Influence of native North American prairie grasses on predation of an insect herbivore of potato

Replacing cropland with non-crop vegetation can affect predation of crop pests through multiple mechanisms, with varying implications for management. Prairie grasses have been planted for on-farm conservation across the United States. We examined the potential impacts of replacing cropland with prairie grasses on predation of Leptinotarsa decemlineata (Say), examining underlying mechanisms. Experiments disentangled changes in the area and edge-to-area ratio of potato patches from the addition of prairie grasses and determined if effects were mediated through herbivore or predator trophic levels. In 2007 and 2008, we compared predation of L. decemlineata eggs, egg and adult densities, and densities of natural enemies between (1) 15 × 20 m monocultures of potatoes, (2) smaller patches (2 × 20 m) surrounded by 15 × 20 m of bare ground and (3) small patches in a 15 × 20 m background of prairie grass. Predation increased fourfold in potatoes surrounded by native grasses compared to potato monocultures, an effect not explained by changes in potato area. Moreover, predation was elevated along edges of monoculture plots within 5 m of adjacent grasses but was consistently lower in their center and opposing edge, suggesting that proximity to grasses, and not edges in general, affected predation. Increases in predation were correlated with increased harvestmen (Opiliones) and spider (Araneae) abundance in native grasses. An experiment suggested the numerical response of harvestmen was sufficient to explain the observed increase in predation. Reducing the distance between crop and non-crop vegetation could benefit generalist predators and increase their impact in crops.

Highlights► We experimentally examined how replacing potato with native prairie grasses affects predation of Leptinotarsa decemlineata. ► Results show native grasses increased predation of L. decemlineata eggs. ► Evidence points towards an increase in spiders and harvestmen as an explanation. ► Reducing distances between non-crop vegetation and crops could increase impacts of generalist predators on pests.