The spatial dynamics of nutrient limitation in communities of generalist predators of crop pests

Generalist arthropod predators vitally control crop pests, but their ability to perform this role relies on them obtaining sufficient nutrients within simplified crop environments. Field margins are thought to provide habitat and resources for predators, such as ground beetles, but how far into fields this provision extends is poorly understood. Since arthropod nutrient contents can vary significantly over small spatial scales, resource scarcity within arable monoculture field interiors may drive nutrient limitation, impacting the fitness of natural enemies of crop pests. Integrating field surveys, colorimetric assays and laboratory feeding trials, we investigated how distance into crop fields and crop stage affect nutrient limitation of ground beetles within arable crop fields. We specifically compared the community structure, nutrient contents and nutritional preferences of beetles at different distances into crops, and between crops before and after harvest to determine likely impacts to nutrient limitation. Ground beetle communities and nutrient contents varied with distance into the crop and crop stage, but variation in nutritional preferences in relation to distance into the crop was species-specific and dependent on crop stage. Nutritional feeding trials indicated broad lipid limitation across all samples in the study. These findings indicate taxon-specific variation in the nutritional dynamics of ground beetle communities at different distances into fields, with some species potentially being better adapted to crop interiors and post-harvest landscapes. Increasing in-field proximity to semi-natural habitat through intercropping and similar management strategies could support nutritionally healthier populations of natural enemies with likely benefits to pest suppression.