The research tested the natural enemies hypothesis in an attempt to explain why lower pest densities are observed in some diversified farming systems. The research evaluated the influence of floral resource provisioning (FRP) and chemical ecology strategies on biological control of Erythroneura leafhoppers (Hemiptera: Cicadellidae) and Planococcus mealybug (Hemiptera: Pseudococcidae) in California vineyards. Field and laboratory studies quantified the impacts on crop damage, pest and natural enemy abundance, and natural enemies fitness theorized to be enhanced through floral resource provisioning in agroecosystems. Multiple two-year studies measured the impact of intercropping three flowering ground covers, lacy phacelia (Phacelia tanacetifolia), bishop's weed (Ammi majus), and common carrot (Daucus carota) on biological control of leafhoppers and vine mealybug by the parasitoids Anagrus spp. (Hymenoptera: Mymaridae) and Anagyrus pseudococci (Hymenoptera: Encyrtidae). Using identical intercropping treatments, the research included three large scale and fully replicated research designs located in the central San Joaquin, the northern San Joaquin, and the Napa Valley of California. Laboratory studies quantified the impacts of FRP on the fitness of Anagyrus pseudococci, a key parasitoid natural enemy of vine mealybug. The central San Joaquin Valley field study measured the impact of FRP and pheromone based mating disruption on biological control of vine mealybug. The northern San Joaquin Valley field study measured the impact of FRP and methyl salicylate on biological control of Erythroneura leafhoppers. The Napa Valley field study measured the effect of methyl salicylate alone on biological control of Erythroneura leafhoppers.

Modern agriculture is characterized by specialized production and the use of monoculture

cropping practices. These agroecosystems are at once concentrating habitat for crop pests and

eliminating habitat for natural enemies. Multiple studies have demonstrated that such changes

can lead to a decrease or total loss of biological control of pests. At the same time, expansion of

monoculture cropping systems across entire agricultural regions has led to the creation of

landscapes that are entirely dominated by a small number of crops and devoid of natural

habitats. In the same way, entire regions can experience a reduction or loss of biological control

to agriculture.

As such, a number of studies have compared crop fields with high and low habitat diversity and

found that diversified cropping systems tend to have enhance natural enemy populations and

increased biological control of pests. At the same time, another set of studies have demonstrated

that monoculture cropping systems can still experience high levels of biological control so long

as they are situated in a landscape with high levels of habitat diversity surrounding them. More

recently, it has been proposed that the use of on-farm habitat diversification to enhance

biological control will likely be influenced by the area and quality of natural habitat surrounding

the farm (i.e. landscape diversity).

This dissertation was designed to evaluate the influence of habitat diversity at the local and

landscape scale on biological control of the Western grape leafhopper (Erythroneura elegantula

Osborn; 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 habitats

that surround vineyards due to the fact that in order for them to successfully overwinter they

must parasitize an alternate leafhopper host that overwinters in an egg stage (E. elegantula

overwinters in the vineyard as an adult). These alternate leafhopper hosts are known to reside in

the natural and semi-natural habitats that surround North Coast vineyards. As such, it is thought

that biological control of E. elegantula in vineyards is particularly sensitive to changes in

landscape diversity. At the same time, the use of monoculture cropping practices results in a

vineyard environment that is very inhospitable to natural enemies of E. elegantula, including

Anagrus spp. Previous studies have demonstrated that without floral nectar (or an analogous

solution) the lifespan of Anagrus parasitoids can be less than two days and it may be that the

introduction of flowering cover crops into vineyards could possibly increase biological control of

E. elegantula by enhancing Anagrus longevity in the field. In this way, increased habitat diversity

at both the field and landscape scale may support increased natural enemy populations which

would lead to increased biological control of E. elegantula.

For this dissertation, a series of studies were conducted in order to evaluate how changes in

habitat diversity at the field and landscape scale could affect natural enemy populations and

ultimately influence biological control of E. elegantula. First, overwintering habitat of Anagrus

spp. was evaluated to identify the specific host plant species that contained leafhopper eggs that

these parasitoids were attacking in natural habitats during the winter, as well as throughout the

rest of the year. Second, vineyards that were adjacent to riparian habitat were studied in order

to evaluate how distance away from a large natural habitat patch influenced the timing, density

and impact of natural enemies in the vineyard. Third, in order to isolate the influence of

landscape 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 and

high diversity landscapes. Finally, over the course of several years the use of flowering summer

cover crops was developed in collaboration with commercial wine grape growers and vineyard

trials were subsequently conducted to evaluate the ability of these flowering cover crops to

enhance biological control of E. elegantula. In order to evaluate how changes in the landscape

influenced the effectiveness of this on-farm habitat diversification practice, these cover crop

studies were conducted at multiple vineyards that were situated in low, intermediate and high

diversity landscapes.

