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Next‐generation sequencing (NGS) is increasingly used for diet analyses; however, it may not always describe diet samples well. A reason for this is that diet samples contain mixtures of food DNA in different amounts as well as consumer DNA which can reduce the food DNA characterized. Because of this, detections will depend on the relative amount and identity of each type of DNA. For such samples, diagnostic PCR will most likely give more reliable results, as detection probability is only marginally dependent on other copresent DNA. We investigated the reliability of each method to test (a) whether predatory beetle regurgitates, supposed to be low in consumer DNA, allow to retrieve prey sequences using general barcoding primers that co‐amplify the consumer DNA, and (b) to assess the sequencing depth or replication needed for NGS and diagnostic PCR to give stable results. When consumer DNA is co‐amplified, NGS is better suited to discover the range of possible prey, than for comparing co‐occurrences of diet species between samples, as retested samples were repeatedly different in prey detections with this approach. This shows that samples were incompletely described, as prey detected by diagnostic PCR frequently were missed by NGS. As the sequencing depth needed to reliably describe the diet in such samples becomes very high, the cost‐efficiency and reliability of diagnostic PCR make diagnostic PCR better suited for testing large sample‐sets. Especially if the targeted prey taxa are thought to be of ecological importance, as diagnostic PCR gave more nested and consistent results in repeated testing of the same sample. [ABSTRACT FROM AUTHOR]

• Landscape had a lower effect on aphids and their parasitoids than fertilisation. • Fertilisation affected plants positively. • Fertilisation affected aphids and primary parasitism rate negatively. • Fertilisation had no effect on secondary parasitism rate. • H 2 ' and generality between parasitoids are affected by landscape and fertilisation. Agricultural intensification can impact agrobiodiversity in several ways such as in terms of population densities, community composition and food web interactions across all trophic levels. This effect can be investigated at two scales: field-scale and landscape scale. Here it was assessed how the impact of fertilisation (within field) and landscape complexity (within landscape) impact cereal aphid-primary parasitoid-secondary parasitoid systems in winter wheat in Germany. A newly developed molecular technique was used to quantify species-specific linkages between aphids, primary parasitoids and secondary parasitoids sampled in fertilised and unfertilised plots in either simple or complex structured landscapes. The results show a stronger effect of fertilisation than landscape complexity on the groups: fertilisation positively affected the crop plants while it negatively affected both the density of the cereal aphid Sitobion avenae and its primary parasitism rates whereas no effect on the level of secondary parasitism rates was observed. Landscape complexity had no effect on plants, aphids, as well as on primary parasitism rate. In case of secondary parasitism rate there was an effect in interaction with sampling date. Field identity accounted for the strongest effect on parasitoid community composition (10.6% of the variance) from all tested variables, while fertilisation and landscape complexity had almost no effect (1.1% and no effect). Nevertheless, a weak cascading effect of both environmental factors could be observed as the primary-secondary parasitoid network structure responded to both. However, these observed effects on food webs strongly depended on species identity, highlighting the need of species-level food web assessment. [ABSTRACT FROM AUTHOR]