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Carabids are abundant in temperate agroecosystems and play a pivotal role as biocontrol agents for weed seed and pest regulation. While there is good knowledge regarding their effects on invertebrate pests, direct evidence for seed predation in the field is missing. Molecular approaches are ideally suited to investigate these feeding interactions; however, the effects of an omnivorous diet, which is characteristic for many carabid species, and seed identity on the detection success of seed DNA has not yet been investigated. In a series of feeding experiments, seeds of six different Central European weed species were fed to beetles of the species Pseudoophonus rufipes, to determine post‐feeding seed DNA detection rates and how these are affected by plant identity, meal size, and chemical seed composition. Moreover, we investigated the effect of a mixed diet of seeds and mealworm on prey DNA detection. Four out of six seed species were detectable for up to five days after consumption, and seed species identity significantly affected post‐feeding detection rates. Detectability was negatively influenced by protein content and seed mass, whereas oil content and meal size had a positive effect. The mixed diet led to both increased detection rates and post‐feeding detection intervals of seed DNA. This suggests that mixed feeding leads to an enhancement of food detection intervals in carabid beetles and that seed identity, their chemical composition, and meal size can affect DNA detection of consumed seeds. These aspects and potential implications of this non‐invasive approach are discussed as they can become highly relevant for interpreting field‐derived data.

Molecular methods have become an important tool for studying feeding interactions under natural conditions. Despite their growing importance, many methodological aspects have not yet been evaluated but need to be considered to fully exploit the potential of this approach. Using feeding experiments with high alpine carabid beetles and lycosid spiders, we investigated how PCR annealing temperature affects prey DNA detection success and how post-PCR visualization methods differ in their sensitivity. Moreover, the replicability of prey DNA detection among individual PCR assays was tested using beetles and spiders that had digested their prey for extended times postfeeding. By screening all predators for three differently sized prey DNA fragments (range 116-612 bp), we found that only in the longest PCR product, a marked decrease in prey detection success occurred. Lowering maximum annealing temperatures by 4 °C resulted in significantly increased prey DNA detection rates in both predator taxa. Among the three post-PCR visualization methods, an eightfold difference in sensitivity was observed. Repeated screening of predators increased the total number of samples scoring positive, although the proportion of samples testing positive did not vary significantly between different PCRs. The present findings demonstrate that assay sensitivity, in combination with other methodological factors, plays a crucial role to obtain robust trophic interaction data. Future work employing molecular prey detection should thus consider and minimize the methodologically induced variation that would also allow for better cross-study comparisons.

The applicability of species-specific primers to study feeding interactions is restricted to those ecosystems where the targeted prey species occur. Therefore, group-specific primer pairs, targeting higher taxonomic levels, are often desired to investigate interactions in a range of habitats that do not share the same species but the same groups of prey. Such primers are also valuable to study the diet of generalist predators when next generation sequencing approaches cannot be applied beneficially. Moreover, due to the large range of prey consumed by generalists, it is impossible to investigate the breadth of their diet with species-specific primers, even if multiplexing them. However, only few group-specific primers are available to date and important groups of prey such as flying insects have rarely been targeted. Our aim was to fill this gap and develop group-specific primers suitable to detect and identify the DNA of common taxa of flying insects. The primers were combined in two multiplex PCR systems, which allow a time- and cost-effective screening of samples for DNA of the dipteran subsection Calyptratae (including Anthomyiidae, Calliphoridae, Muscidae), other common dipteran families (Phoridae, Syrphidae, Bibionidae, Chironomidae, Sciaridae, Tipulidae), three orders of flying insects (Hymenoptera, Lepidoptera, Plecoptera) and coniferous aphids within the genus Cinara. The two PCR assays were highly specific and sensitive and their suitability to detect prey was confirmed by testing field-collected dietary samples from arthropods and vertebrates. The PCR assays presented here allow targeting prey at higher taxonomic levels such as family or order and therefore improve our ability to assess (trophic) interactions with flying insects in terrestrial and aquatic habitats.

Species are embedded in complex networks of ecological interactions and assessing these networks provides a powerful approach to understand what the consequences of these interactions are for ecosystem functioning and services. This is mandatory to develop and evaluate strategies for the management and control of pests. Graphical representations of networks can help recognize patterns that might be overlooked otherwise. However, there is a lack of software which allows visualizing these complex interaction networks. Food Web Designer is a stand-alone, highly flexible and user friendly software tool to quantitatively visualize trophic and other types of bipartite and tripartite interaction networks. It is offered free of charge for use on Microsoft Windows platforms. Food Web Designer is easy to use without the need to learn a specific syntax due to its graphical user interface. Up to three (trophic) levels can be connected using links cascading from or pointing towards the taxa within each level to illustrate top-down and bottom-up connections. Link width/strength and abundance of taxa can be quantified, allowing generating fully quantitative networks. Network datasets can be imported, saved for later adjustment and the interaction webs can be exported as pictures for graphical display in different file formats. We show how Food Web Designer can be used to draw predator–prey and host-parasitoid food webs, demonstrating that this software is a simple and straightforward tool to graphically display interaction networks for assessing pest control or any other type of interaction in both managed and natural ecosystems from an ecological network perspective. Electronic supplementary material The online version of this article (doi:10.1007/s10340-015-0686-7) contains supplementary material, which is available to authorized users.