Results from these studies indicate that the area and composition of natural habitats surrounding

vineyards can have a significant influence on biological control of E. elegantula. Reduced pest

populations in more diverse landscapes is thought to be the result of both reduced crop vigor as

well as increased natural enemy impact during the overwintering period. Early season

populations of Anagrus wasps were found in all vineyards regardless of landscape diversity,

implying a strong dispersal capacity from overwintering sites. The Anagrus demonstrated a

strong density dependent relationship with E. elegantula and this appeared to drive their

densities in vineyards much more so than changes in landscape diversity.

Abstract

Biodiversity and Ecosystem Services in Urban Agriculture: Evaluating Local andLandscape Effects on Parasitic Hymenoptera and Biological Control Services

By Joshua Earl Arnold

Doctor of Philosophy in Environmental Science Policy and Management

University of California, Berkeley

Professor Miguel A. Altieri, Chair

Urbanized areas are the fastest-growing habitat worldwide. In the United States, over 80% of the population now lives in cities. It is expected that urban populations will continue to increase significantly in the coming decades. Matching urban population growth is an increase in urban food production; urban agriculture (UA) has grown 30% in the United States in the last three decades. Growing food in the city has become an increasingly common pathway to affordable, nutrient-rich, and culturally appropriate foods for people who live in high-cost cities.

A myriad of issues complicates urban agricultural production. Once urban farmers gain access to land, they must address many abiotic factors unique to cities that disrupt the ecosystem services many agroecological practices rely on. Increased impervious surface and decreased canopy cover affect hydrological and biogeochemical cycles and increase urban temperatures. Past land uses can affect soil quality and composition. These abiotic factors often exacerbate crop damage from herbivorous insects. Herbivorous pests in urban agriculture can become more persistent and increase in abundance in response to favorable environmental conditions in cities and cause damage to crops.

In urban agriculture, management of pests is almost universally accomplished through cultural practices as pesticides are rejected for environmental and health reasons. Many urban agriculturalists turn to agroecological pest management (APM) practices to increase on-farm beneficial insects and regulate pest populations. Agroecological practices proven on rural farms, such as crop diversification and floral resource provisioning, have been implemented to varying degree in the built environment, often with conflicting results. This research focuses on understanding urbanization impacts on agroecological pest management in urban agriculture. Specifically, how on-farm diversification schemes affect biological control services from parasitic Hymenoptera (PH). Recognizing how these biological control services function in fragmented urban landscapes is vital to urban farmers.

Understanding agroecological pest management practices and factors that may affect ecosystem function on urban farms necessitates understanding urban farm biophysical composition. Over three years, biophysical data were collected on twenty-nine urban farms in the San Francisco Bay Area. The physical composition of urban farms were measured, including overall size, areas of production, and percentage of land not in agricultural production. Indicators of specific management practices, such as type and percentage of mulch and ground cover, floral diversity, and crop and non-crop biodiversity, were recorded, and overall production was assessed. We found that practices associated with APM are widely adopted and are often practiced concurrently. Our research shows that urban farms are highly productive, and most crops grown feed local community members. Land use and spatial composition of urban farms varied, but the production area as a percentage of the total area is often low, and areas set aside for pollination gardens or beneficial habitat are common.

As agroecological pest management in urban agriculture is an understudied topic, a systematic review of research specific to UA and biological control services was conducted. Previous findings recorded significant impacts on both natural enemy and herbivorous pest populations in response to landscape and local effects, but findings remain inconsistent. Local management factors related to agroecological practices, including increased floral abundance, mulch and leaf litter, high plant species richness, and structural diversity, had significant beneficial effects on natural enemy abundance, richness, and biological control services.

We conducted a two-year experiment testing the effects of local management practices and landscape effects on parasitic Hymenoptera, aphids, and crop damage on common Brassica crops. Two fundamental hypotheses in conservation biological control: the enemies and the floral nectar provisioning hypotheses, were tested in novel urban agroecosystems. Local and landscape factors were measured and assessed for their influence on PH populations on eleven San Francisco Bay Area urban farms. Farms were selected to represent a variety of sizes and surrounding imperviousness. Our research indicated that Local factors, including increased mulch coverage, crop richness, and percent of non-crop areas, were predictors of increased PH abundance and aphid parasitism rates.

