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 monoculture/ncropping practices. These agroecosystems are at once concentrating habitat for crop pests and/neliminating habitat for natural enemies. Multiple studies have demonstrated that such changes/ncan lead to a decrease or total loss of biological control of pests. At the same time, expansion of/nmonoculture cropping systems across entire agricultural regions has led to the creation of/nlandscapes that are entirely dominated by a small number of crops and devoid of natural/nhabitats. In the same way, entire regions can experience a reduction or loss of biological control/nto agriculture./nAs such, a number of studies have compared crop fields with high and low habitat diversity and/nfound that diversified cropping systems tend to have enhance natural enemy populations and/nincreased biological control of pests. At the same time, another set of studies have demonstrated/nthat monoculture cropping systems can still experience high levels of biological control so long/nas they are situated in a landscape with high levels of habitat diversity surrounding them. More/nrecently, it has been proposed that the use of on-farm habitat diversification to enhance/nbiological control will likely be influenced by the area and quality of natural habitat surrounding/nthe farm (i.e. landscape diversity)./nThis dissertation was designed to evaluate the influence of habitat diversity at the local and/nlandscape scale on biological control of the Western grape leafhopper (Erythroneura elegantula/nOsborn; Hemiptera: Cicadellidae) in North Coast wine grape vineyards. The key parasitoids of E./nelegantula are Anagrus eryhthroneurae S. Trjapitzin & Chiappini and A. daanei Triapitsyn/n(Hymenoptera: Mymaridae). These Anagrus parasitoids are intimately tied to the natural habitats/nthat surround vineyards due to the fact that in order for them to successfully overwinter they/nmust parasitize an alternate leafhopper host that overwinters in an egg stage (E. elegantula/noverwinters in the vineyard as an adult). These alternate leafhopper hosts are known to reside in/nthe natural and semi-natural habitats that surround North Coast vineyards. As such, it is thought/nthat biological control of E. elegantula in vineyards is particularly sensitive to changes in/nlandscape diversity. At the same time, the use of monoculture cropping practices results in a/nvineyard environment that is very inhospitable to natural enemies of E. elegantula, including/nAnagrus spp. Previous studies have demonstrated that without floral nectar (or an analogous/nsolution) the lifespan of Anagrus parasitoids can be less than two days and it may be that the/nintroduction of flowering cover crops into vineyards could possibly increase biological control of/nE. elegantula by enhancing Anagrus longevity in the field. In this way, increased habitat diversity/nat both the field and landscape scale may support increased natural enemy populations which/nwould lead to increased biological control of E. elegantula./nFor this dissertation, a series of studies were conducted in order to evaluate how changes in/nhabitat diversity at the field and landscape scale could affect natural enemy populations and/nultimately influence biological control of E. elegantula. First, overwintering habitat of Anagrus/nspp. was evaluated to identify the specific host plant species that contained leafhopper eggs that/nthese parasitoids were attacking in natural habitats during the winter, as well as throughout the/nrest of the year. Second, vineyards that were adjacent to riparian habitat were studied in order/nto evaluate how distance away from a large natural habitat patch influenced the timing, density/nand impact of natural enemies in the vineyard. Third, in order to isolate the influence of/nlandscape diversity, a multi-year study was conducted to monitor biological control of E./nelegantula in a number of vineyard monocultures that were situated in low, intermediate and/nhigh diversity landscapes. Finally, over the course of several years the use of flowering summer/ncover crops was developed in collaboration with commercial wine grape growers and vineyard/ntrials were subsequently conducted to evaluate the ability of these flowering cover crops to/nenhance biological control of E. elegantula. In order to evaluate how changes in the landscape/ninfluenced the effectiveness of this on-farm habitat diversification practice, these cover crop/nstudies were conducted at multiple vineyards that were situated in low, intermediate and high/ndiversity landscapes./nResults from these studies indicate that the area and composition of natural habitats surrounding/nvineyards can have a significant influence on biological control of E. elegantula. Reduced pest/npopulations in more diverse landscapes is thought to be the result of both reduced crop vigor as/nwell as increased natural enemy impact during the overwintering period. Early season/npopulations of Anagrus wasps were found in all vineyards regardless of landscape diversity,/nimplying a strong dispersal capacity from overwintering sites. The Anagrus demonstrated a/nstrong density dependent relationship with E. elegantula and this appeared to drive their/ndensities in vineyards much more so than changes in landscape diversity.

Advisor: Altieri, Miguel A.