The bio-efficacy of four plant leaf extracts viz., apple of sodom, Calotropis procera Aiton; Mexican poppy, Argemone mexicana Linnaeus; Mexican marigold, Tagetes minuta Linnaeus and Indian neem, Azadirachta indica were tested against mustard aphid, Lipaphis erysimi on Indian mustard, Brassica juncea. The highest per cent aphid reduction during first, second and third spray were 28.79, 40.52 and 59.32 at 1 : 10 g/ml; 34.70, 44.49 and 66.14 at 1 : 5 g/ml and 53.88, 64.84 and 100.00 at 1 : 2.5 g/ml with Indian neem. However, Mexican marigold was also effective at highest concentration (1 : 2.5 g/ml) and reduced 96.38 per cent L. erysimi. All the treatments of plant leaf extracts showed insecticidal activity, but Indian neem followed by Mexican marigold reduced the aphid population to a great extent.