Host–parasitoid networks across gradients of garden management and urban landscapes

Biological pest control relies on interactions between herbivores and their natural enemies. Maintaining this ecosystem service requires considering herbivore and natural enemy interactions and their response to anthropogenic change at multiple scales. In this study, we use ecological networks to quantify the network structure of interactions between herbivorous hosts and parasitoid wasps, and we relate network structure to local habitat, landscape context, and parasitism rates. First, we ask how do the local habitat and the landscape surrounding gardens affect 1) host abundance, 2) parasitism rates (at the site, plant, individual, and population levels), and 3) host–parasitoid network structure? Additionally, do network structure metrics predict parasitism rates? We sampled herbivores of Brassica oleracea at 22 urban gardens in the California central coast. We reared parasitoids from herbivores and identified all to morphospecies. For each garden, we measured local habitat characteristics (e.g. vegetation, ground cover, canopy cover), quantified the surrounding landscape (e.g. urban, natural, open, and agricultural cover), and calculated three network structure metrics (i.e. interaction richness, vulnerability, and functional complementarity). We used generalized linear and mixed models to examine relationships between host abundance, host parasitism, garden local and landscape characteristics, and network metrics. We sampled 9 herbivore morphospecies (n = 60,245) from which we reared 23 parasitoid morphospecies (n = 612). We found local and landscape effects on host abundance and parasitism, while only local factors affected network structure. Higher network interaction richness correlated with enhanced parasitism rates for two host species and lower parasitism rates for one host species.