www.nature.com/scientificreports OPEN Moderately decreasing fertilizer in felds does not reduce populations of cereal aphids but maximizes ftness of parasitoids Fei Qiao1,2, Quan‑Feng Yang1,3, Rui‑Xing Hou2, Ke‑Ning Zhang1,3, Jing Li2*, Feng Ge1,3* & Fang Ouyang1* Examination of the tradeof between the extent of decreasing nitrogen input and pest suppression is crucial for maintaining the balance between essential yield and an efcient, sustainable pest control strategy. In this study, an experiment with four manipulated nitrogen fertilizer levels (70, 140, 210, and 280 kg N ha−1 = conventional level) was conducted to explore the efects of decreasing nitrogen on cereal aphids (Sitobion avenae and Rhopalosiphum padi) (Hemiptera: Aphididae), Aphidiinae parasitoids (Hymenoptera: Braconidae: Aphidiinae), and body sizes of parasitoids. The results indicated that nitrogen application, in the range of 70–280 kg N ha−1, has the potential to impact the populations of cereal aphids and their parasitoids. However, both diferences between densities of cereal aphids and their parasitoids in moderate (140–210 kg N ha−1) and those in high nitrogen input (280 kg N ha−1) were not signifcant, and the parasitism rate was also unafected. A higher parasitism rate reduced population growth of the cereal aphid (S. avenae). Additionally, a moderate decrease of nitrogen fertilizer from 280 to 140–210 kg N ha−1 maximized the body sizes of Aphidiinae parasitoids, indicating that a moderate decrease of nitrogen fertilizer could facilitate biocontrol of cereal aphid by parasitoids in the near future. We conclude that a moderate decrease in nitrogen application, from 280 to 140–210 kg N ha−1, does not quantitatively impact the densities of cereal aphids or the parasitism rate but can qualitatively maximize the ftness of the parasitoids. Agricultural intensifcation driven predominantly by human activity is benefcial to increase crop yields to meet human needs for food. Nitrogen fertilizer as a critical ingredient of agricultural intensifcation plays a crucial role in agricultural production1,2. However, overuse of nitrogen fertilizer has detrimental impacts on the global environment, including induction of pest outbreaks, loss of biodiversity and degradation of biological controls3–5. Appropriate nitrogen fertilizer is important to implement sustainable agricultural development. Optimizing application of nitrogen fertilizer to maintain agricultural production and lower the pest impact is of global concern. Winter wheat is the most essential grain crop globally. At present, the magnitude of nitrogen input in wheat feld usually features about 200 kg N ha−1 in Europe2,6, and 300 kg N ha−1 in the North China Plain, the single most important wheat production region in China7,8. A recent study considered the balance between wheat yield and environmental quality, the later featured leaching of soil nitrate into the deeper soil layers, recommend- ing 120–170 kg N ha−1 as an optimal application level 2,9. Accordingly, the range of 120–170 kg N ha−1 could be employed as a preliminary criterion to sustain acceptable grain yield while reducing detrimental environment concerns. It is necessary to evaluate the efect of such a moderate amount of nitrogen input on the population of the main insect pests and their natural enemies in winter wheat. 1State Key Laboratory of Integrated Management of Pest and Rodents, Institute of Zoology, Chinese Academy of Sciences, Beijing 100101, China. 2Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China. 3CAS Center for Excellence in Biotic Interactions, University of Chinese Academy of Sciences, Beijing 100049, China. *email: [email protected]; [email protected]; [email protected] Scientifc Reports | (2021) 11:2517 | https://doi.org/10.1038/s41598-021-81855-8 1 Vol.:(0123456789) www.nature.com/scientificreports/ Cereal aphids, including Sitobion avenae and Rhopalosiphum padi (Hemiptera: Aphididae), cause serious economic yield losses of winter wheat. In terms of cultural control, the efects of nitrogen application on the populations of cereal aphids present mixed results5,10–12. For example, both population of cereal aphids (S. avenae and R. padi) are positively responsive to nitrogen levels in the range of 115–170 kg N ha−1 in northern China5. In contrast, nitrogen fertilizer has a negative infuence on the density of cereal aphid (S. avenae) in Germany11. Considering that the levels of nitrogen input in diferent research studies are varied, it is reasonable to assume that adequate coverage of nitrogen levels accounts for the observed variation. Terefore, it is urgently necessary to conduct related research that considers greater coverage of nitrogen levels to ascertain to what extent, if any, decreasing nitrogen fertilizer could markedly reduce the populations of cereal aphids. In terms of biological control, Aphidiinae parasitoids play crucial roles in natural aphid control5,13,14. Two Aphidiinae