ECONOMIC INJURY LEVELS OF RAPESEED (BRASSICA-CAMPESTRIS) APHIDS (LIPAPHIS-ERYSIMI) DETERMINED ON NATURAL INFESTATIONS AND AFTER DIFFERENT INSECTICIDE TREATMENTS

Economic injury levels (EILs) were determined for the aphid Lipaphis erysimi (Kaltenbach) infesting rapeseed variety DYS-3 of yellow sarson under natural infestation during 1987-88 and 1988-89 and with the use of insecticide during 1988-89 and 1989-90 to compare the EILs under different conditions. The natural aphid infestation did not affect yield adversely until 80 and 100 days of crop age in 1987-88 and 1988-89 respectively. Using insecticide application, the significant decline in yield started from 80 and 90 days during 1988-89 and 1989-90 respectively. In a yield-infestation study the period of crop growth between 70 and 100 days was most crucial, when the crop required at least two sprays of Quinalphos at a 20-day interval. Under natural infestation, the EIL with the best fitting relationship was 1.2 aphids per plant in 1987-88 (loglinear), and 14.2 aphids per plant in 1988-89 (quadratic). With insecticide use, EILs for the best fitting relationship (quadratic) were 14.2 and 68.9 aphids per plant in 1988-89 and 1989-90 respectively, The EILs calculated after insecticidal application were generally higher than that for natural infestation levels for the same forms of equation. The EILs computed from the natural infestation seem to be more correct and reliable as there was no external interference.