To test the effects of floral provisioning on PH we sampled thirteen common floral species across community partner sites to link common floral species in urban farms to PH families and subfamilies known to utilize aphids as hosts. We found that PH had no feeding preference, and floral species had little impact on PH abundance. To assess the second criterion of the nectar provision hypothesis, a demonstrable reduction in pests or crop damage, we looked at aphid abundance, rates of parasitism, and overall crop damage on brassicas. Our results show that farms with increased floral richness have lower aphid counts per plant. Our findings indicate that on-farm habitat manipulations can increase ecosystem function, supporting the enemies hypothesis in fragmented urban agriculture sites.

The study of agriculture practiced by the Mixtec people, or “the people of the rain” (Ñuu savi), in the highlands of southern Mexico reveals successful adaptation strategies for growing rainfed crops in an unpredictable and ever changing climate. Both culture and environment have been shaped by a long and challenging history, which continues to bear relevance in this era of globalization and climate change.

My research from 2009 – 2011 focused on farmer strategies for dealing with climatic variability in the Mixteca Alta Region of Oaxaca, Mexico. It was the product of a close collaboration with the farmer-to-farmer training network, the Center for Integrated Small Farmer Development of the Mixteca Alta (Centro de Desarrollo Integral Campesino de la Mixteca Alta, CEDICAM). I was impressed by the depth of farmer knowledge about sustainable agriculture. In fact, what appeared lacking was the grassroots mobilization and political action needed to foster dignified rural livelihoods and environmental stewardship. This topic deserves much greater attention in future research by agroecologists.

My dissertation is organized in four chapters. Chapter 1 contextualizes my research by examining farming traditions of the Mixteca Alta that originated in different historical moments. I also provide a summary of the political economy of labor and sustainable farming.

Chapter 2 examines the social, environmental, and cultural conditions of farming in the Mixteca Alta. Based on in-depth interviews with farmers from two communities, I trace how changes in farming systems has reflected both traditions of farming and an increasingly globalized economy.

I found that a combination of agroecological strategies were important for families to approach self-sufficiency in grain production under highly variable rainfall conditions. Important changes in cropping systems were occurring, particularly shifts towards more precocious crop varieties. It appears that changes in cropping systems have been the result of social disintegration, soil degradation, and climatic changes.

The farmers that I interviewed understood the ecology of their systems. However, were not necessarily farming sustainably. Shift in farming systems were concentrating agricultural labor into the rainy season, thus allowing farmers to work outside their communities for several months during the dry season. While such changes were ostensibly be intended to save labor, they may in fact accomplish the exact opposite. New farming could potentially increase costs in terms of time and money for families, thus diverting attention away from sustainable agricultural practices. Stemming the labor squeeze from farming regions such as the Mixteca Alta likely will require concerted political action, including grassroots mobilization.

Chapter 3 describes participatory research with farmers in the CEDICAM network, as well as climatic studies, that aimed to place climate change mitigation and adaptation into the hands of small farmers. I facilitated workshops in which groups of small farmers described how they had adapted to and prepared for past climate challenges. Farmers reported that their cropping systems were changing for multiple reasons: more drought, later rainfall onset, decreased rural labor, and labor-saving technologies. Examination of climate data found that farmers’ climate narratives were largely consistent with formal climatology data products. There have been increases in temperature and rainfall intensity, and an increase in rainfall seasonality that is likely perceived by farmers as later rainfall onset.

Farmers identified 14 indicators that they subsequently used to evaluate the condition of their agroecosystems. Farmers ranked landscape-scale indicators as more marginal than farmer management or soil quality indicators. From this analysis, farmers proposed strategies to improve the ability of their agroecosystems to cope with climatic variability. Their recommendations, as well as the methodology used in the workshops, holds relevance for farmers and their allies. Notably, they recognized that social organizing and education are required for landscape-level indicators to be improved. This outcome suggests that

climate change adaptation by small farmers involves much more than just a set of farming practices, but also community action to tackle collective problems.

I conclude with Chapter 4, which highlights CEDICAM’s contribution to mobilizing farmers in the Mixteca Alta region. They are taking action to reforest territories and advance agroecological farming. This chapter also reflects on how collaborative research outcomes contributed to CEDICAM’s future outlook on farmer led research. CEDICAM was particularly interested in improving soil cover management and crop selection. CEDICAM as a network continues to promote the social conditions for agroecology to flourish.