Molecular assessment of food web dynamics identifies critical periods for managing resilience in biological pest control

Food webs are not static over time, but our knowledge of their dynamics is extremely scarce due to methodological challenges. In turn, this significantly limits our ability to mechanistically understand the temporal changes that trophic networks annually undergo. Here, we address this gap using DNA-based diet analysis to measure the season-wide dynamics of trophic interactions between invertebrate generalist predators, pest, and alternative prey in replicated cereal fields across 2 years. We used the level of food web specialization as a proxy for predator diet overlap in pest control and hypothesized that it would reach its minimum at the middle point of the season, when primary production should be higher (H1). Conversely, invertebrate diversity would reach its maximum during the same period (H2). Additionally, alternative prey availability would be indirectly increased by adding manure to half of each field to test if this would reduce specialization and increase diversity (H3). In line with our predictions, food web specialization was lowest during the middle of the season, when prey, but not predator, diversity reached its maximum. No significant effects of manure addition were found on food web specialization. Our findings suggest early and late season in cereal systems as the times when generalist predators are behaviorally most constrained, pinpointing these as periods when the pests are eaten by a smaller subset of the predator community. Hence, molecular trophic analyses provide unique insights into the temporal dynamics of food webs and their properties. This allows the generation of temporal roadmaps for when management interventions are expected to be most effective.