Beneficial insects in urban environments: Parasitism rates increase in large and less isolated plant patches via enhanced parasitoid species richness

In urban habitats, plant resources for herbivorous insects and their parasitoids are spatially subdivided and embedded in a matrix of built environment. These conditions could affect insect colonization and persistence, leading to altered trophic interactions. Here we tested whether spatial attributes (size and isolation) of patches of an urban spontaneous plant (Commelina erecta) affect the parasitism rates on a specialist leaf miner (Liriomyza commelinae) from its diverse parasitoid assemblage. To achieve this goal we performed an observational study in Cَrdoba city (Argentina), by sampling 893 naturally established C. erecta patches on 18 neighborhoods. Overall parasitism rates of the leaf miner augmented at increasing patch size and decreasing patch isolation, and those effects were mediated by changes in parasitoid richness. Leaf miners in larger or less isolated plant patches supported more parasitoid species which, independently of host abundance, led to higher parasitism rates. Individual parasitoid species showed different responses to patch attributes, with some species being more efficient in large or less isolated plant patches whereas parasitism rates of other species were independent of patch attributes. Our results highlight the importance of spatial availability of urban plant resources for parasitoid diversity and for the key process of biological control in which they are involved. Understanding how natural enemies respond to the patchiness of herbivore resources in the urban context could contribute to the development of strategies to preserve beneficial insects in these highly modified landscapes.