Agroecology in Action
revised 07-30-00



Plant Biodiversity and Pest Management in a Northern California Vineyard*  

Clara I. Nicholls Department of Entomology, UC Davis
Michael P. Parrella Department of Entomology, UC Davis
Miguel A. Altieri Center for Biological Control, UC Berkeley

One of the consequences of the trend toward expansion of large-scale monocultures is the loss of habitats for natural enemies, which results in increased pest problems. Emerging research shows that one way to reverse this trend is by increasing plant diversity within and around agroecosystems. Some studies have shown that there is enhancement of natural enemies and more effective biological control in vineyards and orchards with cover crops or where wild vegetation remains at field edges. Flowers in such habitats provide pollen, nectar and overwintering sites for a number of predators and parasites.

The goal of this study is to investigate the influence of a 0.3 mile wild flower corridor composed of 66 different flowering plants that cuts across a northern California organic Chardonnay vineyard on the diversity, distribution, abundance and dispersal of key insect herbivores (leafhoppers and thrips) and associated natural enemies. The goal is to test whether changing the landscape layout of the vineyard with this natural flower corridor and the presence of cover crops breaks the monoculture nature of the vineyard, and whether it enhances functional natural enemy biodiversity. The first year of research was directed at determining the species diversity and abundance levels of arthropod fauna associated with the various corridor plant species and at assessing if the corridor influences diversity and abundance of natural enemies in the adjacent vineyard. Arthropod population parameters measured in this vineyard (herein called Block A) were compared to trends observed in a neighboring vineyard that contained no corridor (Block B). Both blocks are managed organically with half of the area of each block planted to summer cover crops (buckwheat and sunflower) which are also a source of flowers.

From May 18 to August 28, 1996 and also during the 1997 growing season, intensive weekly monitoring of herbivorous insects and associated natural enemies was conducted using several sampling methods: yellow and blue sticky traps placed in the corridor and within the vineyards at varying distances from the corridor; direct counting of leafhopper nymphs on leaves; branch shaking; D-vac of vines; sweep netting the covercrops and malaise traps placed across "flight paths" between vineyards and adjacent edges.

Data collected during the 1996 summer shows that leafhoppers and thrips exhibited a clear density gradient reaching lowest numbers in rows close to the corridor and increasing in densities in rows away from the corridor. Predaceous insects such as Orius and generalist predators in the families: Coccinellidae, Chrysopidae and spiders also exhibited and abundance gradient. Leafhoppers were less abundant in vines with cover crops beneath than in vines with bare ground, and this may due to the fact that predators were more numerous and diverse in vines with cover crops.

Data analysis suggests that the corridor serves as a source of natural enemy biodiversity affecting the abundance and distribution of predators in the adjacent vineyard, thus impacting densities of leafhoppers and thrips in vines close to the corridor. Cover crops complement the ecological effects of the corridor by serving as a source of natural enemies, which in turn determine lower pest densities in vines with cover crops beneath. Timing the mowing of cover crops forces movement of Anagrus and Orius and other predators to the adjacent vineyards resulting in further reductions of leafhoppers and thrips.

* Abstract distributed to participants in field day held at Kohn Properties, Hopland, California. August 15, 1997