ENHANCING PLANT DIVERSITY FOR IMPROVED INSECT PEST MANAGEMENT IN NORTHERN CALIFORNIA ORGANIC VINEYARDS

We present the results of studies in organic vineyards in Mendocino and Sonoma counties, California, in an effort to systematize the emerging lessons from our experience on vineyard biodiversity enhancement for ecologically-based pest management. In the Mendocino study, a vegetational corridor connected to a riparian forest channeled insect biodiversity from surrounding habitats into the vineyard, thus overcoming the restricted spatial limits to which the positive influence of adjacent vegetation on vineyard pest dynamics is usually confined. In addition, summer cover crops substantially enhanced biological control of leafhoppers and thrips, by breaking the virtual monoculture that vineyards become in the summer after winter cover crops dry out or are plowed under. In the Sonoma vineyard, an island of flowering shrubs and herbs provided season-long flower resources and alternate preys/hosts for natural enemies, which slowly built up in the adjacent vineyard. The island acted as a push-pull system for natural enemies, enhancing their activity but confining them mostly to the adjacent vine rows. Planting strips of summer cover crops could be a strategy to overcome the push effect of the island. INTRODUCTION Typical grape production in California is in monocultures which are expanding at a rapid rate (nearly 230,000 ha of grapes were grown in California in 2002) resulting in the simplification of the landscape. Since the onset of such simplification, farmers and 2 researchers have been faced with a major ecological dilemma arising from the homogenization of vineyard systems: increased vulnerability of crops to insect pests and diseases which, as in the case of Pierce’s disease, can be devastating when uniform-crop, large-scale monocultures are infested (Hoddle, 2004; Redak et al., 2004). The expansion of monocultures has decreased the abundance and activity of natural enemies due to the removal of critical food resources and overwintering sites (Corbett and Rosenheim, 1996). Many scientists are concerned that, with accelerating rates of habitat removal, the contribution to pest suppression by biocontrol agents using these habitats will decline (Sotherton, 1984; Fry, 1995). It is possible that many pest problems affecting today’s vineyards have been exacerbated by such trends. About 20 million kg of active ingredients of pesticides are used annually in California vineyards to counteract such pest pressure (Department of Pesticide Regulation, 2003). The environmental impact of such pesticide load can be serious (Pesticide Action Network North America, 2005). Concern about these problems has led many people to propose options to rectify this habitat decline by increasing the vegetational diversity of agricultural landscapes. There are many ways in which increased plant biodiversity can contribute to the design of pest-stable agroecosystems by creating an appropriate ecological infrastructure within and around vineyards (Altieri and Nicholls, 2004; Gurr et al., 2004). Biodiversity is crucial to crop defenses: the more diverse the range of plants, animals and soil-borne organisms that inhabit a farming system the more diverse the community of pest-fighting beneficial Proc. IS on Grape Production and Processing Eds.: P.G. Adsule et al. Acta Hort. 785, ISHS 2008 264 organisms (predators, parasitoids, and entomopathogens) the farm can support. In California, farmers usually resort to two main strategies to enhance biodiversity on their vineyards: 1) Many farmers manage floor vegetation or plant cover crops as a habitat management tactic in vineyards to enhance natural enemies. Reductions in mite and grape leafhopper populations have been observed in cover cropped systems (Flaherty, 1969; Daane et al., 1998); however, in many cases such biological suppression has not been sufficient from an economic point of view (Daane and Costello, 1998; English-Loeb et al., 2003). 2) Other farmers manage vegetation surrounding fields to meet the needs of beneficial organisms. Several studies indicate that the abundance and diversity of entomophagous insects within a field is dependent on the plant species composition of the surrounding vegetation and also on the spatial extent of its influence on natural enemy abundance, which is determined by the distance to which natural enemies disperse into the crop (Landis et al., 2000). The role of riparian habitats and especially of wild blackberry patches, near vineyards in enhancing the effectiveness of the wasp Anagrus epos in parasitizing the grape leafhopper is well known (Doutt and Nakata, 1973). Based on this knowledge, Corbett and Rosenheim (1996) found that French prunes (Prunus domestica) adjacent to vineyards could also serve as overwintering sites for A. epos and that leafhopper parasitism was higher in grape vineyards with adjacent prune tree refuges. Other strategies tested experimentally and used by very few farmers include: designing corridors of plants that usher beneficial species from nearby forests or natural vegetation to field centers (Nicholls et al., 2001); selecting non-crop plants grown as strips or islands in fields, whose flowers match the requirements of beneficial species (Gurr et al., 2004). All the above strategies provide alternative food (pollen and nectar) and refuge for predators and parasitoids and increase natural enemy diversity and abundance in vineyards (Altieri and Nicholls, 2004). In the last 7 years, we have applied the above strategies to the design and management of organic vineyards in northern California. In this paper, we present 3 results from some of our previously published studies (Nicholls et al., 2000, 2001), complemented by data from a new case study, in an effort to systematize the emerging lessons from our experience on vineyard biodiversity enhancement for ecologically-based pest management. Our earlier studies took advantage of an existing 600 m corridor with at least 65 flowering species connected to a riparian forest that cut across a monoculture organic vineyard located in Mendocino County in California. This setting allowed for testing the idea whether such a corridor served as a biological highway for the movement and dispersal of natural enemies into the center of the vineyard. We were interested in evaluating if the corridor acted as a consistent, abundant and well-dispersed source of alternative food and habitat for a diverse community of generalist predators and parasitoids, allowing predator and parasitoid populations to develop in the area of influence of the corridor well in advance of vineyard pest populations. We also hypothesized that the corridor would serve as a conduit for the dispersion of predators and parasitoids within the vineyard, thus providing protection against insect pests within the area of influence of the corridor by allowing distribution of natural enemies throughout the vineyard. As the vineyard also contained cover crops, a hypothesis tested in this study was that neutral insects (non-pestiferous herbivores) and pollen and nectar in the summer cover crops provide a constant and abundant supply of food sources for natural enemies. This in turn decouples predators and parasitoids from a strict dependence on grape herbivores, allowing natural enemies to build up in the system, thereby keeping pest populations at acceptable levels. We tested this hypothesis and examined the ecological mechanisms associated with insect pest reduction when summer cover crops are planted early in the season between alternate vine rows. Last year studies capitalized on the