The application of nitrogen (N) fertilizers to agricultural crops is a common practice globally and a crucial component in the current levels of productivity. Excessive N use, however, is costly and damaging to ecosystems. It is recognized that overuse of N fertilizer can promote pest herbivores by enhancing host plant nutritional quality, but less is known of the effects of N on natural enemies of pests via improved quality and availability of prey, and how these may cascade to indirect effects on pests. Here, we explored the effects of N fertilizer on a key egg parasitiod of Nilaparvata lugens (Stål) (Hemiptera: Delphacidae) (brown planthopper, BPH). Application of N to rice plants significantly prolonged the development time of Anagrus flaveolus (Hymenoptera: Mymaridae), increased wing size and decreased fecundity of adult females. Importantly, N application to rice plants significantly decreased the per capita parasitism of BPH eggs by A. flaveolus. Utilizing planthopper prey on N-treated rice plants led to reduced searching efficiency of A. flaveolus. Ultimately, the strength of biological control exerted by A. flaveolus was negatively affected by nitrogen application under field conditions. Parasitoids were able to discriminate between BPH infested rice plants with different levels of nitrogen using visual plant cues. We conclude that N fertilizer use can have profound effects on natural enemy efficiency which potentially increase the dependence on insecticides, another potentially hazardous input.