Natural habitat may deliver ecosystem services to agriculture through the provision of natural enemies of agricultural pests. Natural or non-crop habitat has strongly positive effects on natural enemies in cropland, but the resulting impact on pests is not as well established. This study measured weekly natural enemy (syrphid fly larvae) and pest (cabbage aphid) abundances in Central California broccoli fields for three years. Abundance of syrphid fly larvae increased strongly with the proportion of natural habitat surrounding the farm. As the density of syrphid fly larvae increased, weekly aphid population growth rates slowed, such that aphid densities just prior to harvest were lowest in farms with natural habitat. These landscape-mediated impacts of syrphids on aphids were not evident when data were aggregated into annual averages, a common metric in research on pest control services. We suggest that higher temporal resolution of data for natural enemy and pest abundance can reveal top-down control that is otherwise masked by seasonal and interannual variation in environmental factors.
Bibliographical noteFunding Information:
We are grateful to growers at Mission Organics, Earthbound Farms, Route 1 Organics, Pinnacle Organics, Lakeside Organics, Pure Pacific Organics, Tanimura & Antle, Crown Packing, Swanton Berry Farm, ALBA farms, and the UCSC and USDA experimental farms for their participation in this project. We thank S. Bothwell, K. Tuxen-Bettman, K. Manivong, N. Schowalter, D. Kramer, L. Morandin, D. Karp, and the UC Berkeley's Geospatial Innovation Facility (gif.berkeley.edu) for their support, and three anonymous reviewers for valuable feedback. We are particularly indebted to W. Chaney and H. Smith for introduction to the system, and to M. Hauser for assistance in syrphid identification. Financial support came from the Environ Foundation, the Organic Farming Research Foundation, the USDA's Western-SARE program, and R.C.K. was supported by a National Science Foundation graduate fellowship.
- Biological control
- Brevicoryne brassicae
- Ecosystem services
- Landscape complexity
- Trophic ecology