on sugarbeet yield and its components were studies. The increase in % ISLP decreased root, leaves, theoretical sugar or gross yield, and increased root: leaves ratio or % root/gross yield. No appreciable changes due to % ISLP were observed in % sucrose and % T.S.S. as well. The injury levels of P.mixta were 24.24, 12.12, 42.42, 12.12, 24.24 and 24.24% for root, leaves, sugar, gross yield, root: leaves ratio and % root/gross yield, respectively. The average economic threshold for P.mixta on sugarbeet was 46% infested leaves that ranged between 44 to 48%. This equation should be changed by using the following formula to determine the annu‌al economic threshold as follow: A.C.I.C.) E.T. [(0.13345] / [ 0.0785 ] ( A.T.R.P.) E.T = Economic threshold (% infested sugarbeet leaves by beet fly) A.C.I.0 = Average cost of insect pest control A.T.R.P = Average ton root price according to sugarbeet varieties and weather conditions. From the obtained data, it is important to point out that the farmer should not appy insecticides for controlling such insect, if % ISLP is less than 46.