A Sentinel Pest Experiment On Organic Farms

Previous research has shown that birds can provide pest control in agricultural landscapes, but little is known about what influences the provisioning of this ecosystem service. This study explores the micro-scale variables that may affect bird predation on organic farms throughout Humboldt County, California. A pest outbreak was simulated on 29 farms using cabbage loopers (Trichoplusia ni) glued to 20 kale leaves set out in a 20x15 meter grid in the row crops of each farm. Stations were set out in the early mornings with video cameras that were used to identify and observe the behavior of the predatory species. At each station, we estimated percent vegetation cover, maximum vegetation height, distance to uncultivated habitat, and crop type. We found that bird predation varied strongly both within farms and among farms, and the most commonly detected predators were American Robins (Turdus migratorious) and Western Scrub Jays (Aphelocoma californica). We found a positive association between depredated caterpillars and lower percent vegetation cover. There was mixed support for relationship between distance to uncultivated habitat and predation, but there was strong support for bird predation being more frequent on crops in the Brassica genus. These results may indicate that visual conspicuousness of prey along with mental search images learned by birds may affect the patterns of bird predation within farms.