Conservation Biological Control in Organic Strawberry Crops: The Effects of Landscape Composition, Farm-Scale Diversification, and Non-Crop Vegetation on Arthropod Communities, Pest Control Levels, and Crop Damage

Extensive conversion of terrestrial habitats to agriculture and widespread application of agro-chemicals have transformed ecosystems worldwide, threatening biodiversity, public health, and the ecosystem services on which food production relies. Conservation biological control is an approach to enhancing pest control ecosystem services by conserving and supporting natural enemies of agricultural pests. A clearer understanding of how pest management practices and plant diversity at different scales impact pest control ecosystem services may reduce reliance on chemical pesticides, conserve biodiversity, and enable the transition to more sustainable food production./nData was collected on organically managed strawberry (Fragaria × ananassa) fields in California’s Central Coast. Focal organisms were the lygus bug (Lygus spp., primarily Lygus hesperus), a major strawberry pest, and its suite of arthropod natural enemies. Study sites were selected along orthogonal gradients of landscape composition and local diversification practices. The effects of two pest management practices (insecticide application and tractor vacuum) were also investigated. At each site, arthropod communities were sampled using a hand-held vacuum, sentinel prey experiments were performed to estimate pest control levels (Chapter 1), and crops were surveyed for lygus bug damage (Chapter 2). Piecewise structural equation modeling (PSEM) was used to test causal pathways among landscape and local-scale variables, arthropod communities, pest control levels, and crop damage. Chapter 3 reports a field-scale study that employed novel bi-directional traps to detect spatial and temporal patterns of arthropod movement between crop and non-crop vegetation./nThe PSEM results provide evidence that landscape and local effects on pest control levels and crop damage are largely indirect and mediated through direct effects on arthropod communities. At the landscape scale, higher proportions of woods increased natural enemy abundance, which in turn increased pest control levels and decreased crop damage. Higher grassland proportions directly increased crop damage. Local diversification practices increased natural enemy diversity, which increased pest control levels. Both insecticides and tractor vacuuming were indirectly detrimental to pest control levels through their negative effects on natural enemy abundance. While insecticides also decreased pest abundance, vacuuming did not. The bi-directional movement study corroborated Chapter 1 and 2 results by showing that lygus bugs moved from weedy patches into crop fields more than they did from woody patches, and that natural enemies including spiders and wasps tended to do the opposite, moving more from woods into crops than they did from weeds. The spatial and temporal patterns displayed in the sentinel data, arthropod abundance, and crop damage data provide evidence for the hypothesis that crop damage is determined by pest abundance subject to natural enemy activity, and that natural enemies play a pivotal role in limiting crop damage. The evidence of causal pathways among plant diversity, arthropod communities, and pest control services, in combination with the spatial and temporal patterns detected by bi-directional trapping, can help growers and regional planners develop specific and effective strategies for enhancing pest control services, reducing reliance on insecticides and tractor vacuums, and promoting biodiversity.

Advisor: Kremen, Claire