1. Agricultural land use threatens ecosystem services such as biological control by natural enemies because of simplification of habitat structure and intensification of disturbance and agrochemical inputs. Low parasitism rates of agricultural pests have typically been attributed to a lack of resources for parasitoids in highly simplified landscapes, but this could be confounded by the nearly ubiquitous correlation between landscape complexity and the cover of intensively farmed agricultural crops.
2. Here, we disentangle the mechanisms driving landscape-scale effects on host–parasitoid interactions by taking advantage of a landscape modification gradient in which the diversity of habitat types and annual crop cover in the landscape were uncorrelated. We quantified herbivore densities and parasitism and hyperparasitism rates on two important crop pests (aphids and Plutella xylostella) across 30 landscapes. We used structural equation modelling (SEM) to test whether land-use intensity (insecticide application and habitat disturbance) or resource availability for parasitoids (floral resources and alternative host plants) was mediating the effects of habitat diversity and annual crop cover on the landscape.
3. Rates of primary- and hyperparasitism of aphids and primary parasitism of P. xylostella decreased with increasing annual crop cover, whereas decreasing habitat diversity in the landscape had little effect on parasitism rates. These effects were mediated almost entirely by greater habitat disturbance and greater frequency of insecticide application, rather than by changes in resource availability.
4. Parasitoids were more sensitive to intensive farming practices than were their herbivore hosts, and in turn hyperparasitoids were more sensitive than were primary parasitoids. This supports the theoretical prediction that higher trophic levels should be increasingly sensitive to the disturbances associated with land-use change.
5. Synthesis and applications. Our work suggests that increased land-use intensity (e.g. higher insecticide inputs and greater levels of disturbance associated with increasing area of annual crops) has been underestimated as a driver of landscape effects on host–parasitoid interactions. These findings have important implications for the maintenance of ecosystem services such as biological control. The promotion of low-intensity farming practices that limit the extent and frequency of agrochemical inputs and habitat disturbances will be essential for the maintenance of effective biological control by parasitoids in agroecosystems.