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 e rysimi (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 di fferent conditions. The natural aphid infestation did not affect yield adversely until 80 and 100 days of crop age in 1987-88 and 1988-89 re spectively. Using insecticide application, the significant decline in yield started from 80 and 90 days during 1988-89 and 1989-90 respectiv ely. 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 sp rays of Quinalphos at a 20-day interval. Under natural infestation, th e EIL with the best fitting relationship was 1.2 aphids per plant in 1 987-88 (loglinear), and 14.2 aphids per plant in 1988-89 (quadratic). With insecticide use, EILs for the best fitting relationship (quadrati c) were 14.2 and 68.9 aphids per plant in 1988-89 and 1989-90 respecti vely, The EILs calculated after insecticidal application were generall y higher than that for natural infestation levels for the same forms o f equation. The EILs computed from the natural infestation seem to be more correct and reliable as there was no external interference.