Effects of landscape composition on crop yield mediated by specialist herbivores

Landscape composition affects a variety of arthropod-mediated ecosystem services such as pollination and biological pest control. Yet, the ecological mechanisms that determine how different habitats influence herbivore-natural enemy dynamics, and ultimately crop production are still poorly understood. Here, we examined how landscape composition influences the incidence of two specialist cruciferous pests (flea beetles and leaf-feeding Lepidoptera), and cabbage yield across a gradient of landscape complexity in upstate NY, USA. We expected that an increasing proportion of crop land in the landscape would lead to an increase in pest pressure. However, we found no influence of crop land area on pest pressure or cabbage yield. Rather, pest pressure was best explained by the presence of non-crop habitats (i.e. semi-natural habitats and grasslands) in the landscape. Specifically, we found that landscapes with a higher proportion of semi-natural habitats resulted in fewer Lepidoptera eggs and larvae, possibly due to a landscape indirect effect on the abundance and activity of their predators. An increasing area of grassland correlated positively with the abundance of flea beetles suggesting a spillover of herbivores from grasslands to managed crops. More interesting, we found that cabbage fields surrounded by grassland tended to suffer higher levels of plant damage by flea beetles, and have lower yields. In short, the results of our experiments provide empirical evidence that landscape composition can influence pest populations via spillover from non-crop habitats, with cascading effects on crop productivity.