What do molecular methods bring to host–parasitoid food webs?

•Quantitative host–parasitoid food webs are excellent tools to reveal patterns and generate hypotheses in community ecology.•The application of molecular tools is leading to radically more accurate food webs.•Molecular tools allow food webs to incorporate new dimensions (immunology, phylogeny, microbiology).

Quantitative food webs are valuable tools to detect patterns in community structure and generate hypotheses for experimentation. Quantitative webs of whole communities are usually not feasible to build, and most attention focuses on assemblages of species that interact in similar ways. Hosts and parasitoids are a popular guild for study, and quantitative webs have traditionally been constructed by collecting, rearing, and identifying large samples of individuals from the field. In the past decade molecular methods have begun to be used extensively in studies of host–parasitoid webs to clarify species concepts. We review how this rapidly developing field is using molecular information to detect cryptic interactions between species and to increase our understanding of the phylogenetic and mechanistic processes which structure food webs.