On California's central coast, vegetable producers are establishing hedgerows, made up of native perennial trees, shrubs and herbs, to improve conservation biological control (CBC). Theoretically, perennial vegetation can provide stable refugia and food resources that can enhance arthropod diversity in highly disturbed, annual crop systems. However, two critical criteria for assessing the CBC potential of hedgerows are that key natural enemies are enhanced and that these insects disperse into crop fields to attack pests of agricultural importance. Additionally, the enhancement of natural enemies should outweigh the possible exacerbation of pest levels due to hedgerows. In 2005 and 2006, we used biweekly vacuum samples to monitor key natural enemies and pests of vegetable systems on common hedgerow plants, Achillea millefolium, Baccharis pilularis, Ceanothus spp., Eriogonum giganteum, Heteromeles arbutifolia, and Rhamnus californica, at each of five hedgerow sites adjacent to commercial vegetable fields. We quantified plant bloom rates as an indicator of nectar and pollen resource. To track dispersal patterns of pests and natural enemies from hedgerows into adjacent vegetable fields, in 2006, we sprayed hedgerows with fluorescent pigment to mark foraging insects, captured them in the fields with sticky traps at different distances from the hedgerow, and determined the proportion of marked individuals using a microscope and UV light. Results indicate that (1) arthropod natural enemy and pest species are present in hedgerows, (2) natural enemies and pests are distributed differently across plant species, often based on floral resources, (3) natural enemies are more abundant than pests on hedgerows and on certain plants, like Baccharis pilularis, and (4) natural enemies and pests disperse at least 100m from hedgerows into adjacent crop fields. We discuss the implications of these results for CBC.
