Abstract
The study was carried out to determine the population growth of aphids and their effect on photosynthesis and yield components of late sown varieties of canola (Brassica napus L.) during the growing season of 2010-2011 at Multan, Pakistan. Canola varieties Can Raya, Con-I, Con-II and Westar were sown in a Randomized Complete Block Design with three replications. Six rows were planted in each plot of respective varieties. Two rows were sprayed with imidachloprid 20 SL @ 80ml/acre twice during the season, two were sprayed Sour times (aphid protected) and two were not sprayed (aphid infested) in each plot The mustard aphid, Lipaphis erysimi (Kalt) was the most abundant species on the crop. The cabbage aphid, Brevicoryne brassicae L. was also found in very small numbers. Peak aphid population was on 19th February on the varieties Can Raya and Con-I, whereas on Con-II and Westar the peak population was noted on 5th March. Aphid numbers per plant did not vary among different varieties in both sprayed and unsprayed plots. Photosynthetic rate and chlorophyll was lower in aphid infested plants as compared to that in aphid protected plants in all the varieties. However, water use efficiency, internal CO2 concentration and transpiration rate did not differ in aphid infested and aphid protected plants in any of the varieties. There was a significant yield reduction in aphid infested plots when compared with plots where two sprays were applied in all varieties. Application of two insecticides to late sown B. napus varieties could not recover the total losses caused by aphids. Highest yield was recorded in variety Con I, where plants were sprayed twice and not sprayed. Westar suffered the highest losses in yield from aphids.