parasitoid species, Aphidius uzbekistanicus and A. gifuensis (Hymenoptera: Braconidae: Aphidiinae), predominate in wheat growing area of northern China 15. Tese Aphidiinae parasitoids share both aphid host and hyperparasitoid species 5,15, and are identifed as controphic species 16. Te efects of nitrogen fertilizer on biocontrol by Aphidiinae parasitoids on cereal aphids are varied 5,10,17. For example, the parasitism rate in plots exposed to fertilizer is lower compared with that in fertilizer-free plots in Germany 11, while the parasitism rate increased with nitrogen input in northern China5. A manipulation study in the laboratory showed that parasit- ism rate remained unafected by nitrogen level18. Due to the phenology of cereal aphids and their Aphidiinae parasitoids being closely associated with investigated regions, combined with the fact that adequate coverage of nitrogen levels plays an important role in biocontrol, it is unknown how nitrogen level afects the populations cereal aphids and Aphidiinae parasitoids in cereal felds in northern China. Te body size of parasitoids, especially solitary koinobiont parasitoids, impacts the interaction strength and the structure of the host-parasitoid network19,20. Te hind tibia length and the head width of parasitoids are posi- tively associated with fecundity and ftness of parasitoid adults as well as their ofspring20. Tus, the body sizes of parasitoids characterize the ftness of parasitoids and have been adopted to evaluate the ftness of parasitoids19–21. It has been verifed that the body sizes of parasitoids are stimulated by nitrogen fertilizer application at low aphid densities in the laboratory22,23. However, it is unknown whether such efects of nitrogen fertilizer on the body sizes of parasitoids occur in wheat felds. Systematic tests were conducted to explore the efects of decreasing nitrogen on cereal aphids and Aphidii- nae parasitoids in this study. Tis study combined feld investigation in northern China with measurement of body sizes of parasitoids, under four manipulated nitrogen fertilizer levels (70, 140, 210, and 280 kg N ha−1). Considering the precondition of ensuring the essential grain yield, the goals of this study were to determine to what extent, if any, decreasing nitrogen fertilizer could decrease the densities of cereal aphids and to examine the incidental efects of decreasing nitrogen fertilizer on quantitative and qualitative aspects of the parasitoids. Te specifc goals include ascertaining (1) the efect of decreasing nitrogen fertilizer on the densities of cereal aphids; (2) the efect of decreasing nitrogen fertilizer on the density of parasitoids and the parasitism rate; (3) the efect of decreasing nitrogen fertilizer on the body sizes of parasitoids; (4) the efect of parasitism rate on the population growth of cereal aphid. Results Efect of decreasing nitrogen fertilizer on the densities of cereal aphids. Te densities of cereal aphid S. avenae were decreased signifcantly by decreasing nitrogen fertilizer at both fowering (F3, 22 = 8.62, P = 0.001) and milking (F3, 22 = 4.49, P = 0.013) phases in 2017. However, there were no signifcant diferences between S. avenae densities in the N280 plots and those in the N210 or N140 plots (Fig. 1a). Te diferences of S. avenae densities among all nitrogen levels were not signifcant in either 2016 or 2018, at either fowering or milking phases (Fig. 1a). Te densities of the cereal aphid R. padi in the N70 plots were signifcantly lower than those in the N280 plots at the milking phase in 2016 (F3, 22 = 3.61, P = 0.029), as well as at the fowering (F3, 22 = 3.64, P = 0.028) and milking phases (F3, 22 = 10.98, P < 0.001) in 2017. However, the diferences between R. padi densities in the N280 plots and those in the N210 or N140 plots were nonsignifcant at any phase, including in 2018 when nitrogen levels did not have obvious efects on the densities of R. padi at any phase (Fig. 1b). Efect of decreasing nitrogen fertilizer on the densities of parasitoids and the parasitism rate. Te densities of parasitoids in the N70 plots were signifcantly lower than those in the N280 plots at the milking phase in 2017 (F3, 22 = 6.02, P = 0.004, Fig. 2a). However, the diferences between density of parasitoids in the N280 plots and those in the N210 or N140 plots were nonsignifcant statistically. Te nitrogen levels did not obviously infuence the density of parasitoids in either 2016 or 2018. Nitrogen fertilizer did not signifcantly infuence the parasitism rate in any investigated year (Fig. 2b). Efect of decreasing nitrogen fertilizer on the body sizes of parasitoids. For the parasitoid A. gif- uensis, the hind tibias of the adult males in the N210 plots were signifcantly longer than those in the N70, N140, and N280 plots (F3, 121 = 5.813, P = 0.001, Fig. 3a). Te head widths of adult males in the N210 plots were signif- cantly greater than those in the N70, N140, and N280 plots (F3, 121 = 5.481, P = 0.001, Fig.