Abstract Fertilizers utilization does not only play an important role in increasing the crop yield, but also they may influence the plant nutritional quality, thereby it affects the ecological interactions between herbivores and their natural enemies. In a field study, the effect of nitrogen fertilization (0, 75, 150 and 225 kg N ha−1) on population of mustard aphid, Lipaphis erysimi Kalt. (Hemiptera: Aphididae) and predatory gall midge, Aphidoletes aphidimyza (Diptera: Cecidomyiidae), in canola (Brassica napus L.), as well as their interactive impact on the plant yield was investigated. The population of aphid and predator were positively affected by nitrogen fertilization; the maximum abundances of which were observed in plants with 150 and 225 kg N ha−1 applied. However, predator to prey ratio was lower in the highest N level in comparison with the moderate level. The maximum suppression of the aphid by the predator was also observed in the moderate N level. The interactive effect of nitrogen fertilizing and the predator presence positively influenced the canola grain yield. The lowest yield losses due to aphid infestation were observed in plants fertilized with 150 kg N ha−1 in the presence of the predator. In this fertilization level, the plants could compensate the aphid damage more than the other treatment levels; hence, they produced significantly more grain yield. These findings indicate that adequate plant nutrition can help plants compensate for aphid damage and also positively affect the predator efficiency, leading to an improvement in the canola yield.

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