Effects of shrub crop interplanting on apple pest ecology in a temperate agroforestry system

Pest control by wild arthropods is an important ecosystem service in fruit crops, especially due to markets that value minimal pesticide use. Techniques to augment on-farm habitat for wild arthropods have focused on flowering ground cover planted within orchards and hedgerows on field borders. However, natural enemies found in groundcover often do not favor tree canopy habitat. Conversely, while hedgerows can effectively provide natural enemies that prefer woody microhabitats, their impact diminishes away from field edges. Shrub crops interplanted within orchards could resolve both problems, providing woody habitat for natural enemies directly adjacent to target crop trees. In a multi-layer agroforestry system in Illinois, we vacuum sampled arthropod communities across layers and recorded vegetation characteristics and pest damage on apples. Using generalized linear models, information theoretic model selection, and non-metric multidimensional scaling, we evaluated the effects of three shrub treatments (raspberries, hazelnuts, and both species) on pest and natural enemy guilds in apple trees and shrubs, and on the frequency of pest damage on apples. Shrub composition was an important predictor of arthropod communities on shrubs. However, shrub treatment had only minor impacts on arthropods in apple canopies, indicating the habitats are less similar than anticipated. While two arthropod guilds in apple canopies were linked to pest damage frequency, neither was sensitive to changes in the shrub layer. Results suggest that shrub crop interplanting does not inherently resolve the ecological complexities that impede existing approaches in conservation biological control.