Influence of Landscape Diversity and Flowering Cover Crops on Biological Control of the Western Grape Leafhopper (Erythroneura elegantula Osborn) in North Coast Vineyards

Modern agriculture is characterized by specialized production and the use of monoculturecropping practices. These agroecosystems are at once concentrating habitat for crop pests andeliminating habitat for natural enemies. Multiple studies have demonstrated that such changescan lead to a decrease or total loss of biological control of pests. At the same time, expansion ofmonoculture cropping systems across entire agricultural regions has led to the creation oflandscapes that are entirely dominated by a small number of crops and devoid of naturalhabitats. In the same way, entire regions can experience a reduction or loss of biological controlto agriculture.As such, a number of studies have compared crop fields with high and low habitat diversity andfound that diversified cropping systems tend to have enhance natural enemy populations andincreased biological control of pests. At the same time, another set of studies have demonstratedthat monoculture cropping systems can still experience high levels of biological control so longas they are situated in a landscape with high levels of habitat diversity surrounding them. Morerecently, it has been proposed that the use of on-farm habitat diversification to enhancebiological control will likely be influenced by the area and quality of natural habitat surroundingthe farm (i.e. landscape diversity). This dissertation was designed to evaluate the influence of habitat diversity at the local andlandscape scale on biological control of the Western grape leafhopper (Erythroneura elegantulaOsborn; Hemiptera: Cicadellidae) in North Coast wine grape vineyards. The key parasitoids of E.elegantula are Anagrus eryhthroneurae S. Trjapitzin & Chiappini and A. daanei Triapitsyn(Hymenoptera: Mymaridae). These Anagrus parasitoids are intimately tied to the natural habitatsthat surround vineyards due to the fact that in order for them to successfully overwinter theymust parasitize an alternate leafhopper host that overwinters in an egg stage (E. elegantulaoverwinters in the vineyard as an adult). These alternate leafhopper hosts are known to reside inthe natural and semi-natural habitats that surround North Coast vineyards. As such, it is thoughtthat biological control of E. elegantula in vineyards is particularly sensitive to changes inlandscape diversity. At the same time, the use of monoculture cropping practices results in avineyard environment that is very inhospitable to natural enemies of E. elegantula, includingAnagrus spp. Previous studies have demonstrated that without floral nectar (or an analogoussolution) the lifespan of Anagrus parasitoids can be less than two days and it may be that theintroduction of flowering cover crops into vineyards could possibly increase biological control ofE. elegantula by enhancing Anagrus longevity in the field. In this way, increased habitat diversityat both the field and landscape scale may support increased natural enemy populations whichwould lead to increased biological control of E. elegantula.For this dissertation, a series of studies were conducted in order to evaluate how changes inhabitat diversity at the field and landscape scale could affect natural enemy populations andultimately influence biological control of E. elegantula. First, overwintering habitat of Anagrusspp. was evaluated to identify the specific host plant species that contained leafhopper eggs thatthese parasitoids were attacking in natural habitats during the winter, as well as throughout therest of the year. Second, vineyards that were adjacent to riparian habitat were studied in orderto evaluate how distance away from a large natural habitat patch influenced the timing, densityand impact of natural enemies in the vineyard. Third, in order to isolate the influence oflandscape diversity, a multi-year study was conducted to monitor biological control of E.elegantula in a number of vineyard monocultures that were situated in low, intermediate andhigh diversity landscapes. Finally, over the course of several years the use of flowering summercover crops was developed in collaboration with commercial wine grape growers and vineyardtrials were subsequently conducted to evaluate the ability of these flowering cover crops toenhance biological control of E. elegantula. In order to evaluate how changes in the landscapeinfluenced the effectiveness of this on-farm habitat diversification practice, these cover cropstudies were conducted at multiple vineyards that were situated in low, intermediate and highdiversity landscapes.Results from these studies indicate that the area and composition of natural habitats surroundingvineyards can have a significant influence on biological control of E. elegantula. Reduced pestpopulations in more diverse landscapes is thought to be the result of both reduced crop vigor aswell as increased natural enemy impact during the overwintering period. Early seasonpopulations of Anagrus wasps were found in all vineyards regardless of landscape diversity,implying a strong dispersal capacity from overwintering sites. The Anagrus demonstrated astrong density dependent relationship with E. elegantula and this appeared to drive theirdensities in vineyards much more so than changes in landscape diversity.